How to Use eye Function in MATLAB: Syntax and Examples

In MATLAB, use the eye function to create an identity matrix, which is a square matrix with ones on the main diagonal and zeros elsewhere. You can specify the size by passing one or two arguments, like eye(n) for an n-by-n matrix or eye(m,n) for an m-by-n matrix.

Syntax

The eye function creates identity matrices. Here are the common ways to use it:

• eye(n): Creates an n-by-n identity matrix.
• eye(m,n): Creates an m-by-n matrix with ones on the main diagonal and zeros elsewhere.
• eye(size(A)): Creates an identity matrix with the same size as matrix A.

matlab

I1 = eye(3);
I2 = eye(2,4);
A = [1 2; 3 4];
I3 = eye(size(A));

Output

[
     1     0     0
     0     1     0
     0     0     1
]

[
     1     0     0     0
     0     1     0     0
]

[
     1     0
     0     1
]

Example

This example shows how to create a 4-by-4 identity matrix and a 3-by-5 matrix with ones on the diagonal using eye. It also demonstrates using eye with the size of an existing matrix.

matlab

I4 = eye(4);
I5 = eye(3,5);
B = [5 6 7; 8 9 10];
I6 = eye(size(B));

I4
I5
I6

Output

[
     1     0     0     0
     0     1     0     0
     0     0     1     0
     0     0     0     1
]

[
     1     0     0     0     0
     0     1     0     0     0
     0     0     1     0     0
]

[
     1     0     0
     0     1     0
]

Common Pitfalls

Common mistakes when using eye include:

• Passing non-integer or negative values as size arguments, which causes errors.
• Confusing eye with ones or zeros, which create matrices filled entirely with ones or zeros.
• Expecting eye to create a matrix with ones off the main diagonal (it only sets ones on the main diagonal).

matlab

wrong = eye(3.5); % Incorrect: size must be integer
correct = eye(3); % Correct

wrong2 = eye(-2); % Incorrect: size cannot be negative
correct2 = eye(2); % Correct

Output

Error using eye
Size inputs must be positive integers.

Quick Reference

Usage	Description
eye(n)	Create n-by-n identity matrix
eye(m,n)	Create m-by-n matrix with ones on main diagonal
eye(size(A))	Create identity matrix same size as A

Key Takeaways

Use eye(n) to create an n-by-n identity matrix with ones on the diagonal.

eye(m,n) creates a rectangular matrix with ones on the diagonal and zeros elsewhere.

Size arguments must be positive integers; decimals or negatives cause errors.

eye creates identity matrices, not matrices filled entirely with ones or zeros.

You can use eye(size(A)) to match the size of an existing matrix A.