RogerBW's Blog

Perl Weekly Challenge 93: Max Path 30 December 2020

I’ve been doing the Perl Weekly Challenges. The latest involved colinear points and binary tree sums. (Note that this is open until 3 January 2021.)

TASK #1 › Max Points

You are given set of co-ordinates @N.

Write a script to count maximum points on a straight line when given co-ordinates plotted on 2-d plane.

sub mp {
  my @c=@_;
  my $epsilon=0.0001;
  my $mxp=0;

So I do a nested search: for each (ordered) pair of coordinates A and B, see how many of the others fall on the line they form.

  foreach my $a (0..$#c-2) {
    foreach my $b ($a+1..$#c-1) {
      my @d=map {$c[$b][$_]-$c[$a][$_]} (0,1);

Any two points are in a straight line…

      my $pil=2;
      foreach my $c ($b+1..$#c) {
        my $tp=0;
        my @v=map {$c[$c][$_]-$c[$a][$_]} (0,1);

A bit of special-casing deals with the situation where A and B are on the same horizontal or vertical coordinate.

        if ($d[0]==0) {
          if ($v[0]==0) {
            $tp=1;
          }
        } elsif ($d[1]==0) {
          if ($v[1]==0) {
            $tp=1;
          }

Then it's time for floating-point. Yeah, probably could have done it all in integer. max(v[0],d[0]) % min(v[0],d[0]) == 0 and so on, but it starts getting into special cases quite quickly. So I got lazy. epsilon is there because you can never trust floating-point equality. Python has a Fraction class, Raku has Rat, Ruby has Rational and Rust has fraction, but I basically just implemented this same system again.

        } elsif (abs($v[0]/$d[0]-$v[1]/$d[1])<$epsilon) {
          $tp=1;
        }
        if ($tp) {
          $pil++;
        }
      }
      if ($pil > $mxp) {
        $mxp=$pil;
      }
    }
  }
  return $mxp;
}

Raku is basically the same.

Python should have explicit conversion to float (though the test cases don't check for this).

                elif abs(float(v[0])/float(d[0])-float(v[1])/float(d[1])) < epsilon:

Ruby is much the same again. Rust gets more fun, not least because the copy of the book I have uses ... for inclusive range but the compiler has ..=. (And people complained about Ruby changing its syntax.)

TASK #2 › Sum Path

You are given binary tree containing numbers 0-9 only.

Write a script to sum all possible paths from root to leaf.

Rather than parse an ASCII diagram, I started with an array representation of the tree, with non-existing nodes represented by undef.

is(sp(1,2,undef,3,4),13,'example 1');
is(sp(1,2,3,4,undef,5,6),26,'example 2');

Then it's back to good old FIFO buffer code. Which in Perl is trivial (if sluggish); in other languages, a bit less so. See, the great thing is that a Perl array isn't a first-class type: if I say [@array,$variable] that's simply a reference to a new array consisting of all the values of the old array plus variable on the end. Which is what I want. Each entry in @path is a simple list of all the array indices needed to get to this point. If it has no child nodes, I add the sum of entries to the total.

sub sp {
  my @t=@_;
  my $s=0;
  my @path=([0]);
  while (my $a=shift @path) {
    my $c=($a->[-1])*2+1;
    my $tn=1;
    foreach my $ac ($c,$c+1) {
      if ($ac <= $#t && defined $t[$ac]) {
        push @path,[@{$a},$ac];
        $tn=0;
      }
    }
    if ($tn) {
      $s+=sum(map {$t[$_]} @{$a});
    }
  }
  return $s;
}

This is the sort of thing that did my head in in Raku a few months back. And even here it's a bit dodgy; if I called my temporary variable @a rather than $a it behaved differently. Also note the double flat plus list; all of those are necessary in order to get what seems to me the simple result. Raku doesn't like this use of undef so I substituted -1.

sub sp (**@t) {
  my $s=0;
  my @path=((0));
  while (@path) {
    my $a=shift @path;
    my $c=($a[$a.end])*2+1;
    my $tn=1;
    for ($c,$c+1) -> $ac {
      if ($ac <= @t.end && @t[$ac] != -1) {
        push @path,($a.flat,$ac).flat.list;
        $tn=0;
      }
    }
    if ($tn) {
      $s+=sum(map {@t[$_]},$a.list);
    }
  }
  return $s;
}

For Python I used the deque type for the main buffer. It's happy with None though. I did end up using a temporary array variable, which I suppose is what the other languages are doing internally.

def sp(*t):
    s=0
    path=collections.deque([[0]])
    while len(path)>0:
        a=path.popleft()
        c=a[-1]*2+1
        tn=1
        for ac in range(c,c+2):
            if ac < len(t) and t[ac] is not None:
                b=a.copy()
                b.append(ac)
                path.append(b)
                tn=0
        if tn:
            s += sum(t[i] for i in a)
    return s

Ruby has nil, though it doesn't need a deque type.

def sp(*t)
  s=0
  path=[[0]]
  while (a=path.shift) do
    c=a[-1]*2+1
    tn=true
    c.upto(c+1) do |ac|
      if ac <= t.length && !t[ac].nil? then
        path.push([a,ac].flatten)
        tn=false
      end
    end
    if tn
      s += a.map{|i| t[i]}.sum()
    end
  end
  return s
end

And Rust, ah, beautiful Rust… which gets the deque and type conversions, as well as the explicit temporary variable.

fn sp(t: Vec<i32>) -> i32 {
    let mut s: i32=0;
    let mut path: VecDeque<Vec<i32>>=VecDeque::new();
    path.push_back(vec![0]);
    while path.len() > 0 {
        let a=path.pop_front().unwrap();
        let c=((a.last().unwrap())*2+1) as usize;
        let mut tn=true;
        for ac in c..=c+1 {
            if ac < t.len() && t[ac]>-1 {
                let mut b=a.clone();
                b.push(ac as i32);
                path.push_back(b);
                tn=false;
            }
        }
        if tn {
            s+=a.iter().map(|i| t[*i as usize]).sum::<i32>();
        }
    }
    return s;
}

But this is feeling increasingly like the right way to do things.

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.

Search
Archive
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 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 2022 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