RogerBW's Blog

The Weekly Challenge 160: Balancing Four is Magic 14 April 2022

I’ve been doing the Weekly Challenges. The latest involved English word lengths and array partitioning. (Note that this is open until 17 April 2022.)

Task 1: Four Is Magic Submitted by: Mohammad S Anwar

You are given a positive number, $n < 10.

Write a script to generate english text sequence starting with the English cardinal representation of the given number, the word 'is' and then the English cardinal representation of the count of characters that made up the first word, followed by a comma. Continue until you reach four.

We haven't had string handling in these things for a while - though it turns out to be relatively easy to do even in the languages that are new to me. (My PostScript library set has grown some new functions.) This is probably a relatively ugly way to do it, but it works.

Here's the Perl – which still does a good job of making string handling easy even though I miss the features of later languages.

sub fim {
  my $n=shift;

Establish the lookup table.

  my @words=qw(zero one two three four five six seven eight nine);
  my @p;
  while (1) {

Each phrase starts the same way.

    my $s=$words[$n] . ' is ';

The final phrase ends differently, and leaves the loop.

    if ($n == 4) {
      $s .= 'magic.';
      push @p,$s;

Otherwise, do it the standard way.

    } else {
      $s .= $words[$n];
      push @p,$s;

Join all the phrases for the result.

  return ucfirst(join(', ',@p));

Task 2: Equilibrium Index

You are give an array of integers, @n.

Write a script to find out the Equilibrium Index of the given array, if found.

For an array A consisting n elements, index i is an equilibrium index if the sum of elements of subarray A[0…i-1] is equal to the sum of elements of subarray A[i+1…n-1].

I found a pleasing approach to this. First get the sum of the whole series. Then for each index calculate a partial sums: twice the sum of all previous values, plus this value. At the equilibrium index, this will equal the total. (If the values were guaranteed to be non-negative, you could bail out if this value got higher than the whole-series sum, but the problem specification does not make this guarantee.)


fn equilibriumindex(s: Vec<isize>) -> isize {

Sum of the series. (In Rust, a reference to satisfy the borrow checker.)

    let sm=&s.iter().sum::<isize>();

Start the partial sums.

    let mut sa=0;
    for (i,v) in s.iter().enumerate() {
        sa += v;

Partial sum is now (twice all previous entries) plus (this entry).

        if sa == *sm {
            return i as isize;
        sa += v;

Partial sum is now (twice all entries) ready for the next entry.


If we didn't bail out early, there's no solution.


So in the case of [1,3,5,7,9]:

  • Sum of series (sm) is 25
  • Accumulator (sa) starts at 0
  • index 0, value 1, sa becomes 1; not equal to 25; sa becomes 2.
  • index 1, value 3, sa becomes 5; not equal to 25; sa becomes 8.
  • index 2, value 5, sa becomes 13; not equal to 25; sa becomes 18.
  • index 3, value 7, sa becomes 25; equal to 25. Return index 3.

Full code on github.

Comments on this post are now closed. If you have particular grounds for adding a late comment, comment on a more recent post quoting the URL of this one.

Tags 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s 3d printing action advent of code aeronautics aikakirja anecdote animation anime army astronomy audio audio tech aviation base commerce battletech beer boardgaming book of the week bookmonth chain of command children chris chronicle church of no redeeming virtues cold war comedy computing contemporary cornish smuggler cosmic encounter coup covid-19 crime crystal cthulhu eternal cycling dead of winter doctor who documentary drama driving drone ecchi economics en garde espionage essen 2015 essen 2016 essen 2017 essen 2018 essen 2019 essen 2022 essen 2023 existential risk falklands war fandom fanfic fantasy feminism film firefly first world war flash point flight simulation food garmin drive gazebo genesys geocaching geodata gin gkp gurps gurps 101 gus harpoon historical history horror hugo 2014 hugo 2015 hugo 2016 hugo 2017 hugo 2018 hugo 2019 hugo 2020 hugo 2021 hugo 2022 hugo 2023 hugo 2024 hugo-nebula reread in brief avoid instrumented life javascript julian simpson julie enfield kickstarter kotlin learn to play leaving earth linux liquor lovecraftiana lua mecha men with beards mpd museum music mystery naval noir non-fiction one for the brow opera parody paul temple perl perl weekly challenge photography podcast politics postscript powers prediction privacy project woolsack pyracantha python quantum rail raku ranting raspberry pi reading reading boardgames social real life restaurant reviews romance rpg a day rpgs ruby rust scala science fiction scythe second world war security shipwreck simutrans smartphone south atlantic war squaddies stationery steampunk stuarts suburbia superheroes suspense television the resistance the weekly challenge thirsty meeples thriller tin soldier torg toys trailers travel type 26 type 31 type 45 vietnam war war wargaming weather wives and sweethearts writing about writing x-wing young adult
Special All book reviews, All film reviews
Produced by aikakirja v0.1