Issue #39, July 1997

A quick look at a language designed to manipulate matrices and provide other numerical functions.

For numerical computing, high level languages offer advantages over more traditional languages, such as FORTRAN or C. Built-in graphics capabilities, automatic variable typing and flexible data structures combine to provide an environment in which it is easy to develop your ideas without having to fight with the language. That's not to say that FORTRAN and C are of no use, just that sometimes you want to make life a bit easier.

Matlab is a one such language. It is available on many platforms (including Linux) and provides powerful facilities for manipulating matrices, as well as other numerical functions. Unfortunately, Matlab is commercial software and wasn't available for Linux until recently (in the last twelve months or so). However, there are other, freely-available alternatives, and Octave is one such alternative.

Superficially, Octave looks very much like Matlab, and the
description in its LSM entry reads “GNU Matlab—A numerical matrix
mathematics program.” To begin, type **octave** at
the shell prompt, and Octave greets you with its own prompt. Now we
can start doing math.

As you might expect, entering and manipulating matrices is
one of Octave's strengths. (In order to differentiate Octave
commands from the output, the prompt
**octave:number of the
command** precedes the commands in the
examples below.) We can enter a matrix with the command:

octave:1 a=[1 2 ; 3 4]

Octave then reports the result of the command, namely:

a = 1 2 3 4To suppress output, simply place a semicolon after the command. Note that we didn't have to worry about declaring the size or type of the matrix

octave:2 a' ans = 1 3 2 4Arithmetic operators such as

octave:3 b=[1 0; 3 2] b = 1 0 3 2we can find the matrix product of

octave:4 a*b ans = 7 4 15 8Or define the (

octave:5 a.*b ans = 1 0 9 8Matrix elements can easily be selected by index, so to get the (1,1) entry of

octave:6 a(1,1) ans=1We can select a row or column using the colon operator, exactly as in Matlab. Thus, to select the first column of

octave:7 a(:,1) ans = 1 3And to select the first row of

octave:8 a(1,:) ans = 1 2As well as these elementary operations, Octave provides functions that perform higher-level operations, such as finding the eigenvalues of a matrix, by using a command like the following:

octave:9 eig(a) ans = 5.37228 -0.37228Alternatively, you can find the eigenvalues and eigenvectors by giving the following command:

octave:10 [v,d]=eig(a) v = 0.41597 -0.82456 0.90938 0.56577 d = 5.37228 0.00000 0.00000 -0.37228This is a demonstration of one of the main advantages of using a high level language like Octave. Writing a FORTRAN or C program to find the eigenvalues of a matrix would take a lot more time and effort than it does in Octave. Moreover, Octave routines are usually based on well-known, high quality algorithms, so you can have faith in the results.

Octave provides many other matrix routines, which are detailed in the manual and in the on-line help system.

Octave also lets the user define his own functions via the
**function** keyword. A function definition looks
like this:

function [output values] = name (input values) sequence of commands endfunction

The input and output values are optional, so it is possible to write a function that takes no arguments and returns no values, such as

octave:11 function hello printf("hello\n") endfunctionThe

octave:12 hello helloObviously, functions that don't take arguments or return values aren't all that useful. To accept arguments, list them after the function name in the following manner:

octave:13 function add(x,y) x+y endfunction octave:14 add(1,2) ans = 3The output came from the statement

octave:15 function sum=add(x,y) sum=x+y; endfunctionNow, if we type

octave:16 fred=add(1,2) fred=3A very powerful feature of Octave is the ability to return multiple values from a function. This feature exists in Matlab, but not in FORTRAN. For example, the following function:

octave:17 function [sum,diff]=sumdiff(x,y) sum=x+y; diff=x-y; endfunctionreturns multiple values when invoked as:

octave:18 ns,d]=sumdiff(1,2) s = 3 d = -1Functions can be defined at the keyboard, as we did in these examples, or stored in a file and used again. This lets you build a suite of routines for whatever tasks you want. All you have to do is put the files (identified with a

Octave also provides a full programming language, with flow control and looping constructs, as well as extensive input-output facilities. It is possible to write quite sophisticated programs in Octave, and development time is considerably shorter than you would expect using FORTRAN or C.

Although Octave has very strong matrix capabilities, it has many other features as well. For example, it has routines to manipulate polynomials. A polynomial is entered as a vector of coefficients; so, for example, the polynomial x3+3x2+2x-1 can be represented by the vector:

octave:19 mypoly=[1 3 2 -1] mypoly = 1 3 2 -1

We can then differentiate **mypoly** using the
command:

octave:20 polyderiv(mypoly) ans = 3 6 2or integrate it by:

octave:21 polyinteg(mypoly) ans = 0.25000 1.00000 1.00000 -1.00000 0.00000Note that Octave uses zero as the constant of integration. We can also evaluate the polynomial at a given value; thus, to find the value of

octave:22 polyval(mypoly,2) ans = 23If we want the roots of the polynomial, use:

octave:23 roots(mypoly) ans = -1.66236 + 0.56228i -1.66236 - 0.56228i 0.32472 + 0.00000iNote that Octave is quite happy with complex numbers, even though all the examples I've given have been real. There are also routines to convolve and deconvolve polynomials, form companion matrices and characteristic polynomials, and form a partial fraction representation of the quotient of two matrices.

Other features include functions to solve systems of nonlinear equations, solve differential and differential-algebraic equations, perform quadrature and collocation, as well as statistics, control theory, signal processing, image processing and optimization routines. The manual indicates areas where the developers hope to extend Octave's capabilities.

Octave provides graphics capabilities via the Gnuplot program, which has to be obtained separately. The advantage of this is that Octave supports all the output devices Gnuplot supports, including the Linux terminal, which might be of interest if you have a low-memory system.

Octave provides two low-level graphics functions,
**gplot** and **gsplot**, that behave
almost exactly like the Gnuplot functions **plot**
and **splot**, and also provides several
higher-level plotting functions based on the graphics functions
found in Matlab 3.5. Two and three-dimensional plotting commands
are also available.

If you are familiar with Gnuplot, then you will probably appreciate the flexibility offered by access to Gnuplot's commands. However, the higher-level commands offered by Octave are very easy to use and you may find you don't have to use the Gnuplot commands at all.

Octave is a flexible, powerful, easy-to-use, high-level language designed for numerical computations. It comes with a very readable 200+ page user manual, and a help system based around the GNU info system. The main advantage of a high-level language over a language like FORTRAN is that development time is usually considerably shorter using a high-level language. This allows for easy prototyping and experimentation.

Although the documentation doesn't claim that Octave is intended to be a Matlab clone, or Matlab compatible, Octave is probably the most Matlab-like of the freely available high level languages. It's not exactly the same, but I was able to convert a suite of Matlab m-files that perform finite element analysis of the Navier-Stokes equations to Octave very easily.

Octave has many more features than I have described here, but I've provided an overview of its main strengths. If you're looking for a language for numerical work, Octave is certainly an option. I don't think you can directly compare Octave with such languages as RLaB, SciLab and Yorick—they all do different things, and which you choose depends on what you want to do as well as personal preference. My preference is Octave.

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