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                  Physics and Astronomy                      
              Home  › Our Teaching › Resources › C programming › Conditional statements
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            Conditionals: if() and switch()
            Comments and questions to John Rowe.
                        
                    
            Control statements
                            
            Our very first example of an algorithm, a set of directions          from one place to another, was just a sequence of steps to be          followed ( or "executed") from beginning to end. But very few          algorithms are like that, and our second example, calculating          wages, contained two new features: 
                            
                        
            •  loops, where steps were            repeated several times 
              •           
            • if conditions (more formally            knowns as conditionals) where            certain steps were taken only a particular condition was met            and, optionally, other steps which were taken if it was not.          
              •         
                            
            Another point worth noting is that these were nested one inside the other (the "which          day is it?" condition was inside the "for each day" loop which          was in turn inside the "for every employee" loop. 
                            
            Just about all real algorithms contain these features and          over the next two lectures we shall look at the C statements          most commonly used for this. In this lecture we will study the          conditional statements  if() and switch() which          use logical conditions to control whether code is executed.
                            
            In the next lecture we shall introduce loops          which execute code zero or more times. 
                            
            Like everything else in C, these statements operate in a          simple and consistent way, the key is in how we use them to          create more-complicated programs. 
                            
            Control statements          control which instruction is executed next.
                     
                  
                    
            The if() statement
                            
            The if statement is the primary way for us to          specify that a set of instructions ("statements") are only to          be executed if a certain condition is true. 
                            
            Any statement can have the construction if (expression)          put in front of it. 
                            
              if ( x > 0 ) printf("x is greater than zero\n");
                            
             The sub-statement following the "if()"          is only executed if the controlling expression is true.
                            
            It is conventional to put the sub-statement          on a separate line indented from the original: 
                            
              if ( x > 0 )     printf("x is greater than zero\n");
                            
            There is no semi-colon after the if()          as it is all one statement.
                            
             This is very important: 
                            
              x = -1.4;  if ( x >  0 );         // Subtle but horrible mistake     printf("x is greater than zero\n");
                            
            In this buggy code the if()            statement terminates with the semicolon. Therefore          the printf() will always occur. (Notice that the          indentation has no effect, it's just there to make it easier          for us to understand.) 
                            
             The meaning of > should be          fairly obvious, C also has < (less          than), >= (greater than or equal          to: notice no space) and <= (less          than or equal to), as well other operators which we discuss          below. 
                            
                        
            1.  A simple if() statement 
              2.           
            2.  To practice using the if() statement 
              3.           
            3.  Create a new on-line program in a new window, with a suitable title and opening comment.
              4.           
            4.  Declare two integers and read in their values using scanf()            and %d            
                              
              •  Whenever we read in data from the keyboard we should                always print a message beforehand. In this case it could                be as simple as: 
                                printf("Pease enter two integers\n"); 
                •             
                        
              5.           
            5.  Write an if() statement so that if the first            number is greater than the second your program prints "the            first number is greater than the second" and otherwise does            nothing. 
              6.           
            6.  Obviously you should check this for various numbers you            enter. 
              7.         
                  
                    
            else
                            
            The word else has the meaning          of "otherwise".
                            
             Here we use the "less than" operator for variety: 
                            
              if ( x < 0 )     printf("x is less than zero\n");  else    printf("x is greater than or equal to than zero\n");
                            
                        
            1.  else 
              2.           
            2.  To practice using the else statement 
              3.           
            3.  Modify your previous program so that if the            first number is not greater than the second it prints a            suitable message (using the else statement). That            is, it will always print out one message or the other but            never both. 
              4.         
                  
                    
             else if ()
                            
            More complicated logical conditions can be          expressed using else if () which          introduces an alternative that is only examined if the first if()          was false. It occurs before the else, if any. 
                            
              if ( x > 100 )     printf("x is greater than 100\n");  else if ( x > 10 )    printf("x is greater than 10 but less then or equal to 100\n");  else if ( x > 0 )    printf("x is greater than 0 but less then or equal to 10\n");  else     printf("x is negative\n");
            Step through this code
            Note the order
            if(first condition)  do something;else if(second condition)  // Only if first condition was false  do something else;else if(third condition)   // Only if second condition was also false  do something else again;else                        // Only if third condition was also false  do something else yet again; // 
            The program look at each possibility in turn:          if it finds one that is true it jumps to the end, otherwise it          executes the contents of the else
                            
            The expression else if ()          introduces an alternative that is only examined if the first if()          was false.
                            
