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Asked: 2026-06-05T21:03:26+00:00

How does one write a (Intel) F90 function that converts a string into lowercase

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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 return a character array, e.g. 

program main
    implicit none

    character*32 :: origStr = "Hello, World!"
    character*32 :: newStr

    newStr = to_lower(origStr)
    write (*,*) newStr

end program main

such that this program outputs hello, world!.

I’ve been starting with the to_lower() subroutine found at RosettaCode, but I can’t figure out how to write it as a function.

Thanks in advance!

PS — Bonus points if you can do it with a string of unfixed length!

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1 Answer

  1. Editorial Team

    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.

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