Control Statements

if

Formally:

if ( expression ) statement-1 [ else statement-2 ]

...for example...

if ( colour == 'b' || colour == 'B') /* double-equals */ blue_things++; /* increment: b_t = b_t + 1 */ else other_colours++;

• The else part is optional.
• Statements may be compound:

  if (c == '0') { printf("c was zero \n"); /* compound */ c = 'a'; /* statement */ } else c = '\0';

    if ( expression )
        statement
    else if ( expression )
        statement

        ...

    [ else
        statement  ]

switch

The C switch statement provides multi-way selection based on the value of an integer variable. Formally:

switch ( expression ) { case const-expr : statements case const-expr : statements ... default: statements }

Example

switch (c) { case '0': case '2': case '4': printf("Value was even \n"); break; /* note the break statement */ case '1': case '3': case '5': printf("Value was odd \n"); break; default: printf("No idea \n"); }

Example

#include <stdio.h> main(){ char c = 'a'; switch (c) { case 'a' : printf("\n a"); case 'b' : printf("\n b"); } printf("\n"); }

produces

prompt> ./a.out a b prompt>

while

Formally:

while ( expression ) statement

The expression is evaluated and if it is true (non-zero), statement is executed and expression re-evaluated; the cycle completes when the value of the expression becomes zero (false).

int number_left = 10; while (number_left) { printf("%d \n", number_left); number_left--; }

for

Formally:

for ( expr1 ; expr2 ; expr3 ) statement

• expr1 is executed once at the start;
• expr2 is evaluated and if its result is non-zero (true) the statement is executed;
• expr3 is then executed, expr2 is then re-evaluated and the cycle continues until expr2 yields a zero (false) value.

For example

int i; for (i = 0; i < 3; i++) printf("The value of i is %d\n", i);

Equivalent to:

expr1 ; while (expr2) { statement; expr3; }

Also,

int i; for (i = 0; i < 3; i++) { printf("The value of i is: \n"); printf(" %d\n", i); }

goto

• Never!