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Chapters:
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<a href="Introduction.html">1: Introduction</a><br>
<a href="SimpleExample.html">2: Simple example</a><br>
<a href="InvokingGri.html">3: Invocation</a><br>
<a href="GettingMoreControl.html">4: Finer Control</a><br>
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<a href="Examples.html">8: Examples</a><br>
<a href="Commands.html">9: Gri Commands</a><br>
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<h3>9.3.52: `<font color="#82140F"><code>while</code></font>'</h3>
<!-- latex: \index{while} -->
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<PRE>
<font color="#82140F">
`while .test.|{rpn ...}'
</font></PRE>
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<p>
Perform statements in loop while the value of `<font color="#82140F"><code>.test.</code></font>' or the RPN
expression is nonzero. The end of the loop designated by a line
containing the words `<font color="#82140F"><code>end while</code></font>'. The value `<font color="#82140F"><code>.test.</code></font>' may be
an rpn expression. To leave the loop prematurely, use a `<font color="#82140F"><code>break</code></font>'
statement. Upon encountering a `<font color="#82140F"><code>break</code></font>' statement, Gri jumps to the
line immediately following the loop. If the `<font color="#82140F"><code>-chatty</code></font>' option is
nonzero, a notification is printed every 1000 passes through the loop,
as a debugging measure to catch unintended infinite loops.
<p>
<b>Examples</b>:
<p>
<ul>
<li>
Loop forever, printing a message over and over.
<p>
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<PRE>
<font color="#82140F">
while 1
show "This loops forever. Need to 'break'"
end while
</font></PRE>
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<p>
<li>
Read number pairs from a file, plotting bullets at
the indicated locations. Note the use of an infinite
loop, with a break condition following an
end-of-file test. (Do not be tempted to write
such loops as `<font color="#82140F"><code>while !..eof..</code></font>' because that
would not catch the end of file until the next
time through the loop. The result would be
to draw the last bullet twice, since the `<font color="#82140F"><code>read</code></font>'
will not update the variables when the end of
file is encountered.)
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
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<PRE>
<font color="#82140F">
while 1
read .x. .y.
if ..eof..
break
end if
draw symbol bullet at .x. .y.
end while
<p>
</font></PRE>
</TD>
</TR>
</TABLE>
<li>
Loop 10 times, printing the values of `<font color="#82140F"><code>.i.</code></font>' as they range 0, 1,
..., 9. After exiting from the loop, `<font color="#82140F"><code>.i.</code></font>' will equal 10. Be
<b>careful</b> to use the correct rpn greater-than test to avoid an
infinite loop.
<p>
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<PRE>
<font color="#82140F">
.i. = 0
while {rpn .i. 10 >}
show .i.
.i. += 1
end while
</font></PRE>
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