Type checking (class, isa) in MATLAB

Introduction

Type checking helps you find out what kind of data you have. This is useful to make sure your program works correctly.

When you want to check if a variable is a number or text before doing math.

When you need to confirm if an object belongs to a certain class before calling its methods.

When you want to handle different data types differently in your code.

When debugging to understand what type of data a variable holds.

When writing functions that accept multiple types and you want to act based on the type.

Syntax

MATLAB

class(variable)
isa(variable, 'ClassName')

class returns the exact type name of the variable as a string.

isa checks if the variable is of a certain class or inherits from it, returning true or false.

Examples

This gets the type of x, which will be 'double' for numbers.

MATLAB

x = 5;
className = class(x);

This checks if str is a character array (text), returning true.

MATLAB

str = 'hello';
isChar = isa(str, 'char');

This checks if obj is a datetime object.

MATLAB

obj = datetime('now');
isDateTime = isa(obj, 'datetime');

This checks if A is numeric data (true for arrays of numbers).

MATLAB

A = [1, 2, 3];
isNumeric = isnumeric(A);

Sample Program

This program checks the type of x and str and prints messages accordingly.

MATLAB

x = 10;
if isa(x, 'double')
    disp('x is a double number')
else
    disp('x is not a double number')
end

str = 'hello';
if strcmp(class(str), 'char')
    disp('str is a character array')
else
    disp('str is not a character array')
end

Important Notes

Use class when you want the exact type name as a string.

Use isa when you want to check if a variable belongs to a class or its subclass.

Remember that isa returns a logical true or false, which is useful in conditions.

Summary

class tells you the exact type name of a variable.

isa checks if a variable is of a certain class or inherits from it.

Type checking helps your program handle data safely and correctly.

Practice

1. What does the class function in MATLAB return when used on a variable?

easy

A. True if the variable is numeric

B. The exact type name of the variable

C. The size of the variable

D. The value of the variable

Solution

1. Step 1: Understand the purpose of class

   The class function returns the exact type name of the variable, such as 'double', 'char', or a user-defined class. Other choices describe checks for numeric type, size, or value, but class specifically returns the type name as a string.

2. Final Answer:

   The exact type name of the variable -> Option B

3. Quick Check:

   class = exact type name [OK]

Hint: Remember: class gives type name as text [OK]

Common Mistakes:

• Confusing class with isa function
• Thinking class returns size or value
• Assuming class returns a boolean

2. Which of the following is the correct syntax to check if variable x is of class double using isa?

easy

A. class(x) == 'double'

B. isa('double', x)

C. isa(x, 'double')

D. class(x, 'double')

Solution

1. Step 1: Recall isa function syntax

   The correct syntax is isa(variable, 'classname'), so to check if x is a double, use isa(x, 'double'). Using class(x) == 'double' compares strings unreliably, isa('double', x) reverses arguments, and class(x, 'double') has invalid syntax.

2. Final Answer:

   isa(x, 'double') -> Option C

3. Quick Check:

   isa(variable, 'class') syntax [OK]

Hint: isa(variable, 'classname') checks type safely [OK]

Common Mistakes:

• Swapping arguments in isa
• Using == to compare class strings
• Calling class with two arguments

3. What is the output of the following MATLAB code?

x = 5; y = isa(x, 'double'); disp(y);

medium

A. 'double'

B. 0

C. Error

D. 1

Solution

1. Step 1: Trace code execution

   Variable x is assigned 5, which defaults to class double in MATLAB. isa(x, 'double') returns logical true (displayed as 1), and disp(y) outputs 1.

2. Final Answer:

   1 -> Option D

3. Quick Check:

   isa(5, 'double') = 1 [OK]

Hint: Numeric literals are double by default [OK]

Common Mistakes:

• Expecting 0 because 5 looks like int
• Confusing output with class function
• Thinking isa returns string

4. Identify the error in this MATLAB code snippet:

if isa('double', x)
    disp('x is double')
end

medium

A. Arguments of isa are reversed

B. Missing semicolon after disp

C. isa cannot be used inside if

D. No error, code is correct

Solution

1. Step 1: Check isa function argument order

   The correct order is isa(variable, 'classname'), but here arguments are reversed to isa('double', x). A missing semicolon after disp or using isa in if are not errors.

2. Final Answer:

   Arguments of isa are reversed -> Option A

3. Quick Check:

   isa(variable, 'class') order matters [OK]

Hint: Always put variable first in isa() [OK]

Common Mistakes:

• Swapping arguments in isa
• Thinking missing semicolon causes error
• Believing isa can't be used in if

5. You want to write a MATLAB function that accepts either a numeric array or a table. Which code snippet correctly uses isa to check the input type?

hard

A. if isa(input, 'double') || isa(input, 'table')

B. if isa(input, 'array') || isa(input, 'table')

C. if class(input) == 'numeric' || class(input) == 'table'

D. if isa(input, 'numeric') || isa(input, 'table')

Solution

1. Step 1: Identify correct class names and syntax

   In MATLAB, numeric arrays usually have class 'double', not 'numeric' or 'array'. if isa(input, 'double') || isa(input, 'table') correctly checks either using proper isa syntax and logical OR.

2. Final Answer:

   if isa(input, 'double') || isa(input, 'table') -> Option A

3. Quick Check:

   Use isa with correct class names [OK]

Hint: Use isa with exact MATLAB class names [OK]

Common Mistakes:

• Using 'numeric' instead of 'double'
• Using class() with == for strings
• Assuming 'array' is a class name