**Introduction**- A brief introduction, with links to other valuable IDL sites.
**Getting started**- A few basic hints that will help you get started with the command line interface and a link that will help you get idl running on your display.
**Programs and batch mode**- IDL can be run by typing commands interactively, by creating programs interactively, by reading programs in from files, or it can be run in batch mode.
**Variables and arithmetic**- Examples of setting scalar, vector, and array variables, and performing some basic operations.
**Matrices**- Examples of matrix operations.
**Plots of Y vs. X**- Plots of a function of one variable. One example also shows how to read ASCII data from a file. You can see how to fit a polynomial to data. Also included are examples with MDS access.
**Surface plots**- Examples of surface plots of functions of two variables. Included are examples with MDS access.
**Animation**- Example animation
**Hardcopy**- How to save your images to a postscript file.
**Gotchas**- Some idiosyncrasies of IDL that you should know about.
**Why do my graphics get erased?**- When a window gets covered, then uncovered, someone has to keep a copy of the obscured part of the image.
**Other IDL Topics**- How to find out more about IDL.

This tutorial is organized as a set of simple examples with explanations. It is only a glimpse at what you can do with IDL. Once you start using it, you will want to look at the online and/or hardcopy documentation, and explore.

The most effective way for you to go through this tutorial is by running IDL in a separate window, and trying out the commands and programs as you read the tutorial. Experiment with variations on these commands. Note what happens when you make errors. Alternately, you may prefer using the IDL Basics manual over this tutorial.

First read about How to Setup IDL at PPPL. This will tell you on which machines IDL is running, how to set up your environment, how to set your display, and how to find documentation.

A few basic hints that will help you get started with the command line interface.

Type a question mark "?" for online help if you are running X. If you are running Versaterm on VMS, type "$help idl" at the IDL prompt.

Try typing the following four commands at the IDL prompt:

IDL> a = 5 IDL> print, a IDL> a = [2, 3] IDL> print, a

Observe that commands are followed by a comma, before the parameter list.

To repeat a command, you can go up and down through previous commands using the arrow keys. When you reach the command you want to repeat, hit return (this is like VAX/VMS and the Unix tcsh shell).

IDL programs can be stopped using control-C. (Hold down the control key and hit the letter c).

IDL can be aborted immediately using control-\. (All variables are lost and the state of open files will be uncertain).

IDL can be run by typing commands interactively, by creating programs interactively, by reading programs in from the command line, or it can be run in batch mode.

When you type commands on the command line, each line is executed immediately when you hit the return key. (It is possible to carry over to the next line using a dollar sign "$" at the end of the line).

**Batch mode**

Running in batch mode is similar, except the commands come from a file. You precede the file name with the at-sign "@". Using your favorite editor, create a file called batch_two_prints.pro (in the directory from which IDL is running) that contains the four lines

a = 5 print, a a = [2, 3] print, aType the following at the IDL prompt

IDL> @batch_two_printsThis will execute the four commands exactly as if you had typed them. This can also be done directly from your Unix shell, by typing

% idl batch_two_printsNOTE: This will not work if "idl" invokes a script, rather than the idl binary. Substitute the full pathname for the idl binary, e.g., /usr/local/bin/idl.

To run this code in the background, thus freeing up your terminal:

% idl batch_two_prints &

**Programs**

When typing interactively and running in batch mode, each line is executed immediately. A problem arises with control statements that span lines. Consider the following simple computation of the factorial function. As a batch file it would look like:

f = 1 for k=1,6 do begin $ f = k * f & $ print, f & $ end

The commands must be separated by ampersands "&", and the lines must be continued. The entire loop must essentially be on one line, since each line is executed as soon as it is encountered. Imagine nested loops with long calculations inside.

As a program it is somewhat simpler:

f = 1 for k=1,6 do begin f = k * f print, f endfor end

Files can be created while you experiment with syntax and your algorithm by:

IDL> journal,'mycode.pro' IDL> (many idl commands) IDL> journalThen, edit out the unwanted lines in the file mycode.pro. Add a STOP and END statement. Then .run the file.

Conversely, a good way to debug procedures or functions found in files is to copy lines from them and paste them into an IDL session.

For more information on creating and running programs, including other commands, see Chapter 2 of the User's Guide.

The simplest thing to work with is scalars.

