read-files-and-write-on-files.md

Read and write on files

To read or write on a file in Fortran one must first open it. To open a file name mytextfile.txt one can write

open(unit=10, file="mytextfile.txt", status = new/old, action = read/write)

Only the first two argument (unit and file) are mandatory. You can choose any integer for unit (it can also be a variable integer), as long as it is not used by another file (for example mytextfile_2.txt).

After working on the file, you need to close it with the command

close(10)

Reading

Fortran will read line by line, so if your file is a data file with one observation per line and 5 variables, say, you can get this data into a matrix in the following way

do j=1,15  
   read(10,*) var1(j) var2(j) var3(j) var4(j) var5(j)  
end do

Writing

To write in a file, if you have a matrix with N lines with an identifier and a result (scalar) you can write

do j=1,N
  write(10,*) (matresult(j,i),i=1,2)
end do