                        
            1.  Step through the above "Key example".
              2.           
            2. To understand the else if
              3.           
            3. Step through the above "Key example" in a new window.
              4.           
            4. Step through the     code making sure you always know what the            code will do next.
              5.           
            5. At the end of the code the parts that have been executed            will be highlighted. Make sure you understand the reasons            why some parts were executed and others were not.
                        
              6.         
            There are two things to notice: 
                            
                        
            1.  There is a space in "else if". 
              2.           
            2.  Had we missed out the else s then several            printf()s would have executed, which obviously            would have been an error. 
              3.         
                            
            There can be as many else if ()s          as we like, they are examined in order           but once any one of them evaluates to "true" (non-zero) the          rest are ignored, as is the else if there is one.
                            
            Technically the entire if() ... else          construct forms a single compound statement. 
                            
            Reminder
                            
                        
            •  There is no semi-colon after the if ( ... ).            This would end the if statement at that point. 
              •           
            •  There is also no semi-colon after the { ... }            block. 
              •           
            •  The contents of the block should always be indented for            clarity but the compiler takes no notice of that: it always            folows the syntax. 
              •         
                             
                        
            1.  else if() 
              2.           
            2.  To practice using the else if() statement 
              3.           
            3.  You may find that the easiest way to do            steps 1 and 2 is to copy and paste your initial if()            and printf() statements so that you know have two            of them: 
                          if() 
                            printf() 
                          if() 
                           printf()) 
                          and to edit the result.
                           
                        
              4.           
            4.  Modify your previous program so that the            initial if() condition tests to see if the first            number is greater than twice the second. 
              5.           
            5.  Add an else if() statement that performs the            original test: (first number is greater than the second).            Remember the order: 
                          if()
                          else if()
                          else 
              6.           
            6.  Modify the first two printf() statements so            that their messages now make sense. 
              7.           
            7.  Check your program works properly for all three cases. 
              8.         
            if() ... else if() v several if()s
            The previous example was a single compound if()          statement where only one possible action could occur. In the          following we have replaced all the "else if()"s          with "if()" to create several independent          if() statements where more than one action could occur:
                    
              if ( x > 100 )     printf("x is greater than 100\n");  if ( x > 10 )    printf("x is greater than 10 but less then or equal to 100\n");  if ( x > 0 )    printf("x is greater than 0 but less then or equal to 10\n");  else     printf("x is zero or negative\n");
            Step through this code
            Step through the above two examples and make          sure you understand the difference.
                    
                        
            1.  Mistakes with if() 
              2.           
            2.  To show the effects of ordering and the difference            between if() ... else if() and if() ... if()          
              3.           
            3.  Change the title and initial comment of your if()program            to indicate it will contain some deliberate mistakes. 
              4.           
            4.  Now swap over the two tests so that the if()            checks to see if first number is greater than the second and            the else if() checks to see if first number is            greater than twice the second. 
              5.           
            5.  Again, check your program for all three possible cases.
                              
              •  Does it work properly? 
                •               
              •  Why (not)? 
                •             
                          If you are not sure what's going on step through the            previous example code again. 
              6.           
            6.  Remove both elses so that both just read if(),            rather like the example above. Try all three classes of            values and see what happens. 
              7.         
                  
                    
            Relational operators
                            
                      
            What is truth?
                                
            C does not use a separate "logical" or Boolean (true/false)            class for its conditional expressions. Instead, the            expression in an if statement is simply an            arithmetic expression which is taken to be "true" if it is            non-zero. Thus, in principle we can write: if ( -0.001 ),            which would always be considered to be "true" as -0.001 is            not equal to zero. Needless to say, this wouldn't be a very            good idea! 
                    