IDL> x = 3 IDL> y = 2.5 IDL> z = x + y IDL> w = x^y + sin(z) IDL> print, x, y, z, w 3 2.50000 5.50000 14.8829Square braces are used to define vectors (1-dimensional arrays):

IDL> v1 = [1, 2, 0] IDL> v2 = [1, 0, 0] IDL> print, "v1 = ", v1 v1 = 1 2 0 IDL> print, "v2 = ", v2 v2 = 1 0 0Vectors can be componentwise added, multiplied, etc.:

IDL> v3 = v1 + v2 IDL> print, "v3 = v1 + v2 = ", v3 v3 = v1 + v2 = 2 2 0 IDL> print, "v1 * v2 = ", v1 * v2 v1 * v2 = 1 0 0 IDL> print, "v1 * sin(v3) = ", v1 * sin(v3) v1 * sin(v3) = 0.909297 1.81859 0.00000There are other useful operators, such as min and max:

IDL> min1 = min(v1) IDL> max1 = max(v1) IDL> print, "min(v1), max(v1) = ", min1, max1 min(v1), max(v1) = 0 2Scalars and arrays can be allocated with specific types. Scalar examples:

IDL> x = float(1.3) IDL> sx = fix(x) IDL> lx = long(x) IDL> bx = byte(x) IDL> dx = double(x) IDL> cx = complex(x) IDL> print, x, sx, lx, bx, dx, cx 1.30000 1 1 1 1.3000000 ( 1.30000, 0.00000)Array examples:

IDL> a = fltarr(5) IDL> for i=0, 4 do a(i) = 2*i IDL> b = complex(a) IDL> print, "b = ", b b = ( 0.00000, 0.00000)( 2.00000, 0.00000) ( 4.00000, 0.00000)( 6.00000, 0.00000) ( 8.00000, 0.00000)

IDL> A = dblarr(2, 4) IDL> for i = 0, 1 do begin $ IDL> for j = 0, 3 do begin $ IDL> a(i, j) = 10 * i + j IDL> print, A 0.0000000 10.000000 1.0000000 11.000000 2.0000000 12.000000 3.0000000 13.000000Note that as it is printed, the first index corresponds to the column, and the second index to the row. Another way to think of it is that the way the data is stored, the first index varies fastest, and the second varies the slowest. This agrees with the way the data is printed.

The WHERE function can be extremely useful. Play around with it.

IDL> b = [1, 2, 3, 4, 5, 6, 7, 8] IDL> PRINT, WHERE( b GT 2 AND b le 7) 2 3 4 5 6Watch out, the results of the WHERE function 2-D arrays are confusing, because they return single-valued indices, but they work. (Continuing from the previous example:)

IDL> print, WHERE(A GT 10 AND A LT 13) 3 5

A matrix may be constructed explicitly from vectors:

IDL> v1 = [1, 2, 0] IDL> v2 = [1, 0, 0] IDL> v3 = [4, 5, 6] IDL> A = [[v1], [v2], [v3]] IDL> print, A 1 2 0 1 0 0 4 5 6Create the transpose:

IDL> Atrans = transpose(A) IDL> print, Atrans 1 1 4 2 0 5 0 0 6Take the determinant:

IDL> d = determ(float(A)) % Compiled module: DETERM. IDL> print, d -12.0000Invert:

IDL> Ainv = invert(A) IDL> print, Ainv 0.00000 1.00000 0.00000 0.500000 -0.500000 0.00000 -0.416667 -0.250000 0.166667Multiply vectors by matrices:

IDL> v = [1, 2, 3] IDL> print, A 1 2 0 1 0 0 4 5 6 IDL> print, v 1 2 3 IDL> print, A ## v 5 1 32 IDL> print, v ## A 15 17 18You can solve a linear system Ax = b for x by Cramer's rule (the cramer function expects float or double inputs, requiring an explicit type conversion):

IDL> b = float([1, 2, 16]) IDL> A = float(A) IDL> x = cramer(A, b) IDL> print, x 2.00000 -0.500000 1.75000

The program **fit_y.pro** reads in a set of 10 y values.
It creates 10 x values from 0 to 9 suing the "findgen" command.
It then fits a cubic to the data, and plots the original data as points, and
the fitted function as a curve.

; File: fit_y.pro - Author: Erik Brisson N = 10 y = fltarr(N) openr, 1, 'ex_y.dat' readf, 1, y close, 1 print, 'print y' print, y print, 'another way to print y' for i=0, N-1 do begin print, y(i) endfor plot, psym=1, y x =findgen(N) ypoly = poly_fit(x, y, 3) print, 'coefficients of third degree fit' print, ypoly yapprox = ypoly(0) + ypoly(1)*x + ypoly(2)*x^2 + ypoly(3)*x^3 print, 'values of fit at corresponding x values' print, yapprox oplot, yapprox endFor the above to run, you'll need a file named ex_y.dat

The program **fit_xy.pro** reads in a set of 10 (x,y) pairs,
into a 2 x 10 array.
It separates them into two 1-dimensional arrays, and fits a cubic to it,
and plots this as in the preceding example.

