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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<title>Gri: Debugging Gri programs</title>
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<br>
Chapters:
<br>
&nbsp;&nbsp;<a href="Introduction.html">1: Introduction</a><br>
&nbsp;&nbsp;<a href="SimpleExample.html">2: Simple example</a><br>
&nbsp;&nbsp;<a href="InvokingGri.html">3: Invocation</a><br>
&nbsp;&nbsp;<a href="GettingMoreControl.html">4: Finer Control</a><br>
&nbsp;&nbsp;<a href="X-Y.html">5: X-Y Plots</a><br>
&nbsp;&nbsp;<a href="ContourPlots.html">6: Contour Plots</a><br>
&nbsp;&nbsp;<a href="Images.html">7: Image Plots</a><br>
&nbsp;&nbsp;<a href="Examples.html">8: Examples</a><br>
&nbsp;&nbsp;<a href="Commands.html">9: Gri Commands</a><br>
&nbsp;&nbsp;<a href="Programming.html">10: Programming</a><br>
&nbsp;&nbsp;<a href="Environment.html">11: Environment</a><br>
&nbsp;&nbsp;<a href="Emacs.html">12: Emacs Mode</a><br>
&nbsp;&nbsp;<a href="History.html">13: History</a><br>
&nbsp;&nbsp;<a href="Installation.html">14: Installation</a><br>
&nbsp;&nbsp;<a href="Bugs.html">15: Gri Bugs</a><br>
&nbsp;&nbsp;<a href="TestSuite.html">16: Test Suite</a><br>
&nbsp;&nbsp;<a href="GriInThePress.html">17: Gri in Press</a><br>
&nbsp;&nbsp;<a href="Acknowledgments.html">18: Acknowledgments</a><br>
&nbsp;&nbsp;<a href="License.html">19: License</a><br>
<br>
Indices:<br>
&nbsp;&nbsp;<a href="ConceptIndex.html"><i>Concepts</i></a><br>
&nbsp;&nbsp;<a href="CommandIndex.html"><i>Commands</i></a><br>
&nbsp;&nbsp;<a href="BuiltinIndex.html"><i>Variables</i></a><br>
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<h2>10.13: Debugging Gri Programs</h2>

Here are some hints for debugging Gri programs:
<ul>
<li>
If no data appear on an xy plot, insert `<font color="#82140F"><code>show columns statistics</code></font>'
or `<font color="#82140F"><code>show columns</code></font>' after the `<font color="#82140F"><code>read columns</code></font>' command.  It may be
that you have fixed your axes, and that the axes frame does not include
the data.
<li>
If you get an error message, rerun your Gri program using the
`<font color="#82140F"><code>-trace</code></font>' command-line option, to see which line is causing the
problem.  This often reveals logic errors (e.g. in (`<font color="#82140F"><code>if</code></font>'
statements).  You may also turn tracing on or off at any point in your
Gri program by setting the built-in variable `<font color="#82140F"><code>..trace..</code></font>' to 1 or 0.
Many Gri users have the Gri command aliased to be in trace mode by
default. 
<li>
If Gri complains of a syntax error, consult the printed manual, one of
the online manuals, or the online help facility (see <a href="OnlineHelp.html#OnlineHelp">Online Help</a>).
<li>
Check the version number (printed at startup) to see if a new version of
Gri has been installed, and check the manual for known incompatabilities.
<li>
Sprinkle `<font color="#82140F"><code>show</code></font>' commands throughout your program, to see what's
happening.  Even when you are sure your program works, it is a good idea
to embed `<font color="#82140F"><code>show</code></font>' statements so are they executed if the
`<font color="#82140F"><code>-debug</code></font>' flag is set:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
if ..debug..
  show "X=" .x. "and label is `\label'"
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>

<li> 
If your `<font color="#82140F"><code>draw</code></font>' commands don't draw anything, check to see
whether you've fooled yourself by enforcing an improper scaling; remove
explict scaling (`<font color="#82140F"><code>set x axis ...</code></font>'), clipping (`<font color="#82140F"><code>set clip</code></font>'), data
selection windows (`<font color="#82140F"><code>set input data window x|y</code></font>') and missing values
(`<font color="#82140F"><code>set missing value</code></font>').  Another trick is to read only a portion of
the data set (`<font color="#82140F"><code>read columns 10 x y</code></font>') and then print out all the
values (`<font color="#82140F"><code>show columns</code></font>').
</ul>
<p>
If you determine that the bug is in Gri, not in your program, please
report the bug, so that other users will not have the same hassle;
(see <a href="ReportingBugs.html#ReportingBugs">Reporting Bugs</a>).
<p>


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