Question

In Fortran, is it possible for declaration statements for variables to refer to previously-declared variables? For example, when I try the following:

PROGRAM test3
  IMPLICIT NONE

  INTEGER :: a=2286
  INTEGER :: b=a/3

  WRITE(*,*) a, b
END PROGRAM test3

I get a compile-time error message:

test3.f90:5.16:

  INTEGER :: b=a/3
                1
Error: Parameter 'a' at (1) has not been declared or is a variable, which
does not reduce to a constant expression

On the other hand, if I assign b to a/2 in a statement separate from the declaration of b, it compiles and runs fine:

PROGRAM test3
  IMPLICIT NONE

  INTEGER :: a=2286
  INTEGER :: b
  b=a/3

  WRITE(*,*) a, b
END PROGRAM test3

which gives me the correct output:

2286         762

Why is this the case–that previously-declared variables cannot be included in declaration statements of new variables? Am I doing something wrong? Or is this just a “Fortran fact of life”?

Thank you very much for your time!

Answer 1

The error message is pretty explicit. Initializers used in variable declarations have to be constant values. In your example, a is not a constant.

It should work like this:

PROGRAM test3
  IMPLICIT NONE

  INTEGER, PARAMETER :: a=2286
  INTEGER :: b=a/3

  WRITE(*,*) a, b
END PROGRAM test3

because then a is a constant.