| Paradigm(s) | vectorized |
|---|---|
| Appeared in | 1977 |
| Designed by | David Stern |
| Developer | David Stern & ITT Visual Information Solutions (ITT VIS) |
| Stable release | IDL 8.1 (April 2011) |
| Typing discipline | Dynamic |
| Major implementations | IDL, GNU Data Language |
IDL, short for Interactive Data Language, is a programming language used for data analysis. It is popular in particular areas of science, such as astronomy and medical imaging. IDL shares a common syntax with PV-Wave and originated from the same codebase, though the languages have subsequently diverged in detail. There is also a free implementation, GNU Data Language (GDL).
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IDL is vectorized, numerical, and interactive, and is commonly used for interactive processing of large amounts of data (including image processing). The syntax includes many constructs from Fortran and some from C.
IDL originated from early VAX/VMS/Fortran, and its syntax still shows its heritage:
x = findgen(100)/10 y = sin(x)/x plot,x,y
The findgen function in the above example returns a one-dimensional array of floating point numbers, with values equal to a series of integers starting at 0.
Note that the operation in the second line applies in a vectorized manner to the whole 100-element array created in the first line, analogous to the way general-purpose array programming languages (such as APL, J or K) would do it. This example contains a divide by zero; IDL will report an arithmetic overflow, and store a NaN value in the corresponding element of the y array (the first one), but the other array elements will be finite. The NaN is excluded from the visualization generated by the plot command.
As with most other array programming languages, IDL is very fast at doing vector operations (sometimes as fast as a well-coded custom loop in FORTRAN or C) but quite slow if elements need processing individually. Hence part of the art of using IDL (or any other array programming language, for that matter) for numerically heavy computations is to make use of the built-in vector operations.
The predecessor versions of IDL were developed in the 1970s at the Laboratory for Atmospheric and Space Physics (LASP) at the University of Colorado at Boulder. At LASP, David Stern was involved in efforts to allow scientists to test hypotheses without employing programmers to write or modify individual applications. The first program in the evolutionary chain to IDL that Stern developed was named Rufus; it was a simple vector-oriented calculator that ran on the PDP-12. It accepted two-letter codes that specified an arithmetic operation, the input registers to serve as operands, and the destination register. A version of Rufus developed on the PDP-8 was the Mars Mariner Spectrum Editor (MMED). MMED was used by LASP scientists to interpret data from Mariner 7 and Mariner 9. Later, Stern wrote a program named SOL, which also ran on the PDP-8. Unlike its predecessors, it was a true programming language with a FORTRAN-like syntax. SOL was an array-oriented language with some primitive graphics capabilities.
Stern left LASP to found Research Systems Inc. (RSI) in 1977. The first RSI product was IDL for the PDP-11. In this release, the graphics supported by IDL were primarily Tektronix terminals and raster graphics displays. RSI sold its first IDL licenses to NASA's Goddard Space Flight Center and Ball Aerospace & Technologies Corp. in 1979. Two years later RSI released an initial VAX/VMS version of IDL, which was written in VAX-11 MACRO and FORTRAN. It took advantage of the VAX virtual memory and 32-bit address space. The National Center for Atmospheric Research (NCAR), the University of Michigan, the University of Colorado, and the Naval Research Laboratory started to use IDL with this version.
In 1987 RSI shifted development work of IDL to the Unix environment, which required a complete re-write of the code in C rather than a port of the existing version of VAX IDL. Stern and Ali Bahrami rewrote IDL for Unix on the Sun 3, taking advantage of the re-write to extend and improve the language. Subsequently, IDL was further expanded and ported to several variants of Unix, VMS, Linux, Microsoft Windows (1992), and Mac OS (1994).
Widgets were added to IDL in 1992, providing event-driven programming with graphical user interfaces. In 1997 ION (IDL On the Net), a web server-based system, was commercially released. The first version of ENVI, an application for remote sensing multispectral and hyperspectral image analysis written in IDL, was released in 1994. ENVI was created, developed and owned by Better Solutions Consulting, LLC, until it was purchased from BSC in October 2000 by Eastman Kodak coincident with their purchase of RSI. RSI sold, marketed and supported ENVI under the terms of a license agreement with BSC, LLC from 1994 through October, 2000. New object and pointer types, and limited object-oriented programming capabilities were added to IDL in 1997.
IDL has been applied widely in space science. The European Space Agency used IDL to process almost all of the pictures of Halley's Comet taken by the Giotto spacecraft. The team repairing the Hubble Space Telescope used IDL to help them diagnose anomalies in the main mirror. In 1995, astronauts on board a space shuttle used IDL loaded on a laptop to study ultraviolet radiation. Currently, amongst other applications, IDL is being used for most of the analysis of the SECCHI part of the STEREO mission at NRL, USA, and at the Rutherford Appleton Laboratory, UK.
RSI became a wholly owned subsidiary of ITT Industries in March 2004. As of May 15, 2006, RSI began doing business as ITT Visual Information Solutions. Effective 31 October 2011, as a result of restructuring, that company become Exelis Visual Information Solutions.
As a computer language, IDL:
Some of these features, which make IDL very simple to use interactively, also cause difficulties when building large programs. The single namespace is particularly problematic; for example, language updates that include new built-in functions have on occasion invalidated large scientific libraries. IDL also lacks nested arrays (that is, arrays of arrays are not permitted).
Arrays are passed by reference, and this mechanism is an advertised feature of the language to pass data back out of a subroutine—but array slices are copied before being passed, so that data do not flow back into array ranges, violating the principle of least surprise.
Many historical irregularities survive from the early heritage of the language, requiring individual work-arounds by the programmer. As an example:
The preceding issue can be alleviated using this compiler option:
COMPILE_OPT STRICTARR
Also, that provision of the IDL EULA has no effect in Australia, as a result of sections 47D and 47H of that country's Copyright Act.
The following graphics were created with IDL (source code included):
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