[SOLVED] Fortran do loop variables: restrictions on control variable memberships?
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Fortran do loop variables: restrictions on control variable memberships?
Is there a restriction that the control variable of a DO loop cannot be an element of an array? For example, the following code does not compile on both compilers I've checked:
Code:
program loop
implicit none
integer :: n(2)
do n(1) = 1, 10
do n(2) = 1, 20
end do
end do
end program
However, the following code DOES compile (which makes very little sense):
Code:
program loop
implicit none
integer :: n(2)
associate ( m=>n(1), p=>n(2) )
do m = 1, 10
do p = 1, 20
end do
end do
end associate
end program
Clearly these two loops are precisely the same thing in memory. All ASSOCIATE does is create another name for the same variable. Is there a rule somewhere in the standard that says control variables for DO loops must be declared scalars and not part of a larger object (except, evidently, through association)?
I think it is a grammatical restriction. If you take a look at the source code of `f2c' you will find that control variable should be a `name' (excerpt from src/gram.exec):
Finally SNAME is a lexer symbol, i.e. a word in your program. So loop counter can only be a variable, at least in f2c.
In theory this restriction could be easily thrown, but for some reasons it is not. Semantically, using an array element as loop counter makes sense and in some languages (in C for example) is allowed.
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