RogerBW's Blog

The Weekly Challenge 258: Valuing the Count 03 March 2024

I’ve been doing the Weekly Challenges. The latest involved counting digits and matching bit-counts. (Note that this ends today.)

Task 1: Count Even Digits Number

You are given a array of positive integers, @ints.

Write a script to find out how many integers have even number of digits.

Obviously I could just take the string representation, which is pretty trivial even in a strongly-typed language, but I thought it would be more fun to keep the operations purely numeric. (That way I can change base if it's needed.) Python:

def countevendigitsnumber(a):
  t = 0
  for p in a:
    even = False

Make a copy of the value.

    pt = p

Divide it repeatedly by 10, toggling the evenness flag.

    while pt >= 10:
      pt //= 10
      even = not even
    if even:
      t += 1
  return t

Task 2: Sum of Values

You are given an array of integers, @int, and an integer $k.

Write a script to find the sum of values whose index binary representation has exactly $k number of 1-bit set.

Two parts to this, therefore: a Hamming weight calculation and then a fairly straightforward filter.

For Rust of course the former is built in:

fn sumofvalues(a: Vec<u32>, k: u32) -> u32 {
        .filter(|(i, _n)| i.count_ones() == k)
        .map(|(_i, n)| n)

and ditto in recent Python, but for most languages I implemented the popcount64c function from Wikipedia. Thus in Raku with its unique bitwise operators:

sub popcount64($x0) {
  constant $M1  = 0x5555555555555555;
  constant $M2  = 0x3333333333333333;
  constant $M4  = 0x0f0f0f0f0f0f0f0f;
  constant $H01 = 0x0101010101010101;
  my $x = $x0;
  $x -= ($x +> 1) +& $M1;
  $x = ($x +& $M2) + (($x +> 2) +& $M2);
  $x = ($x + ($x +> 4)) +& $M4;
  return ($x * $H01) +> 56;

sub sumofvalues(@a, $k) {
  return (0 .. @a.end).grep({popcount64($_) == $k}).map({@a[$_]}).sum;

JavaScript got the same algorithm using BigInt. Exceptions came when I couldn't trust the reliability of 64-bit integers (Lua and Ruby), where I implemented Kernighan's method, popcount64d from the same Wikipedia page. (Which, on small numbers like these indices, is probably faster.)

Full code on github.

Add A Comment

Your Name
Your Email
Your Comment

Your submission will be ignored if any field is left blank, but your email address will not be displayed. Comments will be processed through markdown.

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