                            
            The following relational operators all return          the (int) values one          ("true") or zero ("false").
                    
                                  
                          Operator               One if             
                        
                          expression1 >                  expression2               expression1 is                  greater than expression2             
                        
                          expression1 <                  expression2               expression1 is less                  than expression2             
                        
                          expression1 >=                  expression2               expression1 is                  greater than or equal to expression2                          
                        
                          expression1 <=                  expression2               expression1 is less                  than or equal to expression2                          
                        
                          expression1 ==                  expression2               expression1 is                  equal to expression2             
                        
                          expression1 !=                  expression2               expression1 is not                  equal to expression2             
                                          
                                    
             An operand is a fancy name for the          things operators operate on, e.g. in the expression "x * y"          the operator is * and the operands x and y.        
                            
             As with the arithmetic operators, C "does          the right thing" if the operands are of different type, e.g. if (0.001 > 0)          behaves as we would expect ("true"), it doesn't get confused          by the fact that one side is a double and the other          an integer. 
                            
            Note that the equality operator is a double equals sign, a single equals sign          is the assignment operator (see below). There is no space in ==,          !=, <= or >= .
                            
            Other "logical" operators
                            
            C also defines the following operators for          the logical operations "AND", "OR", etc. 
                            
            These operators all have scalar operands,          usually individual relational expressions such as x > 0,          etc.
                            
            In the following examples we use constants to          make clear what the results should be although in practice          some or all of them would be variables, for obvious reasons.
                    
                                  
                          Operator               Meaning                                Example & result                                Example & result             
                        
                           &&                AND                8 > 1 && 7 < 4               0                8 > 1 && 4 < 7               1             
                        
                           ||                OR                8 > 1 || 7 < 4                1                8 > 1 || 4 < 7               1             
                        
                           ( )                Parentheses                ( 6 > 4 || 4 < 6 ) && 6 == 3               0                6 > 4 || ( 4 < 6 && 6 == 3 )               1             
                        
                           !                Negation                ! 1               0                 ! ( 8 > 1 && 4 < 7 )                0             
                                          
                                    
            Note that the OR          operator "||" evaluates to one ("true") if either or both of          its operands are non-zero. 
                    
            The "and" and "or" operators && and ||          combine other expressions. 
                    
            Parentheses "()" group other expressions          and ! is negation.. 
                            
            Example: Game of Life
                            
            Conway's classic Game of Life considers a rectangular grid of          cells, each of which is either alive or dead. Each cell has          eight neighbours; the diagram below shows a live central cell          with four live neighbours and four dead ones: 
                    
                      
                                      
                             *                   
                                        		                 *                
                          
                             *                   *                   
                                        		              
                          
                             
                                        		                  *                   
                                        		              
                                              
                    
                    The game progresses in generations, in each generation a live        cell with either two or three live neighbours stays alive,        otherwise it dies. A dead cell with exactly three live        neighbours becomes alive, otherwise it stays dead. In the above        example the central cell would die.         
             Put another way: a cell with exactly three live neighbours          will be alive in the next generation, as will a cell that is          alive in this generation with exactly two live neighbours. All          other cells will be dead. 
                            
            If we denote a live cell by the value one and a dead cell by          zero, then an if statement to calculate the          next-generation state of a cell would be: 
                            
             We don't actually need the parentheses after          the "||" but there's less chance of us making an          error if we put them in. 
                            
              if ( neighbours == 3 || ( state == 1 && neighbours == 2 ) )    nextstate = 1;  else    nextstate = 0;
                            
            Semi-advanced point: && and || evaluate their          arguments in order
                            
            Normally when we write an expression such as "x + y"          the compiler may choose to evaluate the two arguments in          whatever order it likes. 
                            
            But && and || are different: the computer will always evaluate the left-hand side first.          Why? Because for || if the left-hand side is true it doesn't          need to evaluate the right-hand side to know the overall value          is one (true) and therefore it won't. 
                    
