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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>
<a href="Images.html">7: Image Plots</a><br>
<a href="Examples.html">8: Examples</a><br>
<a href="Commands.html">9: Gri Commands</a><br>
<a href="Programming.html">10: Programming</a><br>
<a href="Environment.html">11: Environment</a><br>
<a href="Emacs.html">12: Emacs Mode</a><br>
<a href="History.html">13: History</a><br>
<a href="Installation.html">14: Installation</a><br>
<a href="Bugs.html">15: Gri Bugs</a><br>
<a href="TestSuite.html">16: Test Suite</a><br>
<a href="Acknowledgments.html">17: Acknowledgments</a><br>
<a href="License.html">18: License</a><br>
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Indices:</br>
<a href="ConceptIndex.html"><i>Concepts</i></a><br>
<a href="CommandIndex.html"><i>Commands</i></a><br>
<a href="BuiltinIndex.html"><i>Variables</i></a><br>
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<h3>8.12.1: Handling headers</h3>
<b>Case 1 -- known number of header lines.</b>
This is easy. If you know that the file has, say, 10 header lines, you
can just do this:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open file
skip 10
read columns x y
...
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<b>Case 2 -- header itself indicates number of header lines.</b>
Quite often the first line of a file will indicate the number of header
lines. For example, suppose the first line contains a single number,
indicating the number of header lines to follow:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open file
read .skip.
skip .skip.
read columns x y
...
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<b>Case 3 -- header lines marked by a textual key.</b>
Sometimes header lines are indicated by a textual key, for example, the
characters `<font color="#82140F"><code>HEADER</code></font>' at the start of the line in the file. The easy
way to skip such a header is to use a system command. Depending on your
familiarity with the operating system (here presumed to be Unix), you
might choose to use Grep, Awk, or Perl. Here are examples:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "grep -v '^HEADER' file |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
For more on the `<font color="82140F"><code>|</code></font>' mechanism, see see <a href="Open.html#Open">Open</a>. The Grep command
prints lines which do not match the indicated string
(because of the `<font color="#82140F"><code>-v</code></font>' switch), and the `<font color="#82140F"><code>^</code></font>' character stands for
the start of the line (see <a href="SystemTools.html#Grep">Grep</a>). Thus all lines with the key word at the
<b>start</b> of the line are skiped.
<p>
<b>Case 4 -- reading and using information in header.</b>
Consider a dataset in which the first line gives the time of
observation, followed by a list of observations. This might be, for
example, an indication of the data taken from a weather balloon released
at a particular time from a fixed location, with the main data being air
temperature as a function of elevation of the balloon. The time
indication might be, for instance, the hour number. One might need to
know the time to print a label on the diagram. You could do that by:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open file
read .time.
read columns x y
draw curve
sprintf \label "Time of observation is %f hour" .time.
draw title "\label"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
where the `<font color="#82140F"><code>sprintf</code></font>' command has been used to change the numerical
time indication into a synonym that can be inserted into a quoted string
for drawing the title of the diagram (see <a href="Sprintf.html#Sprintf">Sprintf</a>). Here the time has
been assumed to be a decimal hour. You might also have three numbers on
the line, perhaps a day, an hour and a minute. Then you could do
something like
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open file
read .d. .h. .m.
read columns x y
draw curve
sprintf \label "Obs. %.0f:%.0f, day %.0f" .h. .m. .d.
draw title "\label"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Here the `<font color="#82140F"><code>%.0f</code></font>' code is used to ensure no numbers will be written
after the decimal point. Naturally, you could convert this to a decimal
day, by e.g.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
...
.dday. = {rpn .day. .hour. 24 / .min. 24 / 60 /}
sprintf \label "Decimal day is %.4f" .dday.
...
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
(Some of you might know how many minutes in a day, but I'm silly so I
kept the extra mathematical step -- nothing is lost by being
straightforward!)
<p>
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