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<title>if-else keywords.</title>
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<h1>The 'if' and 'else' keywords</h1>
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<hr>
<p>
The if-else statement is a two-way decision statement. It is written as
<pre>
if ( expression ) statement1;
[else statement2;]
</pre>
The else portion is optional. If the expression evaluates to
<a href="../CONCEPT/true_false.html">true</a>
(anything other than 0) then statement1 is executed. If there is an <b>else</b>
statement and the expression evaluates to
<a href="../CONCEPT/true_false.html">false</a>
statement2 is executed.
For example
<pre>
(1)
int NumberOfUsers;
.
.
if( NumberOfUsers == 25 )
{ /* No else part */
printf( "There are already enough users. Try later.\n" );
return ERROR;
}
.
.
(2) if( a >= b ) larger = a; /* else part is present */
else larger = b;
</pre>
Consider this code fragment:
<pre>
if( p == 1 )
if( q == 2 ) r = p * 2 + q;
else r = q * 2 + p;
</pre>
Because the <b>else</b> part is optional, there is an ambiguity when an <b>else</b> is
omitted from a nested if sequence. In 'C', this is resolved by associating
the <b>else</b> with the closest previous if that does not have an <b>else</b>.
Therefore, in the above example, the <b>else</b> part belongs to the if(q==2)
statement.
The code can be made more readable by explicitly putting parentheses in
the expression, like this
<pre>
if( p == 1 )
{
if( q == 2 ) r = p * 2 + q;
}
else r = q * 2 + p;
OR
if( p == 1 )
{
if( q == 2 ) r = p * 2 + q;
else r = q * 2 + p;
}
</pre>
Because the statement in the <b>else</b> part can also be an if statement, a
construct such as shown below is possible in 'C' to create a multiple
choice construct.
<pre>
if( expression1 )
statement1;
else if( expression2 )
statement2;
else if( expression3 )
statement3;
.
.
else
statementN;
</pre>
<hr>
<h2>Example:</h2>
<img src="../../GRAPHICS/computer.gif" align=center>
<a href="../EXAMPLES/if.c"> Basic <b>if</b> example.</a>
<br clear=left>
<hr>
<h2>See also:</h2>
<ul>
<li><a href="switch.html">switch</a> keyword.
</ul>
<p>
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<p>
<hr>
<address>Martin Leslie
</address><p>
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