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Chapters:
<br>
<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="GriInThePress.html">17: Gri in Press</a><br>
<a href="Acknowledgments.html">18: Acknowledgments</a><br>
<a href="License.html">19: License</a><br>
<br>
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>9.3.28: `<font color="#82140F"><code>open</code></font>'</h3>
<!-- latex: \index{open} -->
There are two styles of `<font color="#82140F"><code>open</code></font>' command. In the first style, a
simple file is to be opened. In the second style a unix-like "pipe" is
opened, i.e. Gri will read the output of a system command instead of a
file.
<UL>
<LI><a href="Open.html#OpeningSimpleFiles">Opening Simple Files</a>:
<LI><a href="Open.html#OpeningPipes">Opening Pipes</a>:
<LI><a href="Open.html#OpeningURLs">Opening URLs</a>:
</UL>
<p>
<!-- @node Opening Simple Files, Ascii Files, Open, Open -->
<a name="OpeningSimpleFiles" ></a>
<h4>9.3.28.1: Opening simple files</h4>
<UL>
<LI><a href="Open.html#AsciiFiles">Ascii Files</a>:
<LI><a href="Open.html#BinaryFiles">Binary Files</a>:
<LI><a href="Open.html#NetCDFFiles">NetCDF Files</a>:
</UL>
<!-- @node Ascii Files, Binary Files, Opening Simple Files, Opening Simple Files -->
<a name="AsciiFiles" ></a>
<b>Ascii Files</b>
Most applications involve ascii files, and these are very easy to handle
in Gri. For example given a data file named `<font color="#82140F"><samp>foo.dat</samp></font>', just use
the command
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open foo.dat
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
and then you can read the data using various commands. Thus a complete
program might be
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open foo.dat
read columns x y
draw curve
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
If a filename contains blanks or punctuation symbols, you must put it in
double quotes (`<font color="#82140F"><code>"</code></font>'), e.g.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "foo bar.dat"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Indeed, Gri accepts double-quotes on any `<font color="#82140F"><code>open</code></font>' command and some
folks use it on all commands, as a matter of habit.
<p>
Gri can handle compressed files appropriately, e.g.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open foo.data.gz
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
so that there is no need to uncompress data for use with Gri.
<p>
Gri is quite persistant in looking for your file, and if a given file is
not found, it will then check to see if a compressed version is
available, and use that instead. Thus
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open foo.dat
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
will look for a file named `<font color="#82140F"><samp>foo.dat.gz</samp></font>' if `<font color="#82140F"><samp>foo.dat</samp></font>' is not
available. (Only files compressed with the GNU `<font color="#82140F"><code>gzip</code></font>' utility are
handled.)
<p>
If the `<font color="#82140F"><code>open</code></font>' command was successful in opening the file, it will
set the value of the synonym `<font color="#82140F"><code>\.return_value.</code></font>' to the <b>full</b>
pathname of the file. Thus, if `<font color="#82140F"><code>open a.dat</code></font>' is done in directory
`<font color="#82140F"><code>/home/gri</code></font>', then `<font color="#82140F"><code>\.return_value.</code></font>' will equal
the string `<font color="#82140F"><code>/home/gri/a.dat</code></font>'.
<p>
<!-- @node Binary Files, NetCDF Files, Ascii Files, Opening Simple Files -->
<a name="BinaryFiles" ></a>
<b>Binary Files</b>
Like most computer programs, Gri has some trouble with binary files.
One big issue is the so-called "endian" character of the computer. Some
computers store multi-byte values with the most significant bytes first,
while others store them with the most significant bytes last. The
problem is that nothing is stored in data files to indicate which
convention was employed. For this reason, a version of Gri compiled on
a so-called "big-endian" computer will misinterpret multi-byte values
that were created on a so-called "little-endian" computer. Many folks
in the scientific community have converted to using the NetCDF format
(see next section) for precisely this reason, since this format is
independent of the endian character of the computer.
