Flow of Control

Matlab's constructs for controlling the conditional execution of commands are very similar to those found in most popular programming languages. One major difference, however, is that variables created within the if, for, while, switch, and try statements, are not locally scoped but instead share their scope with all variables in the same function. This is quite unlike java, for example, where a variable created inside a loop can only be used inside that loop.

Contents

if, else, elseif

Matlab if statements allow you to execute different code depending on the current state of the program, i.e. the values of certain variables.

test1 = true;
test2 = false;
test3 = false;

if(test1), A = 1; end       % simple if statement on one line. A=1 executed if test1 is true

if(test1)
    A = 2;                  % executed if test1 = true
else
    A = 3;                  % executed if test1 = false
end

if(test1)
    A = 3;                  % executed if test1 = true
elseif(test2)
    A = 4;                  % executed if test1 = false and test2 = true
elseif(test3)
    A = 5;                  % executed if test1 = false, test2 = false, test3 = true
else
    A = 6;                  % executed if test1=test2=test3=false
end

All if statements must end with an end statement.

switch statements

Switch statements are useful when what code to execute depends on a variable that takes on a countable number of values. Most commonly, this value is an integer or a string. Switch statements can be replaced by a long series of if-else statements but this usually results in less readable code. Note that unlike languages such as C or java, switch statements do not fall through; that is, the code from, (at most), one case statement is executed. As such, break statements are not necessary.

color = 'blue';
switch color                    % switching variable
    case 'red'
        A = 1;                  % code for case 'red'
    case 'blue'
        A = 2;
    case {'green','purple'}     % either 'green' or 'purple'
        A = 3;
    otherwise                   % optional 'catch all'
        A = 4;
end

for loops

For loops allow you to execute a block of code a specified number of times. That number can be determined dynamically as the program runs.

n = ceil(100*rand);                 % can be set dynamically
A = zeros(n,1);                     % improve speed by preallocating space
for i=1:n                           % set i = 1, then loop and increment i by 1, until i = n
    A(i,1) = max(i,50);             % execute code within the loop - usually depends on i.
end                                 % both i and A can then be accessed outside the loop.

We can terminate for loops early in several ways. Continue instructs Matlab to skip directly to the next iteration of the current loop without executing the lines directly below the continue command. Break breaks from the current loop completely. Return breaks completely from the current script or function without executing any further code.

A = rand(20,20,20);
counter = 0;
for i=1:size(A,1)
    for j=2:size(A,2)
        for k=3:size(A,3)
            %if k is even, go immediately to beginning of loop
            if(mod(k,2) == 0),          continue; end
            %if j+k is prime, break from this inner loop completely
            if(isprime(j+k)),           break   ; end
            %if all three of i,j,k prime, stop all further execution.
            if(all(isprime([i,j,k])) && false),  return  ; end
            if(isprime(floor(100*A(i,j,k))))
                counter = counter + 1;
            end
        end
    end
end

The continue, break, and return statements should be used sparingly as they can easily obscure the code and can almost always be replaced by if,else,elseif statements.

while loops

While loops are used to execute a block of code until some condition is satisfied. This condition is usually more complicated than simply reaching a set number of iterations as with a for loop. The comments on scope, and the continue, break and return statements apply equally to while loops.

A = true; B = true; C = true;
val = 1;
while(A || B || C)
    val = 2*val +1;
    A = isprime(val);
    B = val < 10;
    C = ((round(sqrt(val)))^2) == val;
end

Here is common code idiom involving break. This effectively allows us to test at the end of the loop, (or in multiple spots).

while(true)
    %execute code
    if(condition)
        break;
    end
end

try/catch statements

Try/catch blocks give you some control over Matlab error handling. They are useful for executing code that might potentially fail, such as writing to a file, allowing you to perform cleanup or recover gracefully. In the example below, ME stands for 'matlab exception'; this is an object of type Exception.

a = rand;
b = a*(a< 0.5);
try
    c = a / b;
    assert(true);                       % set to false to have code throw an error
catch ME                                % disaster recovery, cleanup, inform user, etc...
    display('Something went wrong');
    warning('WARNING:ID','my own warning message');
    display(ME.message);                % ME is a structure with info on the error
    %error('my own error message');     % stops execution
    %rethrow(ME);                       % rethrows the original error and stops execution
end

Published with MATLAB® 7.10