Introduction
1: Introduction
2: Invoking Gre
3: Gre Grammar
4: Commands
5: Cookbook
6: Hints
7: History
8: Plans
9: Concept Index
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1: Introduction
1.1: What `gre ' is
`Gre ' is a program for making science graphs. It is a programming
language, not a GUI-based application. `Gre ' is analagous to
textual markup languages, such as TeX, LaTeX, HTML, etc. This
analogy points out the pros and cons of `gre '. Pro: users get
good control over the output graphs. Con: users must climb a
learning curve before using `gre '.
`Gre ' is based on the `Gri '
`Gri '
language. The main differences between the two are that `gre ' is
faster that `gre ' has a simpler, more powerful, syntax. `Gre '
syntax is based on the Perl language (Perl programmers).
1.2: A simple example
To try `gre ', do the following.
-
Create a datafile by inserting the following text into a file
called `example1.dat':
0.05 12.5
0.25 19
0.5 15
0.75 15
0.95 13
This is a datafile. Each line has an `(x,y) ' pair. If you don't
know what an `(x,y) ' pair is, `gre ' may not be what you need for
your work!
-
Create a command-file by inserting the following text into in a
file called `example1.gre':
open example1.dat;
read columns x=1 y=2;
draw curve;
This is a `gre ' program with three commands. Commands are separated
by semicolons; in this case they are also separated by line separators,
but these are optional, as is all whitespace in a `gre ' program. The
commands do the following things.
-
The first command opens the indicated data-file.
-
The second command reads `
x ' data from the first column and
`y ' data from the second column.
-
The third command indicates to draw the ``curve'' formed by the
`
(x,y) ' points provided in the data-file. The curve comprises
connected line segments -- it is not smoothed, although the word
``curve'' might fool you that it is!
Before the curve is drawn, axes will also be drawn (see below).
-
Run `
gre ' by typing
gre example1
to the operating system. `Gre ' assumes the commandfile has suffix
`.gre', so there is no need to supply the suffix, although nothing
is lost if you do supply it.
The three-line program makes use of many default `gre ' actions,
e.g. automatic drawing of axes and automatic selection of scales. All
defaults may be overridden. The following example illustrates how to
get `gre ' to call the y-axis `foo ' instead of `y ', and to
make this axis range from `0 ' to `20 ', labelled at intervals
of `10 ', instead of `12 ' to `19 ' labelled at unit
intervals (the x-axis still being selected automatically by `gre '):
open example1.dat;
read columns x=1 y=2;
set y name "foo";
set y axis 0 20 10;
draw curve;
For more on `set y axis ', see Set y axis.
A note on execution order. Notice that these axis-control
commands were placed before the `draw curve ' command. This
illustrates an important point: `gre ' executes the commands in a
command-file sequentially, starting at the top. If, for example, the
commands for controlling the y-axis were placed after the
`draw curve ' command, they would have no effect on this
graph.
Note for Matlab users: The `gre ' convention of executing
commands in sequence agrees with most programming languages
except Matlab. In matlab, some commands work non-sequentially,
e.g. an `axis ' command can change an existing graph. However,
other Matlab commands (basically, all non-drawing commands) act
sequentially ... imagine how confusing it would be if arithmetic
commands acted out of sequence, as some Matlab graphics commands do!
(c) 1997-1999 Dan E. Kelley, email
Dan.Kelley@Dal.Ca
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