.. _loops:

Loops
======

.. index:: loops

.. _while:

While loop
-----------

We also discussed this in Topic 1.

    ::

        while( expr ) statement;
        
        while( expr ) {
           code block;
        }

.. _for:

For loop
-----------

The :keyword:`while` and the :keyword:`for` loop are the two most frequently
used looping constructs. The :keyword:`for` loop is most often used when
iterating through a sequence of a pre-defined length.

    ::

        for( expr1; expr2; expr3 ) {
           code block;
        }


Is equivalent to:

    ::

        expr1;
        while( expr2 ) {
           code block;
           expr3;
        }



-  expr1 is the *initializer* of the :keyword:`for` loop.

-  expr2 is the *condition* for continuing with the loop.

- 
   expr3 is evaluated after each execution of the code block and is
   usually used to facilitate *iterating* through the loop.


    ::

        int i, x=0, y=0;
        
        for( i = 0; i < MAX; i++ ) {
           x += i * y*(i - 1);
           y = x*i;
        }
        printf( "x = %d, y = %d\n", x,y );


Variations on :keyword:`for` loops
-----------------------------------

Using empty statements
^^^^^^^^^^^^^^^^^^^^^^^^

An *empty statement* is a statement consisting of just a semicolon
(**;**\ ). It is useful when one statement is needed syntacticly, but
no action needs to be performed.

For example ...

    ::

        /* Variable i is already set in the program */
        for( ; i < MAX; i++ ) {
           block;
        }
        
        /* Some code in the body of the for loop alters the value of i */
        for( i = 0; i < MAX; ) {
           block of code which updates i;
        }
        
        /* The code has a break statement */
        for( i = 0; ; i++ ) {
           block;
           if( some_test ) break;
        }


The comma operator
^^^^^^^^^^^^^^^^^^^^

The *comma operator* allows multiple statements to be executed as
part of either ``expr1`` or ``expr3``.

    ::

        for( sum=0, i=1; i <= n; i++ )
           sum += i;
        
        for( sum=0, i=1; i <= n; sum += i, i++)
           ;


Note: Although seldom used, the comma operator may also be used in a
:keyword:`while` loop construct or in the second expression of a
:keyword:`for` loop. The last expression determines if the body of the loop
is executed.

    ::

        while( i = 0, testvalue < 0 ) {
           statements;
        }

.. _do:

The do loop
------------

.. index:: do

The :keyword:`do` loop is similar to the :keyword:`while` loop except the code
block always executes at least one time. The test of whether to
continue or not is at the end of the loop.

    ::

        do {
           code block;
        }while( expr );