RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved string splitting and testing. (Note that this ends today.)

Task 1: Last Word

You are given a string.

Write a script to find the length of last word in the given string.

Fairly straightforward: split by space, reject zero-length strings (some splitters will include them and I can't say they're wrong), and return the length of the last one.

Ruby:

def lastword(a)

Split and filter:

  w = a.split(" ").select{|x| x.size > 0}

Find last string and take the length.

  w[w.size - 1].size
end

Task 2: Buddy Strings

You are given two strings, source and target.

Write a script to find out if the given strings are Buddy Strings.

If swapping of a letter in one string make them same as the other then they are Buddy Strings.

This is where I find myself fighting with languages that say "oh, a string is just an array of characters, no need to split it into an array, you can just index into it". Yeah, you can index to read, but very rarely to write; and the tutorials omit the splitting because nobody needs to do that.

But Raku is not one such language. It doesn't have array comparisons, though.

sub buddystrings($a, $b) {

Get a list of the characters in the first string.

    my @ac = $a.comb;

Iterate over all possible pair indices.

    for (0 .. @ac.elems - 2) -> $i {
        for ($i + 1 .. @ac.elems - 1) -> $j {

Build a new list that's a copy of the old.

            my @acx = $a.comb;

Swap the characters.

            @acx[$i] = @ac[$j];
            @acx[$j] = @ac[$i];

Compare with the original second string.

            if (@acx.join("") eq $b) {
                return True;
            }
        }
    }
    False;
}

Where I do have an array comparator, as in PostScript (OK I wrote it myself):

/buddystrings {
    0 dict begin

Split both strings to arrays.

    s2a /bc exch def
    s2a /ac exch def

My fallback result is false.

    false

Generate the pairs of indices.

    0 1 ac length 2 sub {
        /i exch def
        i 1 add 1 ac length 1 sub {
            /j exch def

Copy acx directly from ac, and put in the changed characters.

            /acx ac deepcopy def
            acx i ac j get put
            acx j ac i get put

Do a deep value comparison.

            acx bc deepeq {

If that was valid, dump the default false and replace it with true.

                pop true
                exit
            } if
        } for

And if we have a true, drop out of the outer loop too.

        dup {
            exit
        } if
    } for
    end
} bind def

Full code on github.