INTERNAL: Getting Started With IDL: Important Concepts From a Recent New User

Introduction to IDL

This article is designed to give a first time user of IDL a brief description of how to start using the product based on personal experience of what information is really important to know. Some of these topics are from commonly asked questions by people who were also first time users and new to programming. The goal of this article is to provide a bare essentials introduction that will teach the reader about important concepts when programming in IDL, not necessarily the language or native functions/procedures. 

-The IDL Development Environment (IDLDE)
- File naming conventions
- IDL path management
- Useful facts about the IDL programming language
- Types of IDL programs and memory Scope
- Writing a simple procedure, function, and main level program
- Where to look for additional information

The IDL Development Environment (IDLDE)

Before learning about the details behind IDL, it is important to know about the IDLDE, or sometimes called the IDL Workbench. As a quick introduction, a link for the online documentation specifically describing the IDLDE is provided below. This is important to review first so that certain terms will be familiar like the IDL Console and Editor window. 

http://www.exelisvis.com/docs/IDLDEOverview.html

Apart from that link, below is a screenshot with the most important areas boxed in red. See the link for more information on each section. 

IDL> my_pro

% Attempt to call undefined procedure: 'MY_PRO'.

% Execution halted at: $MAIN$

pro my_pro

    print, 'My awesome test procedure!'

end

IDL> my_func

% Compiled module: MY_FUNC.

My awesome test function!

pro my_pro2

    print, 'My awesome test procedure!'

end

IDL> my_pro2

% Attempt to call undefined procedure: 'MY_PRO2'.

% Execution halted at: $MAIN$    

Managing your IDL paths is fairly straightforward, but can cause errors with files not being found. In order to add folders to your path, go to Preferences->IDL->Paths. The image below shows what your preferences page should look like.

From here, if you select the insert button, then you can add a new folder/directory to the search path. When you include this option, continuing with the example above with a .pro file named “my_pro.pro”, IDL will search through each listed directory for the “my_pro.pro” file in addition to the default path. There are three other preferences on this page that are important to know.

First, if you include a new folder/directory to IDL’s search path, there will be the option to include subdirectories. This can be a handy option, but if your file contains many different subdirectories, then it can take IDL longer to initialize and search for different files. 

The second important setting to notice is the option to “Move up” or Move down” additional search directories. This is important because you can alter the order than IDL follows to search for a particular .pro file.  It is recommended that you keep the <IDL_DEFAULT> path as the first search directory, and only manage you additions. This is because if you create a routine with the same name as a native IDL routine, and <IDL_DEFAULT> is not the first search location, then you could start using the wrong function/procedure when you do not mean to. 

The last important setting to note is the check box  that says “Warn when a routine is on IDL’s path more than once.” This is important to let you know that you gave a file a name which has already been used. You can get an error similar to what was referenced in the previous paragraph. If you do have duplicate routines, then one place that you can look is the “Problems” tab by the IDL Console in the IDLDE. From here, the next step is to know some of the technical aspects to using IDL. 


Useful Facts About The IDL Programming Language

p = plot(/test)

p = PLOT(/test)

IDL> my_func

% Compiled module: MY_FUNC.

My awesome test function!

temp = 5

print, temp

end

% Compiled module: $MAIN$.

       5

Another thing to note is that when you define a variable in a main level program, then the variable is known to all main level programs and can be called from the IDL Console. You can confirm this by looking in the Variables tab of the IDLDE and should look like the screenshot below.

The next type of program in IDL is the function. Functions are similar to procedures, but they should have a line which has “return” to return a value. Any code that appears after the line which contains “return” will not be executed. If the “return” statement is left out, then a zero will automatically be returned from the function. Below is a small test function to illustrate how to call a function.  This should be saved as “my_func.pro” and can be called by typing “my_func(5)” in the IDL console.

function my_func, number

    ;make sure to only enter numbers

    temp = number

    return, 'You entered ' + strtrim(string(number),1) + ' as a function input'

end

;define the function

function test_plot_procedure

    p = plot(/test)

end