             A logical sequence such as: 
                            
            expression1 || expression2 || expression3 || ...
                            
            can be thought of a meaning "evaluate expression1,          expression2 etc. until one is true (non-zero), and          return one. If none of them is true the overall answer is          zero". Similarly: 
                            
            expression1 && expression2 && expression3 && ...
                            
            can be thought of a meaning "evaluate expression1,          expression2 etc. until one is zero (false), and return          zero. If all of them are non-zero the overall answer is one".        
                            
            In either case unnecessary expressions are          not evaluated. 
                            
            The "and" and "or" operators && and ||          only evaulate their Right-Hand-Sides when necessary. 
                            
            Precedence
                            
            "And" binds more closely than "or" ie a && b || c is equivalent to (a && b) || c. However, the          "extra warnings" option to our compiler will insist we          explicity put in the parentheses.
                            
             a && b || c          is equivalent to (a && b) ||            c, but the "extra warnings" option to our compiler          requires the parentheses.
                            
            Example of combining expressions with          && and || 
            Here we use expressions involving variables rather than          constants, which of course is what we would do in practice: 
                    
            // Example of using if() with && and ||#include <stdio.h>int main() {  int j = 2, k = 3;  double x = 4.1;  // LHS is true so RHS is NOT evaluated  if ( j + k > x || x > 10 )    printf("Hello 1\n");  // LHS is true so RHS IS evaluated (but is false)  if ( j + k > x && x > 10 )    printf("Hello 2\n");  // Combining && and ||  if ( (j + k < x && x > 10) || j < k )    printf("Hello 3\n");  return 0;}
            Step through this code
                  
                    
            Three common mistakes to look out for
                            
            The compiler will warn you about          all the errors below with the (PCs running Code::Blocks will          flag them as an error, XCode on the Mac will show the Yellow          Triangle of Peril\x99), although the warning may be a little          opaque, for example words to the effect of "use parentheses if          you really want to do this". 
                            
            1. The grandmother of all bugs - missing out an =          sign
                    The equality operator is the double        equals sign ==, and the assignment        operator single equals sign = .         
            This leads to the classic bug: 
                            
                if ( mass = 0 )   // Wrong!    printf("The mass should not be zero!\n"); 
                            
            Step through this code
                            
                        
            1.  Step through the above example.
              2.           
            2. To see the difference between = and == inside an if()
              3.           
            3. Step through the above example in a new window.
              4.           
            4. Step through and look very carefully as it evaluates mass            = 0 inside the if(). Notice how it behaves exactly the same            as it does outside of an if().
              5.           
            5. Now look what happens when it evaluates the correct            expression mass == 0.
                        
              6.         
                    This has two consequences: 
                            
                        
            1.  The variable mass is assigned the value zero. 
              2.           
            2.  This value of the variable mass is used as the            condition of the if() which is of course false, as            mass has just been set to zero. 
              3.         
                            
            Use if ( mass == 0 ) not (            mass = 0 ).
                            
            2. && and || are double too
                    Less common is to have a single & or | when we meant to have        two:         
              if ( j > 0 & j % 2 == 0)     // Wrong     printf("j is positive and even\n");
                            
            This is also legal (but almost certainly          wrong) as a single & is the "bitwise and"          operator which we won't be using until a later lecture.
                            
             && and || are double just like ==
                    
            3. a == b == c doesn't do what we might expect
                            
            Use: 
                            
              a == b && b == c
                            
            instead. 
                            
             This is because C, which places great value          on consistency, treats the expression a == b == c          in the same way as it treats a * b * c.          First it evaluates a == b to obtain either          zero or one, then it checks that c is equal to that          value (zero or one). The expression is therefore equivalent          to:           
            ( a == b && c == 1 ) || (a != b && c == 0 )
                                
            which is probably not what you want! 
                    
                            
            Similarly, use
                            
              a > b && b > c
                            
            instead of: 
                            
              a > b > c     // Wrong 
                            
            Example
                            
            // Demonstrate classic error with if() operators// This code will trigger advanced error warnigs if enabledint main() {  int a = 2, b = 2, c = 2;  if ( a == b == c ) // Wrong!    printf("They are all the same\n");  if ( a == b && b == c ) // Right    printf("They are all the same\n");    if ( a < b < c )  // Wrong!    printf("a < b < c\n");    if ( a < b && b < c ) // Right    printf("a < b < c\n");  return 0;}
            Step through this code 
                            
            use if (a == b && b == c)          not (a == b == c)
                     
                  
                    
            { ... } blocks
                            
            One useful feature of C is that we can put a          { ... } block wherever we can put a single          statement. There is no semi-colon after the closing },          just like there isn't one after the { ... }          block that forms the main() function. 
                            
