5.3: Conditions and Decision Making

A branch starts with a condition. A condition is an expression that MATLAB can evaluate as either true or false. For example, the condition x > 0 asks, "Is x greater than zero?" If the answer is yes, MATLAB treats the condition as true. If the answer is no, MATLAB treats the condition as false.

 

Relational Operators

Relational operators compare values. They are called relational because they describe the relationship between two quantities.

Operator	Meaning	Example
<	less than	x < 10
>	greater than	x > 0
<=	less than or equal to	score <= 100
>=	greater than or equal to	age >= 18
==	equal to	choice == 1
~=	not equal to	answer ~= 0

Notice the difference between = and ==. A single equals sign assigns a value. A double equals sign compares two values.

Caution

Do not write if x = 5. That tries to assign 5 to x, which is not a valid condition in an if statement. To ask whether x is equal to 5, write if x == 5.

 

 

Example \(\PageIndex{1}\)

Using relational operators.

Run the following code. Then change the value of x to 6 and run the code again and observe the displayed message.

x = 5;

% This asks whether x is equal to 5
if x == 5
	disp('x is equal to 5')
end

% This asks whether x is not equal to 5
if x ~= 5
	disp('x is not equal to 5')
end

Solution

x is equal to 5

For x = 6, the displayed message will be : x is not equal to 5

 

Logical Operators

Sometimes one condition is not enough. You may need to check two or more conditions at the same time. MATLAB uses logical operators to combine relational expressions.

Operator	Name	Meaning	Example
&	and	true only if both sides are true	x > 0 & x < 10
|	or	true if at least one side is true	x < 0 | x > 100
~	not	reverses true and false	~isnan(x)

For scalar conditions, you will also often see && and ||. These are short-circuit versions of AND and OR. In beginning MATLAB code, using && and || is often a good habit when your conditions each produce one true/false value.

 

Example \(\PageIndex{2}\)

Using logical operators.

% Prompt the user to enter a number for the diameter of a circle
diam = input('Please enter a diameter for the circule: ');

if diam > 0 && diam <= 10
	area = pi * (diam/2)^2;
	fprintf('The area is %.2f.\n', area);
end

 

Solution

If the input value is 4, for example, then the message will be: The area is 12.57.

 

 

Operator Precedence

When a condition contains several operators, MATLAB follows operator precedence rules. Parentheses are evaluated first, then logical NOT (~), then AND, and then OR. When expressions become long, parentheses make your code easier to read and help prevent mistakes.

 

For example compare the following codes that check for the same conditions. Which version is easier to read?

Version 1:

if diam > 0 && diam <= 10 && diam == round(diam)
    disp('Valid diameter')
end

Version 2:

if (diam > 0) && (diam <= 10) && (diam == round(diam))
    disp('Valid diameter')
end