在 Fortran 中返回不同长度的字符串数组

Question

I would like to create a type to contain an array of strings in Fortran without explicitly assigning lengths so I can return it from a function.

The following is my type:

type returnArr
    Character (*), dimension(4)  :: array
end type returnArr

The following is the function's signature:

type(returnArr) FUNCTION process(arr, mean, stdDev) result(j)

The following is where I try set the result:

j%array(1)= "Test"

But all I get is the following error:

Error: Character length of component ‘array’ needs to be a constant specification expression at (1)

How can I declare the type so that the strings may be of different length?

Answer 1

A character component of a derived type may have either an explicit length, or it may have its length deferred . This latter is how one changes its value (by any of a number of means). It is also necessary to allow the flexibility of different lengths in an array of the derived type.

However, using len=* is not the correct way to say "deferred length" (it is "assumed length"). Instead, you may use len=: and make the component allocatable (or a pointer):

type returnArr
  Character (:), allocatable  :: char
end type returnArr

Then one may have an array of returnArr objects:

function process(arr, mean, stdDev) result(j)
  type(returnArr) :: j(4)
end function

Assignment such as as

  j(1)%char = "Test"

then relies on automatic allocation of the component char .

Note that now the function result is an array of that container type. We cannot do

function process(arr, mean, stdDev) result(j)
  character(:), allocatable :: j(:)
end function

and allocate j(1) to a length different from j(2) (etc.).

Similarly

type returnArr
  Character (:), allocatable  :: char(:)
end type returnArr

doesn't allow those char component elements to be of different lengths.