            So, we could have chosen to write the above as: 
                            
              if ( x > 0 ) {    printf("x is greater than zero\n");  }  else {    printf("x is less than or equal to zero\n");  }
                            
            Whilst it's not necessary in this case, don't            hesitate to use { } if you think it's clearer.          Remember the megaprinciple! 
                            
            We can, of course, put more than one statement in a { ... }          block, which is the main reason for having them. Let's          practice using the "greater than or equal to" operator: 
                            
              if ( x >= 0 ) {    printf("x is greater or equal to zero\n");    y = sqrt(x);  }  else {    printf("x is less than zero\n");  }
                            
            However, the following would be a disaster: 
                            
              if ( x >= 0 )       printf("x is greater than zero\n");      y = sqrt(x);  // ERROR
            Step through this code
                            
            Turn to your neighbour and ask why this          is.
                            
             We can put a { ... } block          wherever we can put a single statement. There is no semi-colon          after the closing }.
                            
            Any { ... } block can have its own          variables
                            
             We have already seen that the main() { ... }          block can have variables declared inside of it. C being          consistent, so can any { ... }          block. These variables are only usable within the block and          their values are not preserved if          the block is re-entered, for example if it is a loop. 
                            
            Any { ... } block can          have its own variables whose values are not            preserved if the block is re-entered.
                     
                  
                    
            Indentation and layout
             In the "Introduction to C" lesson we briefly touched upon          the fact that clarity of our code is enormously helped by indentation. 
                    
            Compare a set of bullet points printed twice, with and          without indentation: : 
                    
                      
                                      
                                              
            Pets: pros and cons
                                  
            •  Cat
                                        
              •  Easy to look after 
                •                         
              •  Kills birds 
                •                       
                                  
              •                     
            •  Dog
                                        
              •  Friendly 
                •                         
              •  Needs daily walk 
                •                       
                                  
              •                     
            •  Great white shark
                                        
              •  Looks great. 
                •                         
              •  Feeds itself 
                •                         
              •  Has a nasty nip:
                                              
                •  Big pointy teeth 
                  •                             
                •  Can open jaws really wide 
                  •                           
                                        
                •                         
              •  Not ideal first pet 
                •                       
                                  
              •                   
                                        		                                                    
            Pets: pros and cons
                                  
            •  Cat
                                        
              •  Easy to look after 
                •                         
              •  Kills birds 
                •                       
                                  
              •                     
            •  Dog
                                        
              •  Friendly 
                •                         
              •  Needs daily walk 
                •                       
                                  
              •                     
            •  Great white shark
                                        
              •  Looks great. 
                •                         
              •  Feeds itself 
                •                         
              •  Has a nasty nip:
                                              
                •  Big pointy teeth 
                  •                             
                •  Can open jaws really wide 
                  •                           
                                        
                •                         
              •  Not ideal first pet 
                •                       
                                  
              •                   
                                        		              
                                              
                    
             The use of so-called white            space, including blank lines, to visually indicate          structure has been used in books for centuries. 
                    
             Our code should be indented in the same way          as a list of bullet points and for the same reason; to          indicate its structure. If we look at our            example programs we will see that all the lines start flush            at the left except for the contents of the { ... } block which are            indented by the same amount. In this case it's two spaces,            four spaces is also common. The closing } is            brought back to the left, level with the code outside of the            { ... } block. 
                    
            When we write functions containing loops and conditionals ("if          statements"), their contents will also be indented by the same          amount, thus making the logical structure of our function          clear at a glace. 
                    
            We also left a couple of blank lines before the start of the          main() function and this is also good practice for          all functions. Minor sub-divisions within a function can be          denoted by a single blank line. 
                    
