isdiag

Determine if matrix is diagonal

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Syntax

tf = isdiag(A)

Description

tf = isdiag(A) returns logical 1 (true) if A is a diagonal matrix. Otherwise, it returns logical 0 (false).

example

Examples

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Open Live Script

Create a 4-by-4 identity matrix. 

I = eye(4)
I = 4×4

     1     0     0     0
     0     1     0     0
     0     0     1     0
     0     0     0     1

Test if the matrix is diagonal.

isdiag(I)
ans = logical
   1

The matrix is diagonal because all of the nonzero elements are on the main diagonal.

Open Live Script

Create a matrix with nonzero elements on the main and first diagonals.

A = 3*eye(4) + diag([2 2 2],1)
A = 4×4

     3     2     0     0
     0     3     2     0
     0     0     3     2
     0     0     0     3

Test if the matrix is diagonal.

isdiag(A)
ans = logical
   0

The matrix is not diagonal because there are nonzero elements above the main diagonal.

Create a new matrix, B, from the main diagonal elements of A.

B = diag(diag(A));

Test if B is a diagonal matrix.

isdiag(B)
ans = logical
   1

The matrix is diagonal because there are no nonzero elements above or below the main diagonal.

Input Arguments

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Input array. isdiag returns logical 0 (false) if A has more than two dimensions.

Data Types: single | double | logical
Complex Number Support: Yes

More About

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Tips

  • Use the diag function to produce diagonal matrices for which isdiag returns logical 1 (true).

  • The functions isdiag, istriu, and istril are special cases of the function isbanded, which can perform all of the same tests with suitably defined upper and lower bandwidths. For example, isdiag(A) == isbanded(A,0,0).

Extended Capabilities

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C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2014a

See Also

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