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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>
<a href="X-Y.html">5: X-Y Plots</a><br>
<a href="ContourPlots.html">6: Contour Plots</a><br>
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<a href="Commands.html">9: Gri Commands</a><br>
<a href="Programming.html">10: Programming</a><br>
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<h2>10.6: If Statements</h2>
Gri has `<font color="#82140F"><code>if</code></font>' statements to make your programs more flexible.
Here's an example:
<p>
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<PRE>
<font color="#82140F">
query \thick "Use thick lines? (0 or 1)" ("0")
if \thick
set line width 2
else
set line width 0.5
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
If you answer 1 to the question, the line thickness will be set at 2
points. If you answer 0 then a thin line will be used. If you press
carriage return a thin line will be used.
<p>
The item following the `<font color="#82140F"><code>if</code></font>' can be
<ul>
<li>
a number (1 means true; anything else means false)
<li>
a variable (1 means true; anything else means false). Example:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
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<PRE>
<font color="#82140F">
if .plot_contours.
draw contour
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<li>
a synonym which expands to a number (1 means true; anything else means
false). Example:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
\plot_contours = "1"
if \plot_contours
draw contour
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
(Don't worry about the fact that synonyms are strings; Gri
expands the string value before interpreting the `<font color="#82140F"><code>if</code></font>' statement.)
<li>
an expression of the form `<font color="#82140F"><code>{string1 == string2 }</code></font>'. The symbol
`<font color="#82140F"><code>==</code></font>' is an operator which tests for string equality. This expands
to `<font color="#82140F"><code>1</code></font>' if the strings are equal, or `<font color="#82140F"><code>0</code></font>' otherwise. The
strings may be either synonyms or string constants. If the string
constant contains only one word, then it is not necessary to enclose it
in quotes, but it is clearer to do so. Examples:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
if {"\variable" == "Salinity"}
set x name "Salinity"
else
set x name "Unknown"
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<li>
a rpn (reverse polish notation) expression
(see <a href="ReversePolishMath.html#rpnMathematics">rpn Mathematics</a>):
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
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<PRE>
<font color="#82140F">
if {rpn .time. 100 <}
# ie, (100 < time), not (time < 100)
show "Time > 100"
else if {rpn .time. 100 >}
show "Time < 100"
else if {rpn "\item" "later" ==}
show "Time ... later babe"
else
show "Time is equal to 100"
end if
if {rpn .time. 10 * 100 ==}
show "Time is equal to 10"
else
show "Time is not equal to 10"
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
</ul>
<p>
There is no need to put the else part in if you don't need it. You
can do
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
set line width 0.5
if \use_thick_lines
set line width 2
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
if you wish.
<p>
If you want just the else part, you can do
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
if ! \use_thick_lines
set line width 0.5
end if
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
(The exclamation point denotes logical negation: `<font color="#82140F"><code>! true</code></font>' equals
`<font color="#82140F"><code>false</code></font>'.)
<p>
If statements may be nested many levels deep. You may also have
`<font color="#82140F"><code>else if</code></font>' blocks, as in:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
if {"\variable" == "S"}
set x name "Salinity"
set x axis 32 33 0.5 .1
else if {"\variable" == "T"}
set x name "Temperature"
set x axis 15 20 1 0.5
else
set x name "Unknown"
end if
</font></PRE>
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