Loops

Most real programs contain some construct that loops within the program, performing repetitive actions on a stream of data or a region of memory. There are several ways to loop in C. Two of the most common are the while loop: 

    while (expression)
    {
      ...block of statements to execute...
    }

and the for loop: 

    for (expression_1; expression_2; expression_3)
    {
      ...block of statements to execute...
    }

The while loop continues to loop until the conditional expression becomes false. The condition is tested upon entering the loop. Any logical construction (see below for a list) can be used in this context.

The for loop is a special case, and is equivalent to the following while loop: 

    expression_1;
 
    while (expression_2)
    {
      ...block of statements...
 
      expression_3;
    }

For instance, the following structure is often encountered: 

    i = initial_i;
 
    while (i <= i_max)
    {
      ...block of statements...
 
      i = i + i_increment;
    }

This structure may be rewritten in the easier syntax of the for loop as: 

    for (i = initial_i; i <= i_max; i = i + i_increment)
    {
      ...block of statements...
    }

Infinite loops are possible (e.g. for(;;)), but not too good for your computer budget! C permits you to write an infinite loop, and provides the break statement to ``breakout '' of the loop. For example, consider the following (admittedly not-so-clean) re-write of the previous loop: 

    angle_degree = 0;
 
    for ( ; ; )
    {
      ...block of statements...
 
      angle_degree = angle_degree + 10;
      if (angle_degree == 360) break;
    }

The conditional if simply asks whether angle_degree is equal to 360 or not; if yes, the loop is stopped.