Repetition

All repetition (iteration) structures control:

a single statement or
a block of statements in {...}

Repetition statements are also called loops.

The controlled statement(s) are called the loop body.

The while Statement

Repetition is controlled by a continuation condition, tested before the loop body is executed. Its general form is:

   while (condition)
      statement

Effect:

test the continuation condition
if FALSE, end the while statement
if TRUE, execute the statements
repeat above three steps

while Example

Compute the sum of the first 50 positive integers:

int sum, num;
...
sum = 0;
num = 1;
while (num <= 50)
    sum = sum + num;

The do..while Statement

Repetition is controlled by a continuation condition, tested after the loop body is executed. Its general form is:

   do {
      statements;
   } while (condition);

Effect:

execute the statements
test the continuation condition
if FALSE, end the do..while statement
if TRUE, repeat above three steps

Use do..while if at least one iteration needed.

The for Statement

The for statement is shorthand for a common pattern of usage of while:

   init;
   while (condition) {
      statements;
      next;
   }

init sets state for first iteration. next sets state for next iteration.

This is expressed using for as:

   for (init; condition; next) {
      statements;
   }

Any of init, condition or next may be omitted.

For Example - A Table of Squares

Print a table of squares.

int i;
...
for (i = 1; i <= 99; i = i + 1) {
    System.out.print(i);
    System.out.print("^2  =  ");
    System.out.println(i*i);
}

which executes as follows

   % java Example
    1^2 = 1
    2^2 = 4
    3^2 = 9
   ...
   10^2 = 100
   ...
   98^2 = 9604
   99^2 = 9801

The break Statement

A break statment must appear within a while,do-while or for loop.

Its execution causes the loop to exit immediately.

It is usually appears in the body of an if statement

This form is common:

   for (init; condition; next) {
      statements₁;
      if (condition₂)
          break;
      statements₂
   }

The break Statement

Simple example of break statement use:

int n, num, count, sum = 0;
... /* infinite loop! */
for (count  = 1 ; ; count = count + 1) {
    printf("Enter a number: ");
    num = Integer.parseInt (stdin.readLine());
    if (num < 0)
    	break; /* non-number -> give up */
    sum = sum + num;
}
System.out.print("Average = ");
System.out.println(sum/count);

The continue Statement

A continue statment must appear within a while,do-while or for loop.

Its execution causes the remainder of the loop body to be skipped - execution skips to the next iteration of the loop.

It is usually appears in the body of an if statement

This form is common:

   for (init; condition; next) {
      statements₁;
      if (condition₂) continue;
      statements₂;
   }

The continue Statement

Simple example of continue statement use:

int n, num, count, sum = 0;
... /* infinite loop! */
for (count  = 1 ; ; count = count + 1) {
    printf("Enter a number: ");
    num = Integer.parseInt (stdin.readLine());
    if (num < 0)
    	break;
    if (num == 0)
    	continue;
    sum = sum + num;
}
System.out.print("Average = ");
System.out.println(sum/count);