How to write Fortran Output as CSV file?

Question

Can any one tell me, how can I write my output of Fortran program in CSV format? So I can open the CSV file in Excel for plotting data.

Answer 1

I thought a full simple example without any other library might help. I assume you are working with matrices, since you want to plot from Excel (in any case it should be easy to extend the example).

tl;dr

Print one row at a time in a loop using the format format(1x, *(g0, ", "))

Full story

The purpose of the code below is to write in CSV format (that you can easily import in Excel) a (3x4) matrix. The important line is the one labeled 101. It sets the format.

program testcsv
   IMPLICIT NONE 

   INTEGER :: i, nrow
   REAL, DIMENSION(3,4) :: matrix

   ! Create a sample matrix
   matrix = RESHAPE(source = (/1,2,3,4,5,6,7,8,9,10,11,12/), &
                    shape = (/ 3, 4 /))

   ! Store the number of rows
   nrow = SIZE(matrix, 1) 

   ! Formatting for CSV
   101 format(1x, *(g0, ", ")) 
   
   ! Open connection (i.e. create file where to write)
   OPEN(unit = 10, access = "sequential", action = "write", &
        status = "replace", file = "data.csv", form = "formatted") 
   ! Loop across rows
   do i=1,3
      WRITE(10, 101) matrix(i,:)
   end do 
   ! Close connection
   CLOSE(10)


end program testcsv

We first create the sample matrix. Then store the number of rows in the variable nrow (this is useful when you are not sure of the matrix's dimension beforehand). Skip a second the format statement. What we do next is to open (create or replace) the CSV file, names data.csv. Then we loop over the rows (do statement) of the matrix to write a row at a time (write statement) in the CSV file; rows will be appended one after another.

In more details how the write statement works is: WRITE(U,FMT) WHAT. We write "what" (the i-th row of the matrix: matrix(i,:)), to connection U (the one we created with the open statement), formatting the WHAT according to FMT.

Note that in the example FMT=101, and 101 is the label of our format statement:

format(1x, *(g0, ", ")) 

what this does is: "1x" insert a white space at the beginning of the row; the "*" is used for unlimited format repetition, which means that the format in the following parentheses is repeated for all the data left in the object we are printing (i.e. all elements in the matrix's row). Thus, each row number is formatted as: 'g0, ", "'. g is a general format descriptor that handles floats as well as characters, logicals and integers; the trailing 0 basically means: "use the least amount of space needed to contain the object to be formatted" (avoids unnecessary spaces). Then, after the formatted number, we require the comma plus a space: **", ". This produces our comma-separated values for a row of the matrix (you can use other separators instead of "," if you need). We repeat for every row and that's it.

(The spaces in the format are not really needed, thus one could use format(*(g0,","))

Reference: Metcalf, M., Reid, J., & Cohen, M. (2018). Modern Fortran Explained: Incorporating Fortran 2018. Oxford University Press.