Programming in Matlab

Interpreters vs. Compiliers

Matlab source code is interpreted, although there are ways of compiling it, that is beyond us here. “Interpreted” means that Matlab reads your code (instructions) one line at a time.

Note:

• In Matlab a carriage return means a new line.
• If you want to supress output then put a ";" at the end of the line.
• If you want to continue a line that is greater than the width of your screen then use "..." at the end of the line.
• If you want to put comments in your program you put a "%" symbol before any text and that text will not be interpreted by Matlab (it is just for you to help understand what you did)

Algorithms

Algorithms are a specific set of instructions to solve a problem. So a recipe is an algorithm, directions to the party tonight is an algorithm, and in this general sense an equation is an algorithm such as g=Gm/r^2. So we can determine the acceleration due to gravity g on object by taking the product of the universal gravity constant G and the mass of whatever is causing the acceleration, such as a planet, and dividing by the square of the distance between the planet and the object.

In Excel, you would represent this algorithm in a formula within a cell.

In Matlab you type it pretty much like that above but you must first give values to G, m and r. In Matlab assigning a value is done with the “=” symbol, just as with Excel.

Program Structure

A program is a series of algorithms that allow us to do what we want to do.

In Matlab you edit a text file in the editor with a distinct structure and save it. It will have a .m extension and if you have set your path (with the menu command File > Set Path) then you should be able to just type the name of the file and it will execute as a function just like any other in Matlab. Here is the basic structure of a program as follows:

function argout = myfunction(argin)

% next you list your commands

argout = 2 + argin;

saving this and running the function would add 2 to whatever you passed in as argin. for example:

>> a=5

a = 5

>> b=myfunction(a)

b = 7

Iterations

If you want to repeat a command over and over you can use a "for" loop. For example:

>> a=[2 5; 3 8; 1 9]

a =

2 5
3 8
1 9

>> b=[1 2]

b =

1 2

>> a + b
??? Error using ==> plus
Matrix dimensions must agree.

>> for i=1:size(a,1)
c(i,:) = a(i,:) + b
end

c =

3 7

c =

3 7
4 10

c =

3 7
4 10
2 11

Conditionals

If you want to do something based on whether something else is true, use an “if” statement. This is similar to the "if" function in Excel. For example:

>> b=[1 2]

b =

1 2

>> if b(1)>1
c=b+1
end
>> if b(2)>1
c=b+1
end

c =

2 3

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C. Connors, 2003