Flow control in C

The C standard defines the following control structures:

Selection

if..else

This structure lets the program perform one of two actions, depending on the value of an expression. Its basic structure is:

if (expression)
    statement1;
else
    statement2;

Multiple statements can be used if they are enclosed by braces, e.g:

if (n==0){
    printf("Done.\n");
} else {
    hanoi(n-1, source, target, aux);
    printf("move from %d to %d.\n", source, target);
    hanoi(n-1, aux, source, target);
}

Statement 2 can be an if statement, which can be used to create an if..else if..else structure, e.g:

if (a=='h')
    printHelp();
else if (a=='f')
    findWord();
else if (a=='q')
    exit(0);
else
    printf("Unknown operation %c\n", a);

switch..case..default

The switch statement will go to one of several locations, depending on the value of an expression. The last example can be written using a switch statement as:

switch (a) {
    case 'h':
        printHelp();
        break;
    case 'f':
        findWord();
        break;
    case 'q':
        exit(0);
        break;
    default:
        printf("Unknown operation %c\n", a);
}

A few notes:

• the control statement (in this example the variable a) has to have integral type (character, short integer, integer bit-field, all of these signed or not, or an object of enumeration type); integral promotion will ensure the type is either int or unsigned int.
• The break statement ensures execution does not fall through to the next statement.
• Control passes to the default label if none of the other case constants match the value of the control expression; if there is no default statement, none of the substatement of the switch is executed.

Iteration

for

The for statement has the format:

   for (variables; condition; step )
       statement

variables is usually variable initialisation; condition is the condition that keeps the loop active, usually a relational expression; step is usually an increment or decrement of one or more variables. Here is an example:

   int i;
   for (i = 1; i <= 10; i++)
       printf("i = %d\n", i);

statement can be replaced by a block, which is a sequence of statements enclosed by braces. Any of variables, condition, or step can be omitted. When condition is omitted, the condition is assumed permanently true.

while

A while loop has the following format:

  while ( expression )
     statement

Expression is evaluated. If it is true (non-zero) the body of the loop, the statement, is executed; expression is then re-evaluated and if true the body is executed again. Only when expression is false (zero) will statement be skipped and the loop terminated.

Statement can be replaced by a block. A block is a sequence of statements enclosed by braces.

do..while

This is the format of the do iteration statement:

   do 
       statement
   while ( expression ) ;

statement is executed repeatedly as long as expression remains unequal to zero. statement can be replaced by a list of statements enclosed by braces (a block) as demonstrated in this example:

   do {
       printf("Press 'q' to exit: ");
       scanf("%s", str);
   } while ( str[0] != 'q' && str[0] != 'Q' );

This will loop repeatedly, asking for input, until either 'q' or 'Q' is entered.
Note: a do..while loop is guaranteed to run at least once.

Project: Topic:C

Previous: Variables and Expressions — Flow control in C — Next: Functions and Methods

Go to the School of Computer Science