             The google style guide specifies that { ... }          blocks are indented by two spaces          and that lines are a maximum of eighty          characters long. 
            This is extremely important and teams          of programmers have instructions ("style guides") that specify          exactly how code should be formatted. 
                    
            In general C doesn't care about white-space at all outside of          character strings, although the compiler will get upset if we          try to break up words ("pri  ntf") or          numbers ("1.2  34"). 
                    
            Also remember that we will sometimes need to print our          program, so follow the google guidelines and limit our lines          to eighty characters. We can break long statements over          several lines, making sure that the continuation lines are          themselves properly indented. 
                    
             We are also allowed to have more than one statement on the          same line but again this is highly disapproved-of. 
                    
            Layout summary
                        
            •  Indent the contents of  { ... }            blocks by either two or four spaces, always using the same            amount. 
              •           
            •  Bring the final } back in line            with the outside of the block. 
              •           
            •  Leave two or three blank lines between functions. 
              •           
            •  Leave a single blank line within a            function when we wish to emphasise internal divisions. Note            the logic here: gaps between lines within functions are less            than those between functions so that a quick glance tells            the brain the code is divided into divsions (in this case            functions) which in turn have (informal) subdivisions. 
              •         
            Correct layout of our code, particularly the          indentation and blank lines, makes it much easier to          understand. 
                    
            The markers will be            instructed to be extremely strict on this as it's          very important and not hard to get right. 
                  
                  
                    
            Nested if()s
                            
             We can put if() statements inside each other. This          is useful when we have more than one criterion to choose          between, as in the following example. 
                            
            Example
                            
             In this example we wish to do different things according to          whether a number ("j") is positive or negative and          also whether it is even or odd: 
                            
             The {}s are unnecessary here but          again I think they make the code look much clearer - remember          the mega-principle! 
                            
              if ( j > 0 ) {    if ( j % 2 == 0 )      printf("j is even and greater than zero\n");    else      printf("j is odd and greater than zero\n");  }  else {               // j <= 0     if ( j % 2 == 0 )      printf("j is even and negative\n");    else      printf("j is odd and negative\n");  }
                            
            Armed with "else if()" we could rewrite this as          follows: 
                            
              if ( j > 0 && j % 2 == 0 )    printf("j is even and greater than zero\n");  else if ( j > 0 && j % 2 != 0 )    printf("j is odd and greater than zero\n");  else if ( j <= 0 && j % 2 == 0 )    printf("j is even and negative\n");  else if ( j <= 0 && j % 2 != 0 )      printf("j is odd and negative\n");  else      printf("I seem to have made a mistake!\n");
                            
            One minute discussion: turn to          your neighbour and discuss which version of the even/odd,          positive/negative code is clearer. 
                     
                  
                    
            The switch() statement
                            
            The switch() statement chooses one          of several choices, with an optional default, based          on an integer controlling expression. 
                                    
             A situation that can occur quite often is the need to choose          between one of several integer values and take the appropriate          action for each. This could be done with a large if()          statement but it is rather clumsy: 
                            
            // This works but is a bit clumsy   int number;  // Assign a value to number...   if ( number == value1 ) {    // ... Do something   }  else if ( number == value2 ) {    // ... Do something else again   }  else if ( number == value3 || number == value4 ) {  // ... Do something else yet again   }  else {    // ... Do yet another thing   }
                            
            C provides a special statement for dealing          with this situation, the switch()          statement, subject to two conditions: 
                            
            The expression that controls the switch()          choice must be an integer.
                            
             The options must be integer constants and must          all be different. 
                            
             The outside of a switch()          statement has the same familiar form as an if() or while()          statement: 
                            
              switch (integer-expression) {    ...  }
                     
                  
                    
            The case labels
                            
             The { ... } block          contains the options to be chosen (switched) between. They          have the general form: 
                            
              case integer-constant:    statements;
                            
            The choices ("labels") are followed by colons:            case 2:
                            
            It's conventional to "pull back" the labels to align them          with the switch() statement itself.
                            
             Within the switch() statement, the          program jumps to the choice (technically referred to as a label)          that matches the value of the expression. 
                            
