RogerBW's Blog

I’ve been doing the Weekly Challenges. The latest involved numerical searching and letter substitution. (Note that this ends today.)

Task 1: Three Power

You are given a positive integer, $n.

Write a script to return true if the given integer is a power of three otherwise return false.

All right, I ignored the positive bit, but so did the test cases. The obvious approach would be to take a cube root, round down to integer, and cube it, but I felt like writing a bisecting search instead. (Newton-Raphson would converge faster, but would probably need floating point.)

Ruby:

def threepower(n0)

Apparently zero isn't a power of three (test cases).

  if n0 == 0 then
    return false
  end

This will work for negative numbers too.

  n = n0.abs

Set up the extents of the search.

  lo = 1
  hi = n.div(2)
  while true do

Find a new midpoint, and its cube.

    t = (lo + hi).div(2)
    c = t * t * t

Is it the answer?

    if c == n then
      return true
    end

Have we converged without an answer?

    if lo == t then
      return false
    end

Otherwise pull the edge of the range into the midpoint and go again.

    if c < n then
      lo = t
    else
      hi = t
    end
  end
end

Task 2: Reverse Vowels

You are given a string, $s.

Write a script to reverse all the vowels (a, e, i, o, u) in the given string.

This is very easy… if you ignore case. So I chose not to. In Raku:

First, a vowelness tester.

sub is_vowel($c) {
    if ($c ~~ m:i/<[aeiou]>/) {
        return True;
    } else {
        return False;
    }
}

Then the main function:

sub reversevowels($a) {

Break the string into a list. (I don't like subscripted string access, Python-style, though I used it in Python.)

    my @p = $a.comb();

Have a sublist of that with just the vowels, in order.

    my @q = @p.grep({is_vowel($_)});

Initialise an index to point past the end of that.

    my $qi = @q.elems;
    my @o;

For each character:

    for @p -> $c {

If it's a vowel:

        if (is_vowel($c)) {

Take the last unused vowel from the vowel list.

            $qi--;
            my $nc = @q[$qi];

Flip it to match the case of the character we're looking at.

            if ($c eq uc($c)) {
                $nc = uc($nc);
            } else {
                $nc = lc($nc);
            }

Add it to the output.

            @o.push($nc);
        } else {

If the character is not a vowel, add it as-is.

            @o.push($c);
        }
    }
    return @o.join('');
}

Full code on github.