CODEWITHSUNDEEP
matrixfortran
Guy Roy
Guy Roy

Reputation: 1

Question about the behavior of the fortran function MATMUL()

I am a newbie in fortran and i have to multiply matrices of different shapes with MATMUL() and the result is not what i expected...

Here is my fortran code:

  integer, dimension(3,2) :: a
  integer, dimension(2,2) :: b
  integer :: i, j

  a = reshape((/ 1, 1, 1, 1, 1, 1 /), shape(a))

  b = MATMUL(a,TRANSPOSE(a))

  do j = 1, 2
     do i = 1, 2
        print*, b(i, j)
     end do
  end do

I expected this matrix as a result:

b =
| 3 3 | , a 2x2 matrix
| 3 3 |

Instead, i got this error message:

matmlt.f90(9): error #6366: The shapes of the array expressions do not conform. [B] b = MATMUL(a,TRANSPOSE(a)) ------^

To make this code work properly i had to switch the MATMUL arguments like this:

b = MATMUL(TRANSPOSE(a), a)

And this way, i obtain what i was expecting at the beginning. But this is not intuitive.

On paper,

a =
| 1 1 1 |
| 1 1 1 |

transpose(a) =
| 1 1 |
| 1 1 |
| 1 1 |

a x transpose(a) =
| 3 3 |
| 3 3 |

and

transpose(a) x a =
| 2 2 2 |
| 2 2 2 |
| 2 2 2 |

What is wrong with my code?

Thank you.

Upvotes: 0

Views: 607

Answers (1)

Max R
Max R

Reputation: 61

your matrix definition for the variable 

     integer, dimension(3,2) :: a

means, that you have 3 rows and 2 cols (different ofyour assumption). Subsequently

a=
|11|
|11|
|11|

and

transpose(a) =
|111|
|111|

matmul(a,transpose(a)) =
|2 2 2|
|2 2 2|
|2 2 2|

so your variable b should defined like

 integer, dimension (3,3) :: b

instead of 

 integer, dimension (2,2) :: b

what is the reason of the

matmlt.f90(9): error #6366: The shapes of the array expressions do not conform. [B] b = MATMUL(a,TRANSPOSE(a)) ------^

Error

Upvotes: 2

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