Why Does My Matrix Vector Multiplication in FORTRAN Not Work as Expected?

[Milentije]

I have a problem with something that should be very simple,I do not no if it is programming issue or my ignorance.
If I have matrix c(5,5)
3 0 0 0 0
1 2 0 0 0
0 1 2 0 0
0 0 1 4 1
0 0 0 1 2
and vector b(1,2,3,4,5)

c*b=a

a=(3,5,8,24,14)
FORTRAN MATMUL
C=MATMUL(B,A)

Correct?

 

[CompuChip]

a=(3,5,8,24,14)

That is correct.

C=MATMUL(B,A)
I don't know Fortran at all, but here you are multiplying B and A to get C. This is different from what you said earlier (multiplying C and B to get A). I don't know that MATMUL does with two vectors, but you will either get a number (the inner product) or a matrix (but not the one you expect).

Did you mix up your naming, or ... ?

 

[SteamKing]

Here is information on the intrinsic function MATMUL:

http://h21007.www2.hp.com/portal/download/files/unprot/fortran/docs/lrm/lrm0285.htm

BTW, if you have knowledge, you 'know' something. 'no' means negation or the absence of something.

 

[HallsofIvy]

I do not no if it is programming issue or my ignorance

'no' means negation or the absence of something.

Perhaps here he used it correctly!

 

[blue_raver22]

I understand your frustration with this issue. Matrix vector multiplication can be confusing, especially when dealing with larger matrices and vectors. It is possible that there could be a programming issue, but it is also important to make sure you understand the mathematical principles behind matrix vector multiplication.

In your example, the matrix c is a 5x5 matrix, meaning it has 5 rows and 5 columns. The vector b is a 5x1 vector, meaning it has 5 rows and 1 column. In order to perform matrix vector multiplication, the number of columns in the matrix must equal the number of rows in the vector. In this case, c has 5 columns and b has 5 rows, so the multiplication is possible.

To perform the multiplication, you need to multiply each element in the first row of c by the corresponding element in b, and then add them together. This will give you the first element in the resulting vector a. Repeat this process for each row in c, and you will have your final vector a.

In terms of programming, you may need to use a specific function or syntax to perform matrix vector multiplication. The FORTRAN MATMUL function you mentioned appears to be a valid way to perform this operation, but it is always important to double check your code to make sure it is working correctly.

I hope this explanation helps to clarify the process of matrix vector multiplication. It is always important to understand the underlying principles in order to troubleshoot any issues that may arise. If you continue to have difficulty, it may be helpful to consult with a colleague or a programming expert for assistance.