RogerBW's Blog

The Weekly Challenge 225: Diffs on the Right, Words on the Left 16 July 2023

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

Task 1: Max Words

You are given a list of sentences, @list.

A sentence is a list of words that are separated by a single space with no leading or trailing spaces.

Write a script to find out the maximum number of words that appear in a single sentence.

Not having to account for multiple, leading, or trailing spaces makes this much easier than it might be. Broadly I used two approaches depending on which was easier in each language: counting the spaces (then adding 1), as in Lua:

function maxwords(a)
   local m = 0
   for i, v in ipairs(a) do
      local ct = 0
      for n, m in string.gmatch(v, "%s") do
         ct = ct + 1
      if ct > m then
         m = ct
   return m + 1

or splitting the string, as in Javascript:

function maxwords(a) {
    return Math.max( => x.split(" ").length));

Task 2: Left Right Sum Diff

You are given an array of integers, @ints.

Write a script to return left right sum diff array as shown below:

(I.e. for a sequence [a..e], produce a sequence [-(c+d+e), a-(d+e), (a+b)-(e), (a+b+c)] - taking the absolute value of each term.)

My basic approach is in two stages: build up the list of sums (from front and back), then produce the differences.

In Raku:

sub leftrightsumdiff(@a) {
    my $al = @a.end;

@left is 0, a, a+b, etc.; @right is 0, e, e+d, etc.

    my @left = [0];
    my @right = [0];
    for 0 .. $al-1 -> $i {
        @left.push(@left[$i] + @a[$i]);
        @right.push(@right[$i] + @a[$al - $i]);

I reverse @right and combine the two.

    return Array(zip(@left, @right.reverse).map({abs($_[0] - $_[1])}));

Basically every modern language seems to have something like zip for iterating two arrays in parallel. The exceptions were Perl, Lua, PostScript and JavaScript, for which I used an explicit index. (Probably I should add one to my PostScript libraries, though I haven't often needed it outside these problems.)

Full code on github.

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