Edited Version at A18532488.

MATLAB, short for Matrix Laboratory, is a 4th generation¹ programming language that specialises in mathematical computing², but can also be expanded to include functions that allow image processing, distributed and parallel computing, and various other applications. Its main use is the manipulation of matrices³ in a great many ways, with the underlying language of MATLAB being fine-tuned for this purpose. The first version was written in Fortran by Cleve Moler at the University of New Mexico, but was rewritten in C after the language gained acceptance in various fields of applied mathematics. Together with Jack Little and Steve Bangert, Moler founded The MathWorks in 1984, with MATLAB having since become widely used in teaching and research as well as in commercial areas. MATLAB is now available for Windows, Unix, Linux and Mac OS X.

The Basics

Compared to other programming languages, MATLAB is a lot less scary to the newcomer, this being one of the reasons why it is used for teaching. There is no need to declare variables before using them, and most mathematical expressions⁴ don't require much fiddling with to turn them into something MATLAB can understand. As well as allowing the creation and manipulation of vectors and matrices of various descriptions, the language also includes programming favourites such as 'if', 'switch⁵', 'for' and 'while', allowing the creation of innocent-looking scripts which can take hours on end to execute. The following is a MATLAB function⁶ that follows the rules of the 3n+1 Conjecture, which would be run by saving it as threenplusone.m and then typing threenplusone(χ) at the command line, where χ would be the number you wanted to test:

function [result] = threenplusone(input)

while input ~= 1

  if rem((input),2) == 1 % if 'input' is odd⁷

    input = (input.*3)+1; % multiply by 3, add 1

    disp(input)

  end

  while rem(input,2) == 0 % while 'input' is even⁸

    input = input./2; % divide by 2

    disp(input)

  end

end % will return to start of loop unless input = 1

result = input;

disp(result)

disp('Success!');

Running the program displays each value of 'input' followed by a notification that 'input' has reached 1, assuming that this is possible⁹. If it's not, then you'll either have to press Ctrl + Break, which will halt execution of the function, or wait an infinitely long time for the program to stop. For convenience, the above lacks any way of catching numbers that aren't positive integers, but we're assuming the user has the sense not to make the input anything except for a positive integer. The function could be improved to run faster but has been kept simple for the purposes of the Entry, and could also be expanded to accept a whole matrix of numbers, returning a matrix containing the result of each implementation of the main code.

Notice that the multiply (.*) and divide (./) symbols are preceded by dots - this is to indicate simple multiplication and division as opposed to the more complicated operations often used on matrices¹⁰ The '%' symbol indicates that the rest of that line is a comment which will MATLAB will ignore, while the ';' at the end of some lines stops MATLAB from displaying the result of the calculation in the command window. The latter is very useful when dealing with large matrices, as displaying thousands of columns of numbers in a little window is very time consuming and also rather messy. However, the ';' symbols could be removed in this example, with the command window then displaying each value of 'input' produced while running the program. Finally, it should be noted that '~=' stands for 'not equal to'.

Vectorisation

MATLAB is designed to work with vectors¹¹ as opposed to loops of code which repeat the same operation on a series of scalars¹². This means that a script which makes use of loops is much slower than the equivalent 'vectorised' script, which puts all the numbers to be worked on into a single list rather than working on them one by one. This is due to the fact that MATLAB can easily work on a whole vector at once without interruption, but is forced to cycle through logical statements and other operations if the code uses a loop. The following is an example of a looped script and a vectorised script that do the same thing:

A = [1 4 9 16 25];

for x = 1:5

  A(x) = realsqrt(A(x));

end

disp(A)

A = [1 4 9 16 25];

A(1:5) = realsqrt(A(1:5));

disp(A)

Both scripts find and display the square root of each number in the vector 'A', but the latter is much quicker. A script which squares and then square roots the numbers between 1 and 1,000,000 takes around ten times longer to run in looped form, and so a vectorised script is more ideal than one which contains loops, with the act of rewriting a script thus being known as vectorisation. The run-time can be measured using the 'tic' and 'toc' functions, which measure the time taken for the section of a program in-between them to run. One important part of vectorisation is the colon operator ':', with '(1:5)' indicating that the square root should be found for columns one to five of 'A' and then used to replace the values that were there before.

More Complicated Stuff

MATLAB also includes commands to allow programmers to use MATLAB data and functions in programs written in C and Fortran so that they can get their fix of object-oriented programming, and the basic package also includes GUI¹³ development software for the production of colourful little programs with clickable buttons which make functions produced in MATLAB more accessible to lay users. While the core of MATLAB contains many hundreds of commands and functions, 'toolboxes' can be added such as the distributed computing toolbox, which allows code to be split up and run on several processors at the same time so as to increase the running speed of software.

Problems

One particular problem with MATLAB is that licenses for it are quite costly, with the language being subject to copyright and being different from alternatives such as GNU Octave in many subtle ways. This means that once a long and complicated program has been written in MATLAB, the costs in time and money of switching to a different supplier and thus another programming language can be prohibitive. Also, MATLAB has had several versions, some of which are not entirely compatible. On the programming side, MATLAB can be irritating due to the fact that all matrices start with row 1, column 1, so that all references to a matrix have to be made using positive integers. For instance, if a matrix represented data from -100 to +100 on the x and y axes, the programmer would need to store each value in the column and row of the matrix equal to (value on x axis) + 101 and (value on y axis) + 101 respectively.

MATLAB Tips

• Make the layout of your program clear by using indents in the same manner as the example above, and use lots of comments to make clear notes on what you're doing.

• Use the Matlab Editor when writing code - it uses various colours for different parts of the code, helps you to pair up brackets, and automatically applies indents after 'for', 'if', 'while' etc.

• Add checks to ensure that your code is running properly, and add error messages that will display if something goes wrong.

• Start off by getting the small yet important parts of your program to work, then aim bigger. Learn to use the Matlab Editor's cell mode, which allows you to run small sections of code on their own straight from the editing window.

• Make use of the built-in functions, as they have been tried and tested and will run quite quickly without many problems. Ensure you know the syntax of the functions you are using so as to avoid wasting time while looking for a missed comma.

• Don't be scared by the number of commands available in MATLAB - help is available for each one and there's often a better way of doing something, provided you know how.

• If run-time is important, try to vectorise (see above) the section of the program which takes the most time.

Also, have a look at Seven Secrets of Successful Programmers.

- Alex Ashman