Fortran 90 data statement does not overwrite data

Question

I am trying to initialize an array using DATA statement in Fortran 90. The code is following:

PROGRAM dt_state
IMPLICIT NONE
INTEGER :: a(4), b(2:2), c(10)

DATA a/4*0/
WRITE (6,*) a(:)    

DATA a/4,3,2,1/
WRITE (6,*) a(:)
END PROGRAM dt_state

I expected that results on the screen will be 0 0 0 0 and 4 3 2 1. However, what I got is 0 0 0 0 and 0 0 0 0. It means DATA statement does not overwrite the values of a, does it?

Answer 1

A variable can appear in a DATA statement only once. The DATA statement is for initialization, which is done only once at program start.

In executable code use assignment to set array values

a = (/ 4, 3, 2, 1 /)

(in Fortran 90)

or

a = [ 4, 3, 2, 1 ]

(in Fortran 2003).

It is better to use this syntax also for initialization.

Answer 2

Your code is not standard-compliant. That is: from F2008 5.2.3:

A variable, or part of a variable, shall not be explicitly initialized more than once in a program.

The DATA statement (is one thing that) performs such explicit initialization (5.4.7.1), and so in particular two cannot appear for the same variable.

For your second "initialization", use assignment. [As given by @VladimirF who is a faster typist than I.] Further, while one can put a DATA statement with executable statements, as in this case, the standard goes as far as making that obsolescent (B.2.5):

The ability to position DATA statements amongst executable statements is very rarely used, unnecessary, and a potential source of error.

As the code is non-standard, and the error is not one the compiler is required to detect, the compiler is free to whatever it likes with the code. I thought it would be interesting to see what we do see with a small selection:

• One refused to compile (and also pointed out the obsolesence);
• Two quietly went ahead, using the first initialization;
• One used the second initialization, warning.

Of course, one wouldn't want to rely on any of these behaviours, even if it was a desired one.