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<h3 class="section">10.1 The <code>if</code> Statement</h3>
<p><a name="index-g_t_0040code_007bif_007d-statement-556"></a><a name="index-g_t_0040code_007belse_007d-statement-557"></a><a name="index-g_t_0040code_007belseif_007d-statement-558"></a><a name="index-g_t_0040code_007bendif_007d-statement-559"></a>
The <code>if</code> statement is Octave's decision-making statement. There
are three basic forms of an <code>if</code> statement. In its simplest form,
it looks like this:
<pre class="example"> if (<var>condition</var>)
<var>then-body</var>
endif
</pre>
<p class="noindent"><var>condition</var> is an expression that controls what the rest of the
statement will do. The <var>then-body</var> is executed only if
<var>condition</var> is true.
<p>The condition in an <code>if</code> statement is considered true if its value
is non-zero, and false if its value is zero. If the value of the
conditional expression in an <code>if</code> statement is a vector or a
matrix, it is considered true only if it is non-empty and <em>all</em>
of the elements are non-zero.
<p>The second form of an if statement looks like this:
<pre class="example"> if (<var>condition</var>)
<var>then-body</var>
else
<var>else-body</var>
endif
</pre>
<p class="noindent">If <var>condition</var> is true, <var>then-body</var> is executed; otherwise,
<var>else-body</var> is executed.
<p>Here is an example:
<pre class="example"> if (rem (x, 2) == 0)
printf ("x is even\n");
else
printf ("x is odd\n");
endif
</pre>
<p>In this example, if the expression <code>rem (x, 2) == 0</code> is true (that
is, the value of <code>x</code> is divisible by 2), then the first
<code>printf</code> statement is evaluated, otherwise the second <code>printf</code>
statement is evaluated.
<p>The third and most general form of the <code>if</code> statement allows
multiple decisions to be combined in a single statement. It looks like
this:
<pre class="example"> if (<var>condition</var>)
<var>then-body</var>
elseif (<var>condition</var>)
<var>elseif-body</var>
else
<var>else-body</var>
endif
</pre>
<p class="noindent">Any number of <code>elseif</code> clauses may appear. Each condition is
tested in turn, and if one is found to be true, its corresponding
<var>body</var> is executed. If none of the conditions are true and the
<code>else</code> clause is present, its body is executed. Only one
<code>else</code> clause may appear, and it must be the last part of the
statement.
<p>In the following example, if the first condition is true (that is, the
value of <code>x</code> is divisible by 2), then the first <code>printf</code>
statement is executed. If it is false, then the second condition is
tested, and if it is true (that is, the value of <code>x</code> is divisible
by 3), then the second <code>printf</code> statement is executed. Otherwise,
the third <code>printf</code> statement is performed.
<pre class="example"> if (rem (x, 2) == 0)
printf ("x is even\n");
elseif (rem (x, 3) == 0)
printf ("x is odd and divisible by 3\n");
else
printf ("x is odd\n");
endif
</pre>
<p>Note that the <code>elseif</code> keyword must not be spelled <code>else if</code>,
as is allowed in Fortran. If it is, the space between the <code>else</code>
and <code>if</code> will tell Octave to treat this as a new <code>if</code>
statement within another <code>if</code> statement's <code>else</code> clause. For
example, if you write
<pre class="example"> if (<var>c1</var>)
<var>body-1</var>
else if (<var>c2</var>)
<var>body-2</var>
endif
</pre>
<p class="noindent">Octave will expect additional input to complete the first <code>if</code>
statement. If you are using Octave interactively, it will continue to
prompt you for additional input. If Octave is reading this input from a
file, it may complain about missing or mismatched <code>end</code> statements,
or, if you have not used the more specific <code>end</code> statements
(<code>endif</code>, <code>endfor</code>, etc.), it may simply produce incorrect
results, without producing any warning messages.
<p>It is much easier to see the error if we rewrite the statements above
like this,
<pre class="example"> if (<var>c1</var>)
<var>body-1</var>
else
if (<var>c2</var>)
<var>body-2</var>
endif
</pre>
<p class="noindent">using the indentation to show how Octave groups the statements.
See <a href="Functions-and-Scripts.html#Functions-and-Scripts">Functions and Scripts</a>.
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