Three-way comparison function for strings

[ChetanKarwa]

I went through the stdlib_string_type module and I want to recommend some changes. Correct me if I am wrong at any point.

• The lgt, llt, lge and lle work very similarly to the operators >, <, >=, <= respectively according to me there is not much need for all these functions.
• Instead of all these we can have a general function compare(str1,str2) this function will return an integer value:
  • 1 if str1 is lexicographically greater than str2,
  • 0 if str1 is lexicographically equal to str2,
  • -1 if str1 is lexicographically smaller than str2.
• We can also include a function like compare_ignore_case(str1,str2) this function is similar to compare but is not case sensitive. (can improve nomenclature of the functions)

[ivan-pi]

• The lgt, llt, lge and lle work very similarly to the operators >, <, >=, <= respectively according to me there is not much need for all these functions.

Quoting the gfortran Fortran language reference:

In general, the lexical comparison intrinsics LGE, LGT, LLE, and LLT differ from the corresponding intrinsic operators .GE., .GT., .LE., and .LT., in that the latter use the processor’s character ordering (which is not ASCII on some targets), whereas the former always use the ASCII ordering.

On most modern processors the native character ordering will be ASCII, so the two will be de facto the same, but this is not mandated the standard. Hence, the need to maintain two sets of character comparison functions for consistency with the standard.

[ivan-pi]

The new functions you propose sound like they could be useful in some circumstances, even if they offer the same functionality. For example they can be used in a select case construct:

select case (compare(str1,str2))
case (1) ! greater than
...
case(-1) ! less than
...
case default ! equal
...
end select

This can be weighed against the following if-else

if (lgt(str1,str2)) then
...
else if (llt(str1,str2)) then
...
else
...
end if

[awvwgk]

I wonder if Fortran would allow us to use operator(<=>)? But operator(compare) would work for me as well.

An implementation would be most suitable in stdlib_ascii with a thin wrapper in stdlib_string_type to allow normal character variables as input as well.

[ivan-pi]

Do you have some reason to prefer the operator syntax? Personally, I find the functional version easier to grasp and also has less characters when importing. But I'm fine with having both.

use stdlib_string_type, only: compare, operator(.compare.)

if (compare(str1,str2) > 0) then
...
end if

if ((str1 .compare. str2) > 0)
...
end if

I assume these functions would follow the ASCII ordering and not the default processor one?

[ChetanKarwa]

I would vote for the functional version, it would be easier to get used to it.
ASCII order is widely used and accepted so it's the best option for implementing the compare function.

[ChetanKarwa]

I also propose to add a few other functions that can be commonly used.

• Replace function, this can be implemented in several ways:

  • First implementation: string.replace(pos , len, new_string)
    • pos = starting position of the sub-string to be replaced.
    • len = length of the substring
    • new_string = the string that will replace the substring
  • Second implementation: string.replace(pos, old_string, new_string)
    • old_string = substring to be replaced.
    • pos = position to start the search of old_string
    • new_string = string that will replace old_substring
    • This function will replace only the first occurrence of the old_string.
  • Third implementation: string.replaceall(pos, old_string, new_string)
    • This meaning of variables is similar to the second implementation.
    • The difference is it will find and replace all the occurrences of old_string after pos and not just one.

• We already have an operator to Concat a string with another but I feel we should also implement a function for the same.

The community can have a discussion over the need for such functions and if needed also suggest some other implementation of Replace function.

[awvwgk]

There is also a proposal for sub and gsub at #69 (comment).

[ivan-pi]

Thanks @ChetanKarwa for suggesting. I've opened a new issue for the replace.

I suggest we keep this issue open for the compare and you can rename the issue "Three-way comparison function for strings".