; File: fit_xy.pro ; Author: Erik Brisson N = 10 xy = fltarr(2,N) openr, 1, 'ex_xy.dat' readf, 1, xy close, 1 x = xy(0,*) y = xy(1,*) print, 'x' print, x print, 'y' print, y plot, psym=1, x, y ypoly = poly_fit(x, y, 3) yapprox = ypoly(0) + ypoly(1)*x + ypoly(2)*x^2 + ypoly(3)*x^3 print, 'yapprox' print, yapprox oplot, x, yapprox endFor the above to run, you'll need a file named ex_xy.dat

Also check out the contour examples with MDS access.

An example of drawing a surface plot directly, rendered as a wire mesh,

a = findgen(35) b = 45 -findgen(45) c = a # b window, 0, retain=2, title='surf_wire',xsize=500, ysize=500 SURFACE, c

IDL provides an interactive viewer for surface plots, called xsurface.

XSURFACE,c

An example of drawing a surface plot directly, rendered as a shaded surface. ; File: shade_surf.pro - Author: Erik Brisson

SHADE_SURF, cThese plots can be combined with various contour plots in various ways.

Producing surface plots from scattered data is demonstrated in hydro.pro. This is data in which the (x,y) locations for which we have the function evaluated do not lie on a regular gird. To make a surface plot, IDL needs to have the function evaluated on a regular rectangular grid. There are two steps involved. The first is to form a triangulation using the input (x,y) points to use for interpolation, and the second is to produce a mesh from that interpolation. Another example, at scv.bu.edu/SCV/Tutorials/IDL/examples/idl/pro/triangulate.pro, shows this process and renders the result as a wire mesh surface plot. http://scv.bu.edu/SCV/Tutorials/IDL/examples/idl/pro/tri_shade.pro, does the same thing, and in addition, draws a shaded surface plot, using two different kinds of shading.

; File: making_waves.pro - Author: Erik Brisson pro making_waves a = fltarr(256,3,63) for i=0,255 do for j=0,2 do for k=0,62 do $ a(i,j,k) = sin(float(i-k)/10.)/exp((float(i)/200.)) frames=bytarr(400,300,63) window,retain=2,/free,title='making waves',xsize=400,ysize=300 for k=0,62 do begin $ shade_surf, a(*,*,k) & $ frames(0,0,k) = tvrd() & $ end movie, frames, order=0 end

IDL> set_plot,'PS' IDL> device, filename='your_filename.ps' IDL> ... IDL> ... plot some stuff ... IDL> ... IDL> device, /close IDL> set_plot,'X'Alternately, the local routines setup_ps and setup_x can span the plot commands you want to send to a file when using x-windows. When using Versaterm on VMS at PPPL, the SGLIB graphing system can be used, or the standard printing of Tektronix files from Versaterm. Great tips on IDL postscript output (as well as many other things) can be found at http://dfanning.com.

- IDL leaves you in the procedure when an error is detected (unless the ONERROR routine is called within the procedure). Enter RETALL to return to the top-level.
- Array indices are zero based, e.g.,
a = BYTARR(100) FOR i=0,99 DO a(i) = i

- If you don't use LONG(0), the maximum value is 64K, so:
FOR i=LONG(0),100000 DO sum = sum+i

would give an error without the use of LONG(0). - It may not be obvious how to read files written by other programs. There are user routines to read columns of ASCII numbers.
- > and < are not what they may seem.
a = 5 < 3 ; sets a to 3 (the lesser value) a = WHERE (array < 2) ; sets a = array (if first element is < 2) a = WHERE (array LT 2) ; is probably what you want

In any case, this topic is called "backing store". It can be done by IDL, done by the windowing system, or not done. By default, the X window system does not have backing store turned on.

In IDL, there is a keyword RETAIN, for specifying which kind of backing store to use.

- RETAIN = 0 implies no backing store,
- RETAIN = 1 IDL asks the window system to do it,
- RETAIN = 2 IDL does it

window,0,retain=2,xsize=500,ysize=500

Backing store will now be maintained for this window by IDL.

There is an excellent **demo** supplied with idl, which shows many of
the advanced things IDL can do.
Just type "demo" at the IDL prompt.

You may also want to look at the IDL supplied examples
in **/usr/local/rsi/idl/examples** on Unix or IDL_DIR:[EXAMPLES] on
VMS.

You should also visit these valuable IDL sites, especially the ones that let you search for IDL routines written by others (no such search exists for fusion, or PPPL-specific software, but it should).

Other examples are in the Class2 Directory. For a simple, but useful example of a widget for plotting MDS signals, see mdsw_noch.pro

Return to the NSTX Software Page

*Borrowed heavily from http://scv.bu.edu/SCV/Tutorials/IDL/idl_webtut.html

Edited for NSTX: 25-Apr-2005

by: Bill Davis