             The switch() statement jumps to the          case whose value equals that of the controlling expression,          skipping over any intervening code.
                            
            Example: a simple menu
                            
            A common example of this is printing a simple menu with          choices number 1, 2, 3 etc. 
                            
            Useful tip: A common problem with          menus is that the string of options can be extremely long. C          solves this for us by joining strings separated by spaces,          without commas, into one large string. We illustrate this          below. 
                            
            C joins several strings separated by spaces,          without commas, into one large string. 
                            
            Reminder: '\t' is the tab character. 
                            
            // A simple menu with a switch()  printf("\n Menu\n\n"         "\t 1. Some option\n"         "\t 2. Another option\n"         "\t 3. A third option\n\n"         "\t 0. Quit\n");  scanf("%d", &option);    switch (option) {  case 0:    printf("Goodbye, thanks for using the switch program\n");    return 0;      case 1:    printf("You chose option one.\n");    break;      case 2:    printf("You chose option two.\n");    break;      case 3:    printf("You chose option three.\n");    break;      default:    printf("Unknown option: %d\n", option);        }
            Step through this code
                     
                  
                    
            The default label
                            
             Rather like the else of an if()          statement, the default label applies if none of the          other values match. Just like with else it is          optional. The default label can occur anywhere          inside the switch(), but for obvious reasons it is          usual to put it at the end. If there is no default          label and none of the other labels match, the program jumps          over the entire { ... } block. 
                            
            If no case matches then control jumps          to the default: label, if there is no default          label the program skips over the entire { ... }          block.
                     
                  
                    
            Falling through the cases
                    
                            
            The program does not jump out          of the switch() statement when it meets the next          label. 
                            
             The break statement breaks out of the          switch() to the statement following the switch()          statement's { ... } block.
                            
                        
            1. Step through the above example.
                        
              2.           
            2. to see what happens if we miss out the break statement
                        
              3.           
            3. Step through a version of the above            menu code where we have "accidentally" missed out the break            statement. 
              4.           
            4. Notice how the code executes two printf() statements, not            just the one and how it just skips over the "case: 2" statement.
                        
              5.         
                            
             A common use of this is to assign the same action to two or          more cases. We shall see some examples of this below. 
                            
             There is no need for for a break statement after          label 0 as the return returns from the whole          function so label 1 would never be reached if option          had had the value zero. 
                            
             A common bug is to forget the break          statement. If you deliberately omit the break          statement I suggest putting in a comment to indicate this,          except in the case where one label immediately follows          another. An example of this is shown next week. 
                            
            Example: multiple labels with the same          action
                            
             Consider a test if a prime number less than 10. In this case          either of the numbers 2, 3, 5 and 7 gives rise to the same          action. 
                            
            // Demonstrate switch() consecutive labels#include <stdio.h>int main() {  int num = 7;  // Is num a prime less than 10?  switch(num) {  case 2: case 3: case 5: case 7:    printf("%d is a prime number less than 10\n", num);    break;  default:    printf("%d is not a prime number less than 10\n", num);      }  return 0;}
            Step through this code
                     
                  
                    
                        
            1. Days in a month using switch()
              2.           
            2. To practice using a switch()
              3.           
            3. We will write a switch() statement            that prints out the number of days in a month using the            integer values 1-12 for the months January to December. We            will do this in stages. As usual with the final            mini-exercise, if you find it a little difficult finish it            off in the lab class.
                        
              4.           
            4. Create a new on-line program in a new window, with a suitable title and opening comment..
              5.           
            5. Declare an int variable with a suitable name to            represent the month under consideration.
              6.           
            6. Print a message to the screen telling the user to enter a            month in the range 1 to 12, then read in the value of the            month using scanf("%d", &your_variable_name).
              7.           
                            
              • Remember that the format to the scanf() should              just be "%d" with no extra characters (eg not              "%d\n").
                            
                •           
                        
            7. As a check, immediately after reading in the integer value            print it out to the screen. Build & run and check the            program reads in the number successfully.
                        
