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<a name="The-while-Statement"></a>
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<p>
Next: <a href="The-do_002duntil-Statement.html#The-do_002duntil-Statement" accesskey="n" rel="next">The do-until Statement</a>, Previous: <a href="The-switch-Statement.html#The-switch-Statement" accesskey="p" rel="prev">The switch Statement</a>, Up: <a href="Statements.html#Statements" accesskey="u" rel="up">Statements</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="The-while-Statement-1"></a>
<h3 class="section">10.3 The while Statement</h3>
<a name="index-while-statement"></a>
<a name="index-endwhile-statement"></a>
<a name="index-loop"></a>
<a name="index-body-of-a-loop"></a>

<p>In programming, a <em>loop</em> means a part of a program that is (or at least can
be) executed two or more times in succession.
</p>
<p>The <code>while</code> statement is the simplest looping statement in Octave.
It repeatedly executes a statement as long as a condition is true.  As
with the condition in an <code>if</code> statement, the condition in a
<code>while</code> statement is considered true if its value is nonzero, and
false if its value is zero.  If the value of the conditional expression
in a <code>while</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 nonzero.
</p>
<p>Octave&rsquo;s <code>while</code> statement looks like this:
</p>
<div class="example">
<pre class="example">while (<var>condition</var>)
  <var>body</var>
endwhile
</pre></div>

<p>Here <var>body</var> is a statement or list of statements that we call the
<em>body</em> of the loop, and <var>condition</var> is an expression that
controls how long the loop keeps running.
</p>
<p>The first thing the <code>while</code> statement does is test <var>condition</var>.
If <var>condition</var> is true, it executes the statement <var>body</var>.  After
<var>body</var> has been executed, <var>condition</var> is tested again, and if it
is still true, <var>body</var> is executed again.  This process repeats until
<var>condition</var> is no longer true.  If <var>condition</var> is initially
false, the body of the loop is never executed.
</p>
<p>This example creates a variable <code>fib</code> that contains the first ten
elements of the Fibonacci sequence.
</p>
<div class="example">
<pre class="example">fib = ones (1, 10);
i = 3;
while (i &lt;= 10)
  fib (i) = fib (i-1) + fib (i-2);
  i++;
endwhile
</pre></div>

<p>Here the body of the loop contains two statements.
</p>
<p>The loop works like this: first, the value of <code>i</code> is set to 3.
Then, the <code>while</code> tests whether <code>i</code> is less than or equal to
10.  This is the case when <code>i</code> equals 3, so the value of the
<code>i</code>-th element of <code>fib</code> is set to the sum of the previous two
values in the sequence.  Then the <code>i++</code> increments the value of
<code>i</code> and the loop repeats.  The loop terminates when <code>i</code>
reaches 11.
</p>
<p>A newline is not required between the condition and the
body; but using one makes the program clearer unless the body is very
simple.
</p>
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Next: <a href="The-do_002duntil-Statement.html#The-do_002duntil-Statement" accesskey="n" rel="next">The do-until Statement</a>, Previous: <a href="The-switch-Statement.html#The-switch-Statement" accesskey="p" rel="prev">The switch Statement</a>, Up: <a href="Statements.html#Statements" accesskey="u" rel="up">Statements</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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