1 For Loops

A for loop will run a piece of code again and again for a certain number of iterations:

% Add up all the number from 1 to 100
running_total = 0;
for k = 1:100
    running_total = running_total + k;
end

disp(['The total is ', num2str(running_total)])

## The total is 5050

2 While Loops

A while loop will to continue to run while a certain condition is met. In this example, the cosine of an angle is calculated using the Taylor series with additional terms being added until Octave runs out of precision (ie until the new term does not change the value stored in Octave’s memory due to it being too small):

% Calculates cosine of x using the Taylor series
% cos(x) = 1 - x^2/2! + x^4/4! - x^6/6! + x^8/8! - x^10/10! + ...

% Angle in degrees
xdeg = 4;

% Convert to radians
x = pi * xdeg / 180;
% Initialise counter
k = 0;
% Initialise stopping conditions
oldsum = -1;
newsum = 0;

while newsum ~= oldsum
    k2 = 2 * k;
    oldsum = newsum;
    newsum = newsum + (-1)^k * x^(k2)/factorial(k2);
    k = k + 1;
end

disp(['The cosine of ', num2str(xdeg), '° converges to ', num2str(newsum)])
disp(['using ', num2str(k - 1), ' terms of the Taylor series.'])
disp('')
cosx = cos(x);
disp(['The exact solution is ', num2str(cosx), '.'])

## The cosine of 4° converges to 0.99756
## using 5 terms of the Taylor series.
## 
## The exact solution is 0.99756.

If you accidentally write a while loop that runs forever, press Ctrl+C to cancel it.

3 If Statements

Code that follows an if statement will only run if the statement is true:

if true
    disp('This code will run')
end

if false
    disp('This code will not run')
end

## This code will run

3.1 Relational Operators

You can use relational operators to construct statements:

< : less than
<= : less than or equal
> : greater than
>= : greater than or equal
== : equal
~= : not equal

if 1 == 1
    disp('This code will run')
end

if 10 > 20
    disp('This code will not run')
end

## This code will run

3.2 Logical Operators

Multiple logical statements can be joined together by using logical operators:

& : and
| : or
~ : not

if (1 == 1) & (10 < 20)
    disp('This code will run')
end

if (1 != 1) | (10 > 20)
    disp('This code will not run')
end

## This code will run

3.3 Elseif and Else statements

An if statement can be followed by one or more elseif statements and these can be followed by an else statement:

If the original if statement is true, it will run
Else if the original statement is false and the first elseif statement is true, then only the first elseif statement will run
Else if the original if statement and the first elseif statement are both false and the second elseif statement is true, then only the second elseif statement will run
This logic goes on for each elseif statement
If all the if and elseif statements are false, then the else statement will run

% Enter a person's height
height = 1.75  % m

if height > 1.96
    disp('You are taller than Stephen Fry')
elseif height > 1.77
    disp('You are taller than Charlize Theron')
elseif height > 1.64
    disp('You are taller than Daniel Radcliffe')
elseif height > 1.47
    disp('You are taller than Danny DeVito')
else
    disp('You are very short!')
end

## height = 1.7500
## You are taller than Daniel Radcliffe

4 Breaking out of a Loop

The break command will break out of a loop (either a for loop or a while loop) if it is run. It can be put inside an if statement to allow the loop to run until the condition is met:

% Escape from a velociraptor

% Your initial position
x_you = 2  % m
% Your speed
v_you = 5;  % m/s
% The velociraptor's initial position
x_raptor = 0  % m
% Velociraptor's speed
v_raptor = 3;  % m/s

while true
    % Check for an outcome
    if x_you > x_raptor + 20
        disp('You escaped the velociraptor!')
        break
    end
    if x_you <= x_raptor
        disp('The velociraptor caught you!')
        break
    end

    % Update the situation
    x_you = x_you + v_you
    x_raptor = x_raptor + v_raptor
end

## x_you = 2
## x_raptor = 0
## x_you = 7
## x_raptor = 3
## x_you = 12
## x_raptor = 6
## x_you = 17
## x_raptor = 9
## x_you = 22
## x_raptor = 12
## x_you = 27
## x_raptor = 15
## x_you = 32
## x_raptor = 18
## x_you = 37
## x_raptor = 21
## x_you = 42
## x_raptor = 24
## x_you = 47
## x_raptor = 27
## x_you = 52
## x_raptor = 30
## You escaped the velociraptor!

⇦ Back

Introduction to Octave:Loops and If Statements