<p>
Presuming an appropriate endian character, however, reading is
straightforward. A command of the form
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open foo.dat binary
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
tells Gri that the data are stored in a binary format. With the above
syntax, Gri expects images to be in `<font color="#82140F"><code>unsigned char</code></font>' (8 bits), while
other data, such as columns and grids, are expected to be in 32-bit
format (suitable for reading into a so-called "float" variable in the C
programming language).
<p>
You may also specify the format directly, as in the following examples;
Gri then interprets all data as being in the indicated format and then
converts to the internal format before using the data.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open \filename binary uchar
open \filename binary 8bit
open \filename binary int
open \filename binary float
open \filename binary double
open \filename binary 16bit
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
As with ascii files, Gri will automatically uncompress any files that
are compressed, and if it fails to find a given filename, it will try to
open a compressed version of it (i.e. one with a `<font color="#82140F"><samp>.gz</samp></font>' suffix).
<p>
<!-- @node NetCDF Files, Opening Pipes, Binary Files, Opening Simple Files -->
<a name="NetCDFFiles" ></a>
<b>NetCDF Files</b>
The NetCDF format provides the best of both worlds. It is binary, so
that data are relatively compact, and may be read very quickly.
(Reading ascii data is time-consuming in C++, the language in which Gri
is written.) But it does not suffer the endian problem problem of
normal binary files (see previous section), since information about the
endian character is stored in the file itself, and Gri uses this
information to decode the data without difficulty, regardless of the
endian characteristics of the computer on which Gri is running and of
the computer that created the data.
<p>
For more information on netCDF format, see
<p>
`<font color="#82140F"><code>http://www.unidata.ucar.edu/packages/netcdf/index.html</code></font>'
<a href="http://www.unidata.ucar.edu/packages/netcdf/index.html">
here </a>.
<p>
The syntax of opening NetCDF files is as below
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open foo.nc netCDF
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
and the syntax for reading such files is described in sections on the
various `<font color="82140F"><code>read</code></font>' commands (see e.g. see <a href="Read.html#ReadColumns">Read Columns</a>).
<p>
<!-- @node Opening Pipes, Opening URLs, NetCDF Files, Open -->
<a name="OpeningPipes" ></a>
<h4>9.3.28.2: Opening pipes</h4>
Sometimes it makes sense to get Gri to work with the results of another
command in the OS. Gri handles this by creating a so-called "pipe",
thus reading the output from the other command. (Readers familiar with
the unix OS will know what pipes are all about, and especially why they
are a good thing. Other readers might wish to skip this section.)
<p>
Suppose we wish to plot an x-y plot using just the first few lines of a
datafile named `<font color="#82140F"><samp>foo.dat</samp></font>'. Unix users will know that a good way to
see the first few lines of such a file would be to type the command
`<font color="#82140F"><code>head foo.dat</code></font>'. They also know that these lines could be provided
to a second unix command, named `<font color="#82140F"><samp>do_foo</samp></font>' say, by the command
`<font color="#82140F"><code>head foo.dat | do_foo</code></font>'. This uses a so-called "pipe", designated
by the vertical line (called a pipe symbol below).
<p>
Gri can read the output from system commands by using a syntax in which
the (quoted) system command ends in a pipe symbol, e.g.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "head foo.dat |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
as in the example above.
<p>
<b>Aside</b>: When pipe-open commands are used, Gri creates a temporary
file (often located in `<font color="#82140F"><samp>/usr/tmp</samp></font>', but that varies with machine).
This is automatically cleaned up when Gri completes executation, but if
Gri dies (or is interrupted) before it finishes, you'll be left with an
extra file in this temporary-storage directory. It's up to you to clean
that directory up from time to time.
<p>
Some common examples of pipe-open commands are given below.
<p>
<ol>
<p>
<li>
<b>Comma-separated values</b> are common in files created by, or
intended for, spreadsheets. Since Gri expects data elements to be
separated by blanks (or tabs), you'll have to convert the commas into
blanks. There are many ways to do that using pipes, e.g.
