Switch Case Structures

1. Switch Case Structures Lecture 9 Winter Quarter

2. Switch Multiple Selection Structure • A multiple selection structure is useful when an algorithm contains a series of decisions in which a variable or expression is tested separately for one of several possible integral values. • Each integral value represents a different action to be taken in the algorithm. • C provides the switch multiple selection structure to implement this type of decision making. Winter Quarter

3. Note use of colon! Switch-Case Structures • The switch - case syntax is: switch (integer expression test value) { case case _1_fixed_value : action(s) ; case case_2_fixed_value : action(s) ; default : action(s) ; } Winter Quarter

4. Switch-Case Structures • The switch is the "controlling expression" • Can only be used with constant integer expressions. • Remember, a single character is a small positive integer. • The expression appears in ( ) • The case is a "label" • The label must be followed by a " : " • Braces, { }, not required around statements Winter Quarter

5. Switch-Case Structures • Unlike if-else if-else structures, when the value in a case matches the test value, all of the actions in the rest of the structure take place. • This is shown in the following program where the user enters a value that matches the first case and every action in the structure is executed. Winter Quarter

6. A Sample Program to Illustrate Switch-Case Problem: Write a program to ask the user to enter his/her letter grade and then respond with an appropriate message regarding his/her academic status. Winter Quarter

7. A Sample Program to Illustrate Switch-Case Algorithm: 1. Set up the environment 2. Prompt user to enter his/her letter grade 3. Get user’s response 4. If grade is a or A say “Good Job” and go to 9 5. If grade is b or B say “Pretty good” and go to 9 6. If grade is c or C say “Better get to work” and go to 9 7 If grade is d or D say “You are in trouble” and go to 9 8. Say “You are failing” 9. Terminate program Winter Quarter

8. A Sample Program to Illustrate Switch-Case /* This program associates a letter grade with a message appropriate to the score. */ #include <stdio.h> intmain( ) { char grade ; printf("Enter your current letter grade\n") ; grade = getchar( ) ; Winter Quarter

9. A Sample Program to Illustrate Switch-Case switch (grade) { case('a') : case('A'): printf("Good Job!\n") ; case('b'): case('B'): printf("Pretty good.\n"); Winter Quarter

10. A Sample Program to Illustrate Switch-Case case('c'): case('C'): printf("Better get to work.\n"); case('d'): case('D'): printf("You are in trouble.\n"); default: printf("You are failing!!\n"); } /* End of switch-case structure */ } /* End of main program */ Winter Quarter

11. Switch-Case StructuresResultant Output from Grade Program /* The following results are produced when the user enters an "A" as input to the program prompt. */ Good Job! Pretty good. Better get to work. You are in trouble. You are failing! Winter Quarter

12. Switch-Case Structures break ; • The problems with the previous program can be corrected by use of the break statement. • It can be used in either a repetition structure or a selection structure to break out of (that is, to exit from) the structure. • The syntax is: break ; • The following program is the previous one with the addition of the break statements. Winter Quarter

13. Fixed Program using Switch-Case Structures #include <stdio.h> int main ( ) { int grade ; printf ("Enter your current letter grade\n") ; while ( ( grade = getchar ( ) ) != EOF) { switch (grade) { case ('a') : case ('A') : printf ("Good Job!\n") ; break ; Winter Quarter

14. Fixed Program using Switch-Case Structures case ('b') : case ('B') : printf ("Pretty good.\n") ; break ; case ('c') : case ('C') : printf ("Better get to work.\n") ; break ; case ('d') : case ('D') : printf ("You are in trouble.\n") ; break ; Winter Quarter

15. Fixed Program using Switch-Case Structures case ('f') : case ('F'): printf ("You are failing!!\n") ; break ; case (' ') : case ('\n') : //< See note pg 17 break ; default : printf ("Invalid grade. Try again.\n") ; } /* End of switch/case */ } /* End of while loop */ } /* End of "main" function */ Winter Quarter

16. Comments on Last Example Program • Use of the whilerepetition structure -- more discussion on repetition structures later this week. • Use of the end-of-file, or EOF, test. Note that EOF (a DEFINED constant) is a negative integral value, usually a -1 on most (but not all) systems. (EOF is actually defined in the <stdio.h> header file.) • Use of ints (instead of chars). Why? • From the keyboard, a <return> <cntrl-d> generates an EOF signal on most UNIX systems. Winter Quarter

17. Comments on Last Example Program • The statements: case (' ') : case ('\n') : break ; were used to clear the keyboard input buffer. • Another way to clear it is with the statement: fgets(input_flush,256,stdin); where fgets(char_strg, len_char_strg, file_pointer); • This fgets statement can prove very useful in today’s daily assignment. • The two sample programs which follow show why flushing the input stream is important Winter Quarter

18. Flawed Sample Character Input Program • #include <stdio.h> • Int main() • { • char ans2; • while(ans2 != ‘E’) // Here ‘E’ is a character constant • { • printf(“\n Input a character followed by <enter>”); • ans2 = getchar(); // getchar takes one char from input buffer • // and leaves the <enter> in the buffer • printf(“ ans2 >> %c <<\n”,ans2); • } • } This program runs until you put in an E. This program does not handle multiple inputs correctly. Winter Quarter

19. Output from Flawed Sample Program input a char followed by <enter> e ans2 = >> e << input a char followed by <enter> ans2 = >> << In this case a newline character which is produced by the <return> or <enter> is read by getchar on the second time through the loop Winter Quarter

20. Correct Sample Input Program • #include <stdio.h> • Int main() • { • char ans2,input_flush[256]; • while(ans2 != ‘E’) • { • printf(“\n Input a character followed by <return>”); • ans2 = getchar(); • fgets(input_flush,256,stdin); // Input buffer is flushed • // The <return> is not in buffer • printf(“ ans2 >> %c <<\n”,ans2); • } • } Winter Quarter

21. Output from Correct Input Program input a char followed by <enter> e ans2 = >> e << input a char followed by <enter> f ans2 = >> f << Winter Quarter

22. Comments on Last Example Program • Example of use offgets(input_flush,256,stdin); char figure,input_flush[256] ; float size ; printf ("Enter figure type>") ; scanf ("%c", &figure) ; //or figure=getchar( ) ; fgets(input_flush,256,stdin); printf ("Enter size of figure>") ; scanf ("%f", &size) ; fgets(input_flush,256,stdin); Winter Quarter

23. Assignment G08 • Use a switch-case structure to select from among the shapes for which calculations are to be made. • May use just first character of shape name to select which calculation to make. • Program only does one shape, and then exits. No looping required for today's assignment. Winter Quarter