How to write matrix row by row with implied do loop?

Question

My IDE is CodeBlocks with MinGW 9.2.0.

I need a help with writing matrix row by row in textual file.

This is my simple code:

program matrix
 
 integer :: i, j 
 integer, dimension(2,2) :: a_mat
 
 forall( i = 1 : 2, j = 1, 2 )
    
   a_mat(i,j) = i + j

 end forall 
 
 open( unit = 15, file = 'matrix_utput.txt', action = 'write' )
  
   write(15,'(*(i2.2,1x))') ( ( a_mat(i,j), j = 1, 2 ), i = 1, 2 )
 
 close( unit = 15 )

end program matrix

In my .txt file i got this: 02 03 03 04 How to change format to get this:

02 03

03 04

The linked question is the opposite problem, but the solution is the same, you loop over loop using a normal do loop and us the implied loop for the individual rows. — Vladimir F Героям слава, Dec 10 '20 at 21:24
Yes, with format reversion. But it is less clear and you must insert the size into the format - when it is not fixed. Maybe we can reopen the question and answer about that. — Vladimir F Героям слава, Dec 10 '20 at 21:54
I spent a lot of time lookong for duplicates but missed this one. It was quite a specific problem so the answer was also quite specific. I will keep my answer undeleted because there is a small difference in reading vs. writing. — Vladimir F Героям слава, Dec 11 '20 at 08:42

score 1 · Accepted Answer · answered Dec 10 '20 at 21:56

1

The normal way is to loop in a do loop line by line as shown at Write matrix with Fortran

But one can also do it in an implied loop using format reversion.

 write(15,'(2(i2.2,1x))') ( ( a_mat(i,j), j = 1, 2 ), i = 1, 2 )

Basically, the number in front of the parenthesis must be the row length (number of columns). If you put the * there, it will consume the whole array. This way, it will open a new record each time the format found all its items.

answered Dec 10 '20 at 21:56

Vladimir F Героям слава

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One can also get rid of the trailing space, but I assumed it is not necessary. – Vladimir F Героям слава Dec 10 '20 at 22:02
So, just to conclude: this `'(*(1x,i2.2,:))'` can not be used for a matrix with any numbers of columns and the only way is to put the number of columns into character variable and use `//` to create `'(987(1x.i2.3))'. Is that correct?` – Harry Kastorp Dec 10 '20 at 23:27
If you can use an explicit loop instead of implied, you can do this with advance=no, an empty write to trigger a linefeed and atomic writes see this https://stackoverflow.com/q/25714322/12150218 – Cocofalco Dec 11 '20 at 00:13
@Cocofalco There is no need for such things with an explicit loop as the duplicate link and the link in the answer show.. – Vladimir F Героям слава Dec 11 '20 at 08:23
1

@HarryKastorp Yes, or just use the normal do loop as people normally use. – Vladimir F Героям слава Dec 11 '20 at 08:24
@VladimirF I was just pointing out that - No - using a computed format statement is not the only way to do this – Cocofalco Dec 13 '20 at 02:02

How to write matrix row by row with implied do loop?

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