`<font color="#82140F"><samp>sed</samp></font>' system utility, e.g.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "sed -e 's/,/ /g' foo.dat |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Other unix facilities, such as `<font color="#82140F"><code>tr</code></font>' will also work, of course. If
the file has headers, you'll want to remove them also. This can be done
with the `<font color="82140F"><code>skip</code></font>' command (see <a href="Skip.html#Skip">Skip</a>) but you could also do it at
the open stage, e.g. to remove the first two lines, use
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "sed -e 's/,/ /g' foo.dat | tail +2 |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<li>
<b>Manipulating column data</b> is done by e.g.
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "cat foo.dat | awk '{$1, $2 * 22}' |"
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
where `<font color="#82140F"><samp>awk</samp></font>' has been used to multiply the second column in the file
named `<font color="#82140F"><samp>foo.dat</samp></font>' by 22.
<p>
<li>
<b>Time-based and geographical data</b> are sometimes encountered. For
an example, suppose that longitude/latitude (i.e. x/y) data are stored
in Hour.minutesecond format, e.g. 12.2133 means hour 12, minute 21,
second 33. Gri doesn't read HMS format, but gawk can be told to:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "cat datafile.HMS | \
awk '{ \
split($1, hms, \".\"); \
h = hms[1]; \
m = int(hms[2] / 100); \
s = hms[2] - 100 * m; \
x = h + m / 60 + s / 3600; \
split($2, hms, \".\"); \
h = hms[1]; \
m = int(hms[2] / 100); \
s = hms[2] - 100 * m; \
y = h + m / 60 + s / 3600; \
print(x,y) \
}' | "
read columns x y
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
<li>
<b>Timeseries data</b> are often stored in formats that blend letters
and numbers. For one thing, using letters (e.g. `<font color="#82140F"><code>aug</code></font>') removes an
ambiguity in numerically-based data. (Example: 02/03/2000 means one
thing to an American and another thing in the rest of the world.
However, everybody agrees on what 2000-Feb-03 means.) Suppose, for
example, that we have data in a format such as
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
Tue_Jul_25_11:07:51 0.62
Tue_Jul_25_11:22:51 0.59
Tue_Jul_25_11:37:51 0.56
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
(stored in a file called `<font color="#82140F"><samp>foo.dat</samp></font>' say) and we want a graph of the
y-variable (0.62, 0.59, 0.56) versus x-variable, time expressed say as
seconds in the day. Then here is how that could be done:
<p>
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "cat foo.dat |\
sed -e 's/_/ /g' -e 's/:/ /g' |\
awk '{print ($4*3600+$5*60+$6, $7)}' |"
read columns x y
draw curve
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
Note that the actual day information is skipped in this example;
seasoned `<font color="#82140F"><code>awk</code></font>' users could easily fill in the code to handle
datasets spanning several days.
</ol>
<p>
<!-- @node Opening URLs, Postscript, Opening Pipes, Open -->
<a name="OpeningURLs" ></a>
<h4>9.3.28.3: Opening URLs</h4>
Gri can open a URL, <em>if</em> you have the `<font color="#82140F"><code>wget</code></font>' program on your
machine. (`<font color="#82140F"><code>wget</code></font>' is available from the GNU website
<a href="http://www.gnu.org/software/wget/">`<font color="#82140F"><code>http://www.gnu.org/software/wget/</code></font>'</a>.)
<p>
The URL must be enclosed in quotes (since otherwise,
Gri will interpret the `<font color="#82140F"><code>//</code></font>' sequence as indicating an old way
of denoting comments). For example,
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
open "http://gri.sourceforge.net/gridoc/examples/example1.dat"
read columns x y
show columns
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
If you don't have `<font color="#82140F"><code>wget</code></font>' installed on your machine, the above won't
work, but you can always use another fetching program, with a system
call, as in the following:
<TABLE SUMMARY="Example" BORDER="0" BGCOLOR="#efefef" WIDTH="100%">
<TR>
<TD>
<PRE>
<font color="#82140F">
\url = "http://gri.sourceforge.net/gridoc/html/examples/example1.dat"
open "lynx -dump \url |"
read columns x y
draw curve
</font></PRE>
</TD>
</TR>
</TABLE>
<p>
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