More on characters and strings

Overview

Teaching: 10 min
Exercises: 10 min

Questions

• How are character variables compared in Fortran?

• How can I create and use flexible strings?

Objectives

• Understand relational operators and some of the intrinsic functions that can be used with character variables.

• Be able to create and use deferred length character variables.

• Work with arrays of character variables.

• Be able to use characters as procedure dummy arguments and results.

Useful character operations

Relational operators

The meaning of relational operators == and so on in the context of characters is largely what one would expect from the standard ASCII sequence. E.g.,

   "A" <  "b"      ! true
   "A" == "a"      ! false
   "A" /= "]"      ! true

Note the ordinal position of a single character in the ASCII sequence can be found via an intrinsic function, e.g.:

   integer             :: i
   character (len = 1) :: c
   i = iachar("A")           ! i = 65
   c = achar(i)              ! c = "A"; requires 1 <= i <= 127

Remember that in ASCII the upper case characters A to Z have integer values 65 to 90, while the lower case characters a to z have integer values 97 to 122.

If character variables are compared, then each letter is compared left-to-right until a difference is found (or not). If the variables have different lengths, then the shorter is padded with blanks.

Some other intrinsic functions

The length of a character variable is provided by the len() intrinsic function.

The length with trailing blanks removed is len_trim(). To actually remove the trailing blanks, use trim(). This is often seen when concatenating fixed-length character strings using the // operator:

  print *, "File name: ", trim(file_stub)//"."//trim(extension)

It’s also useful if you want to perform an operation on each individual character, e.g.,

  do n = 1, len_trim(string)
    if (string(n:n) == 'a') counta = counta + 1
  end do

Note that the colon is mandatory in a sub-string reference, so a single character must be referenced, e.g.,

   print *, "Character at position i: ", string(i:i)

Various other intrinsic functions exist for lexical comparisons, justification, sub-string searches, and so on.

Deferred length strings

The easiest way to provide a string which can be manipulated on a flexible basis is the deferred length character:

  character (len = :), allocatable :: string

  string = "ABCD"         ! Allocate and assign string
  string = "ABCDEFG"      ! Reallocate and assign again

  string(:) = ''          ! Keep same allocation, but set all blanks

This may be initialised and updated in a general way, and the run time will keep track of book-keeping. This also (usually) reduces occurrences of trailing blanks.

If an allocation is required for which only the length is known (e.g., there is no literal string involved), the following form of allocation is required:

  integer :: mylen
  character (len = :), allocatable :: string

  ! ... establish value of mylen ...

  allocate(character(len = mylen) :: string)

Allocatable strings will automatically be deallocated when they go out of scope and are no longer required. One can also be explicit:

  deallocate(string)

if wanted.

Arrays of strings

We have seen that we can define a fixed length parameter, e.g.,:

  character (len = *), parameter :: day = "Sunday"

Suppose we wanted an array of strings for “Sunday”, “Monday”, “Tuesday”, and so on. One might be tempted to try something of the form:

  character (len = *), dimension(7), parameter :: days = [ ... ]

Exercise (2 minutes)

Check the result of the compilation of example3.f90.

The are a number of solutions to this issue. One could try to pad the lengths of each array element to be the same length. A better way is to use a constructor with a type specification:

[character (len = 9) :: "Sunday", "Monday", "Tuesday", "Wednesday", ...]

Here the type specification is used to avoid ambiguity in how the list is to be interpreted.

Check you can make this adjustment to example3.f90.

Strings as dummy arguments, or results

If a character variable has intent in in the context of a procedure, it typically may be declared:

  subroutine some_character_operation(carg1, ...)

     character (len = *), intent(in) :: arg1
     ...

This is also appropriate for character variables of intent inout where the length does not change.

For all other cases, use of deferred length allocatable characters is recommended. E.g.,

  function build_filename(stub, extension) result(filename)

    character (len = *), intent(in)  :: stub
    character (len = *), intent(in)  :: extension
    character (len = :), allocatable :: filenane
    ...

A matching declaration in the caller is required.

Exercise (10 minutes)

Exercise name

Write a subroutine which takes an existing string, and adjusts it to make sure it is all lower case. That is, any character between “A” and “Z” is replaced by the corresponding character between “a” and “z”.

Write an additional function to return a new string which is all lower case, leaving the original unchanged.

You can use the accompanying templates exercise_module1.f90 and exercise_program1.f90.

Solution

A solution to the exercise can be found in solution_program1.f90 and solution_module1.f90.

Key Points

• String-handling can be problematic if there is no mechanism to keep track of the length of the string.

• Remember that Fortran provides intrinsic functions such as trim() and operators such as concatenation // which can help with string manipulation.

• Pay attention when using strings as dummy arguments or procedure results.