Dynamic allocation of string arrays in Fortran does not resize

Question

Consider the following Fortran program:

program test
character(len=:), allocatable :: str
allocate(character(3) :: str)
print *, len(str)
str = '12345'
print *, len(str)
end program

When I run this I get the expected result:

   3
   5

That is, the character string was resized from 3 to 5 when str was set to '12345'. If, instead, I use an array of dynamic strings this is not true. Example:

program test
character(len=:), allocatable :: str(:)
allocate(character(3) :: str(2))
print *, len(str(1)), len(str(2))
str(1) = '12345'
print *, len(str(1)), len(str(2))
end program

When I run this I get:

   3           3
   3           3

So the set of str(1) did not change the length the string. I get the same behavior with ifort 16.0.2 and gfortran 5.3.1. My question is this behavior consistent with the latest Fortran standard or is this a bug in the compilers?

Answer 1

This is the correct behaviour. An element of an allocatable array is not itself an allocatable variable and cannot be re-allocated on assignment (or in any other way). Also, all array elements must have the same type - including the string length.

Answer 2

Expanding on @Vladimir F's answer, you could achieve what you have in mind with a few changes to your code, like the following (which creates a jagged array of character vectors):

program test

type :: CherVec_type
    character(len=:), allocatable :: str
end type CherVec_type

type(CherVec_type) :: JaggedArray(2)

allocate(character(3) :: JaggedArray(1)%str)
allocate(character(3) :: JaggedArray(2)%str)
print *, len(JaggedArray(1)%str), len(JaggedArray(2)%str)

JaggedArray(1)%str = "12345"
print *, len(JaggedArray(1)%str), len(JaggedArray(2)%str)

end program

which creates the following output:

$gfortran -std=f2008 *.f95 -o main
$main
       3           3
       5           3