RogerBW's Blog

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

Task 1: Group Position

You are given a string of lowercase letters.

Write a script to find the position of all groups in the given string. Three or more consecutive letters form a group. Return "" if none found.

Well, I'm using typed languages, so I'm happy to return an empty list in that last case. This is basically going to be another pass character by character through a string. Raku:

sub groupposition($a) {
    my @mx;
    my $lc = 'z';
    for $a.comb.kv -> $i, $c {

For the first character, set "most recent character" to something that isn't this.

        if ($i == 0) {
            $lc = ($c.ord + 1).chr;
        }

If the character hasn't changed, add one to the count of the active group.

        if ($lc eq $c) {
            my $mc = @mx.elems;
            @mx[$mc - 1][1] += 1;

Otherwise, start a new group, mapping this character to a count of 1.

        } else {
            @mx.push([$c, 1]);
            $lc = $c;
        }
    }

Then filter the list of groups and format them appropriately (e.g. ['a', 3] becomes "aaa").

    my @out;
    for @mx -> @ms {
        if (@ms[1] >= 3) {
            @out.push(@ms[0] x @ms[1]);
        }
    }
    @out;
}

Task 2: Reverse Equals

You are given two arrays of integers, each containing the same elements as the other.

Write a script to return true if one array can be made to equal the other by reversing exactly one contiguous subarray.

So basically it's a matter of checking every possible single swap. JavaScript lacks an array content comparator:

function arrcmp(a, b) {
    if (a.length != b.length) {
        return false;
    }
    for (let i = 0; i < a.length; i++) {
        if (a[i] != b[i]) {
            return false;
        }
    }
    return true;
}

Then check using a standard traverse through half of the possible combinations.

(Arguably if the arrays are equal the result should be false but I'm going by the examples.)

function reverseequals(a, b) {
    if (arrcmp(a, b)) {
        return true;
    }
    for (let i = 0; i < a.length - 1; i++) {
        for (let j = i + 1; j < a.length; j++) {
            let c = [...a];
            for (let x = i; x <= j; x++) {
                c[x] = a[j - (x - i)];
            }
            if (arrcmp(c, b)) {
                return true;
            }
        }
    }
    return false;
}

Full code on github.