How to print only non-zeros of a matrix?

Question

I have a matrix, but I am trying not to print the 0 values in the matrix. I wrote the logic but it is not working.

do i=1,42

if (massmat(i,j).ne.0) then

write(20,*)i,(massmat(i,j),j=1,42) 

end if  

end do

It still prints out all the zeros and all the values. Can someone please help?

Answer 1

You could write

do i = 1, 42
   write(20,*) i, pack(massmat(i,:), massmat(i,:)/=0)
end do

This will loop over the rows of massmat and print all the non-0 elements in each. I'll leave you to consult your Fortran documentation for details of the pack function.

Answer 2

Let's read through the main do loop to see what's going wrong:

do i=1,42

This will loop over all the columns of the matrix, as you suggested in the comments above.

if (massmat(i,j).ne.0) then

This line tests whether massmat(i,j) is strictly equal to 0. But the value of j has not yet been set. In order to test all the elements of the matrix, there should be another do statement above that loops over all values of j.

write(20,*)i,(massmat(i,j),j=1,42)

This line writes a single index i and an array, which is a column of the matrix. So if the element massmat(i,j) is not 0, the entire column containing massmat(i,j) is written, even though it may contain elements which are zero. Instead, this line should probably write i, j, and a single matrix element massmat(i,j), so that only the element which has just been tested will be written.

Answer 3

write (20,*) pack(massmat,massmat /= 0) 

prints the nonzero elements.