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<h3 class="section">10.3 The <code>while</code> Statement</h3>
<p><a name="index-g_t_0040code_007bwhile_007d-statement-564"></a><a name="index-g_t_0040code_007bendwhile_007d-statement-565"></a><a name="index-loop-566"></a><a name="index-body-of-a-loop-567"></a>
In programming, a <dfn>loop</dfn> means a part of a program that is (or at least can
be) executed two or more times in succession.
<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 non-zero, 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 non-zero.
<p>Octave's <code>while</code> statement looks like this:
<pre class="example"> while (<var>condition</var>)
<var>body</var>
endwhile
</pre>
<p class="noindent">Here <var>body</var> is a statement or list of statements that we call the
<dfn>body</dfn> of the loop, and <var>condition</var> is an expression that
controls how long the loop keeps running.
<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>This example creates a variable <code>fib</code> that contains the first ten
elements of the Fibonacci sequence.
<pre class="example"> fib = ones (1, 10);
i = 3;
while (i <= 10)
fib (i) = fib (i-1) + fib (i-2);
i++;
endwhile
</pre>
<p class="noindent">Here the body of the loop contains two statements.
<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>A newline is not required between the condition and the
body; but using one makes the program clearer unless the body is very
simple.
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