              8.           
            8. Now have a very small switch() statement with one case (2            for February) that prints "28" and a default that prints a            helpful error message saying that the number is not a valid            month. Build & run and check it works OK.
              9.           
            9. Now add the cases for the other months. Check it works for            a number of valid and invalid integer values. (NB: do not            try entering non-integers when the code is trying to read in            an integer, that's a much more advanced issue.)
                        
              10.         
            Fall-through after other statements
                            
            The above example            contains several adjacent cases and the "fall-through"            behaviour is fairly intuitive. Much less intuitive, and much            less common, is when a case has some executable statements            and then falls through to the next one. A          fall-through after another statement is well worth a comment          as otherwise it can look like a mistake as in this somewhat          contrived example:
                            
            // Demonstrate switch() consecutive labels#include <stdio.h>int main() {  int num = 2;  // Is num a prime less than 10?  // Take special note of 2, the only even prime  switch(num) {  case 2:     printf("Even thought it is even, ");    /* Fall through */  case 3: case 5: case 7:    printf("%d is a prime number less than 10\n", num);    break;  default:    printf("%d is not a prime number less than 10\n", num);      }  return 0;}
            Step through this code
                     
            Summary
            The text of each key point is a link to the place in the web page.
             
            The if() statement
             
            Relational operators
             
            { ... } blocks
             
            The switch() statement
            For study after the lecture
                
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                                            • Time - Idioms by The Free Dictionary 
                                              https://idioms.thefreedictionary.com/time
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                                              time
                                               
                                              Also found in: Dictionary, Thesaurus, Medical, Legal, Financial, Acronyms, Encyclopedia, Wikipedia.
                                              Related to time: Time zones
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                                              time (out)
                                              Stop everything for just a minute! "Hey, stop a minute! Time out!" yelled Mary as the argument grew in intensity. Right in the middle of the discussion, Alice said, "Time!" Then she announced that dinner was ready.
                                              See:
                                              12
                                              References in classic literature?
                                              `Now, it is very remarkable that this is so extensively overlooked,' continued the Time Traveller, with a slight accession of cheerfulness.
                                              `Yes, I think I see it now,' he said after some time, brightening in a quite transitory manner.
                                               View in context
                                              We speak of what is white as large, because the surface over which the white extends is large; we speak of an action or a process as lengthy, because the time covered is long; these things cannot in their own right claim the quantitative epithet.
                                               View in context
                                              (2) However much we approximate the time of judgment to the time of the deed, we never get a conception of freedom in time.
                                               View in context
                                              So as time went on these well-known tales came to be told in many different ways, changing as the times changed.
                                               View in context
                                              Hence in all such cases, we should be unable to recognise the parent-form of any two or more species, even if we closely compared the structure of the parent with that of its modified descendants, unless at the same time we had a nearly perfect chain of the intermediate links.
                                               View in context
                                              The murderer was about to strike her on the head with the mutton-bone--a terrible weapon in the hands of a Larsan or Ballmeyer; but she fired in time, and the shot wounded the hand that held the weapon.
                                               View in context
                                              "And did your worship eat anything all that time, senor?" asked the cousin.
                                               View in context
                                              `If you knew Time as well as I do,' said the Hatter, `you wouldn't talk about wasting IT.
                                               View in context
                                              He was the first of the writers of great Anna's time whom I knew, and he made me ready to understand, if he did not make me understand at once, the order of mind and life which he belonged to.
                                               View in context
                                              From the time of our parting till their death we kept up a correspondence with the older members of the family, and in later years we have kept in touch with those who were the younger members.
                                               View in context
                                              Just about this time, I got hold of a book entitled "The Columbian Orator." Every opportu- nity I got, I used to read this book.
                                               View in context
                                              She was also so spiteful that she gladly devoted all her time to carrying out all the mean or ill-natured tricks of the whole body of fairies.
                                               View in context
                                              The patient had obtained a `card' from the hospital some time before; and when her time came it was taken to the porter by a messenger, generally a little girl, who was then sent across the road to the house in which Philip lodged.
                                               View in context
                                              I stood still for some time. The sight of her had made me very happy.
                                               View in context
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