The if structure

    if ( condition ) 
    {
        statement(s)
    }

Example :

    /* make sure x is positive */
    if (x < 0)
    {
        x = -x;   
    }

The if/else structure

    if ( condition ) 
    {
        statement(s) 
    }
    else 
    {
        statement(s)
    }

Example :

    /*assign a grade */
    if (score >= 70)      
    {
        grade = 'P';
    } 
    else 
    {
        grade = 'F';
    }

The switch structure

  switch (E)
  {
      case C1 :
          statements_1;
          break;
      case C2 :
          statements_2;
          break;
      .
      .
      .
      default :
          statements_n;
          break;    
  }

More about switch

E is the control expression which should evaluate to an integer.

the Ci's should be integer constants.

case C1: statements_1; break; is a case clause

default: statements_n; break; is a default clause

Semantics

E is evaluated
If E's value is equal to one of the Ci's, then control is immediately transfered to that case.
...Otherwise, if there is a default clause, control is transferred there.
......Otherwise, exit the switch.
The statements in that selected case are evaluated.
If a break is evaluated, control is transfered to the next statement after the switch

Good programming practice

include a break in every clause
always include a default clause

More switch examples

The Program

/**************************************************** * File: cardrank.c * Author: S. Bogar * Date: Spring 1942 * Section 0101 * E-Mail: bogar@cs.umbc.edu * * Reads in a number between 1 and 13 and writes * out the appropriate symbol for a playing card * of that rank. *******************************************************/ #include <stdio.h> int main() { int rank; /* Prompt user and get card rank */ printf("What is the rank of the card (1-13)? "); scanf ("%d", &rank); /* Print appropriate name */ switch (rank) { /* Special card names for 1 and 11-13 */ case 1: printf("Ace\n"); break; case 11: printf("Jack\n"); break; case 12: printf("Queen\n"); break; case 13: printf("King\n"); break; /* 'default' handles 2 - 10; just print the value */ default: printf("%d\n", rank); break; } return 0; }

The Sample Run

linux1[75]% a.out What is the rank of the card (1-13)? 11 Jack linux1[76]% a.out What is the rank of the card (1-13)? 1 Ace linux1[77]% a.out What is the rank of the card (1-13)? 8 8 linux1[78]% a.out What is the rank of the card (1-13)? 12 Queen linux1[79]% a.out What is the rank of the card (1-13)? 13 King linux1[80]% a.out What is the rank of the card (1-13)? 7 7 linux1[81]% a.out What is the rank of the card (1-13)? 14 14 linux1[82]%

Oops

We need to check for bad input.

cardrank2

The Program

/********************************************** * File: card2.c * Author: Sue Bogar * Date: Summer 1942 * Section: 0101 * E-Mail: bogar@cs.umbc.edu * * * Reads in a number between 1 and 13 and writes * out the appropriate symbol for a playing card * of that rank. * Checks for and handles invalid input *****************************************/ #include <stdio.h> int main() { int rank; /* Prompt user and get card rank */ printf("What is the rank of the card (1-13)? "); scanf ("%d", &rank); /* Print appropriate name */ switch (rank) { /* special card names for 1, 11-13 */ case 1: printf("Ace\n"); break; case 11: printf("Jack\n"); break; case 12: printf("Queen\n"); break; case 13: printf("King\n"); break; /* for 2 - 10, just print rank */ case 2: case 3: case 4: case 5: case 6: case 7: case 8: case 9: case 10: printf("%d\n", rank); break; /* 'default' catches invalid input */ default: printf("Bad card number: %d\n", rank); break; } return 0; }

The Sample Run

linux1[83] % a.out What is the rank of the card (1-13)? 1 Ace linux1[84] % a.out What is the rank of the card (1-13)? 8 8 linux1[85] % a.out What is the rank of the card (1-13)? 14 Bad card number: 14 linux1[86] % a.out What is the rank of the card (1-13)? 13 King linux1[87] %