.. _control:

Control Constructs
==================

.. index:: selection statement, loop

Control constructs determine the flow of execution of the program.  With no
control constructs, statements execute sequentially.  Some control constructs
are :dfn:`selection statements` meaning that it selects which code to
execute.  Other control constructs are :dfn:`loops`, where the control
construct determines how many times the loop will execute.

C keywords related to selection statements include: :keyword:`if`,
:keyword:`else`, :keyword:`switch`, :keyword:`case`, :keyword:`break`,
:keyword:`default`.

C keywords related to loops include: :keyword:`for`, :keyword:`while`,
:keyword:`do`, :keyword:`break`, :keyword:`continue`.

if block
-----------

    ::

        if( condition ) statement;
        
        if( condition ) {
           statement block;
        }
        
        if( condition ) {
           statement block1;
        } else {
           statement block2;
        }


    ::

        if( x < 10 && y < x ) 
           printf( "x is less than 10, but greater than y" );
        else {
           if( x < 10 )
              printf( "x is less than 10, and not greater than y" );
           else {
              if( y < x )
                 printf( "x is not less than 10 and is greater than y" );
              else
                 printf( "x is not less than 10 and not greater than y" );
           }
        }


Note that the two pairs of brackets are not needed, but they add to
the clarity of the program. Avoid the ambiguous else ...

    ::

        if( condition1 )
           if( condition2 ) 
              statement;
        else {          /* WRONG */
           statement block;
        }


Relational and Logical Operators
------------------------------------

Critical to any control construct are conditions statements that evaluate to
either **True** or **False** (boolean statements).

.. note::  

    In C, extend the Boolean statements to integers.  0 and *NULL* are
    **False**, and any other valid data value is **True**.  

Relational Operators
~~~~~~~~~~~~~~~~~~~~~~~

Relational operators evaluate how two items compare to each other.

========  =====================================================
Operator  Meaning
========  =====================================================
==        Equal to each other (note: ``a == b`` NOT ``a = b``)
!=        Not equal
>         Greater than
<         Less than
<=        Less than or equal
>=        Greater than or equal
========  =====================================================

.. note::

    C does not have relational operators to evaluate strings or other data
    structures -- only the fundamental data types. See :ref:`data_types1`.
    Other functions may be used for these evaluations, such as 
    :func:`strcmp()`.

Logical Operators
~~~~~~~~~~~~~~~~~~~~~~~

Logical operators allow us to combine other boolean expressions to form one
based on the desired logic.


========  ======== ==========
Operator  Meaning   Example
========  ======== ==========
&&        AND       a && b
||        OR        a || b
!         NOT       !a  
========  ======== ==========

Parenthesis **( )** may also be used to control the order in which boolean
expressions are evaluated.

while loops
----------------

    ::

        while( operator ) {
           statement block;
        }


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

Here we present another looping construct. It 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 `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.


::

        #include <stdio.h>

        #define MAX 5

        int main(void)
        {
            int i, x=1, y=1;

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

            return 0;
        } 

Now compete :ref:`hw3`.