Type checking (class, isa) in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the class function do in MATLAB?

The <code>class</code> function returns the type (class name) of a variable as a string, such as 'double', 'char', or a user-defined class name.

beginner

How does the isa function differ from class in MATLAB?

<code>isa</code> checks if a variable is of a specified class or inherits from it, returning true or false. <code>class</code> returns the exact class name as a string.

beginner

What will isa(x, 'double') return if x is a 3x3 matrix of doubles?

It will return true because x is of class 'double'.

intermediate

If obj is an object of a subclass, what does isa(obj, 'SuperclassName') return?

It returns true if obj inherits from SuperclassName, even if obj is not exactly of that class.

beginner

What is the output of class(42) in MATLAB?

The output is 'double' because numeric literals like 42 are stored as double precision by default.

What does class(x) return in MATLAB?

A The class name of x as a string

B True if x is a double

C The size of x

D The value of x

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

Which function checks if a variable belongs to a class or its subclass?

A class

B isa

C size

D typeof

isa returns true if the variable is of the specified class or inherits from it.

What will isa('hello', 'char') return?

A false

B char

C true

D error

A string like 'hello' is of class 'char', so isa returns true.

If obj is exactly class 'MyClass', what does class(obj) return?

A false

B true

C obj

D 'MyClass'

class returns the exact class name as a string.

Which is true about isa(x, 'double') when x is a double array?

A Returns true

B Returns false

C Returns the size of x

D Returns the class name

isa returns true if x is of class 'double'.

Explain how to check the type of a variable in MATLAB using class and isa.

Think about when you want the exact type name versus when you want to check if it belongs to a class or subclass.

Describe a situation where isa is more useful than class.

Consider object-oriented programming with subclasses.

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