In FORTRAN, what is the use of FMT without parameters?

Question

I check an old FORTRAN 77 program.
It has an FMT character array, lets say:

CHARACTER*4 FMT(4)

which gets some elements like:

/'9,','1X','A','0'/ ...etc...  

During execution, FMT elements might get different values e.g.:

FMT(4) = '3x'

I m confused because the WRITE statement is like this

WRITE (xOUT,FMT) 'Result: ',(trek(j),j=1,Nev)

The FMT is an array. Right ? Not a string.
So what FORMAT exactly will the WRITE read from FMT ?
Any help is much appreciated.

Answer 1

The variable FMT is a character array. When an array is used as the format of an input/output statement, it is treated as though (Fortran 2018, 13.2.2 p3) each element is specified and concatenated. So in this case

write(xOUT,FMT) ...

is interpreted like

write(xOUT,FMT(1)//FMT(2)//FMT(3)//FMT(4)) ...

Which is unfortunate, because FMT(1) must have a ( as its first non-blank character.

Outside a character edit descriptor blanks in a format are not significant.

---

This, although it initially seems odd, is a handy way to create dynamic formats. Consider an extreme example where we want to write a string used as a format which could be (I5.5) or (F26.14).

This could be:

character(6) fmt(3)
fmt(1) = '('
fmt(3) = ')'

if (something) then
  fmt(2) = 'I5.5'
  print fmt, 113
else
  fmt(2) = 'F26.14'
  print fmt, 123.56
end if

(Yes, you wouldn't do this, I hope, but one can easily extend this to situations where it does seem appealing.)

Compare with

character(8) fmt
fmt = '(      )'

if (something) then
  fmt(2:7) = 'I5.5'
  print fmt, 113
else
  fmt(2:7) = 'F26.14'
  print fmt, 123.56
end if

to see what the author was trying to avoid.