how do i use the in-built inv function to calculate a matrix inverse and then calculate A-1A and AA-1.

how do i use the in-built inv function to calculate a matrix inverse and then calculate A-1A and AA-1.

[Stephen23]

@David Craven: are you trying to solve some linear equations?

In general matrix inversion a very poor way of performing numeric calculations, and the inv documentation describes better ways of solving equations.

Answer 1

[Cam Salzberger]

Hello David,

Strictly speaking, you would do it like this:

Ainv = inv(A);
Ainv*A
A*Ainv

However, as Stephen pointed out, there are faster and more accurate methods for calculating the inverse when you plan to solve linear equations or include it in some matrix algebra. These two commands are mathematically equivalent, but will be faster and more precise:

A\A   % equivalent to Ainv*A
A/A   % equivalent to A*Ainv

-Cam

[David Craven]

I run this function

[Inverse,Result1,Result2] = matrixinverse(A)
 Inverse = %calculate the inverse of a matrix with the inv function
 Result1 = %calculate Inverse times A
 Result2 = %calculate A times Inverse

Inverse = inv(A);
Result1 = A*Inverse;
Result2 = Inverse*A;

end

and get this

A =

    9.6800    9.6100    2.2700
    2.4100    5.3600    4.7900
    9.7300    8.2000    9.2400

Error: File: matrixinverse.m Line: 2 Column: 70 Expression or statement is incomplete or incorrect.

[Cam Salzberger]

Well, if line 2 of that file is:

Inverse = %calculate the inverse of a matrix with the inv function

then that looks incomplete to me. Maybe you should just remove those lines, and keep it to the other lines only.

-Cam

Answer 2

[James Tursa]

The comment character % needs to be at the beginning of the comments. So these lines

 Inverse = %calculate the inverse of a matrix with the inv function
 Result1 = %calculate Inverse times A
 Result2 = %calculate A times Inverse

should be this instead

 % Inverse = calculate the inverse of a matrix with the inv function
 % Result1 = calculate Inverse times A
 % Result2 = calculate A times Inverse