How can I write a to_upper() or to_lower() function in F90?

Question

How does one write a (Intel) F90 function that converts a string into lowercase (or, alternatively, uppercase)? I want to pass a character array to the function and have it retu

Answer 1

Here's one that doesn't rely on the ASCII representation

Pure Function to_upper (str) Result (string)

!   ==============================
!   Changes a string to upper case
!   ==============================

    Implicit None
    Character(*), Intent(In) :: str
    Character(LEN(str))      :: string

    Integer :: ic, i

    Character(26), Parameter :: cap = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    Character(26), Parameter :: low = 'abcdefghijklmnopqrstuvwxyz'

!   Capitalize each letter if it is lowecase
    string = str
    do i = 1, LEN_TRIM(str)
        ic = INDEX(low, str(i:i))
        if (ic > 0) string(i:i) = cap(ic:ic)
    end do

End Function to_upper

You can easily change this to to_lower by switching the low and cap strings in the loop.

Answer 2

As the original author of this code, I'm pleased that it's of some help. I used to wonder why these functions were not built in to Fortran. My guess is that they only work for a rather restricted set of letters, i.e. the ones used in English. If you have text in almost any other European language you will have characters with accents, and then converting them to upper or lower case is much harder. For example e-grave in French turned into upper case is usually shown as just plain E (the grave accent gets lost), but in e-acute it does not. The designers of Fortran have always tried to provide facilities which suit a wide range of languages, and doing upper/lower case conversion in a multi-language way is not at all easy. At least that's my guess as to why you have to do it yourself.

Answer 3

Wow -- even though I'd searched for over an hour, immediately after posting this, I found an answer here (under "Miscellaneous Fortran Hints and Tips").

The code I used is as follows (for to_upper):

function to_upper(strIn) result(strOut)
! Adapted from http://www.star.le.ac.uk/~cgp/fortran.html (25 May 2012)
! Original author: Clive Page

     implicit none

     character(len=*), intent(in) :: strIn
     character(len=len(strIn)) :: strOut
     integer :: i,j

     do i = 1, len(strIn)
          j = iachar(strIn(i:i))
          if (j>= iachar("a") .and. j<=iachar("z") ) then
               strOut(i:i) = achar(iachar(strIn(i:i))-32)
          else
               strOut(i:i) = strIn(i:i)
          end if
     end do

end function to_upper

Hope this helps somebody!