RogerBW's Blog

Perl Weekly Challenge 96: Reverse Distance 22 January 2021

I’ve been doing the Perl Weekly Challenges. The latest involved reversing words in a string and calculating Levenshtein distance. (Note that this is open until 24 January 2021.)

TASK #1 › Reverse Words

You are given a string $S.

Write a script to reverse the order of words in the given string. The string may contain leading/trailing spaces. The string may have more than one space between words in the string. Print the result without leading/trailing spaces and there should be only one space between words.

In Perl, this reduces trivially to "do you know how the split function works", a bit like the tr test back in #90.

sub rw {
  my $n=shift;
  return join(' ',reverse split ' ',$n);

In other languages it gets a little more interesting. Raku lets me put things in a sensible order:

  return $n.comb(/\S+/).reverse.join(' ');

Python makes me separate them:

def rw(n):
    return ' '.join(t)

Ruby lets me keep them in order:

  return n.split(' ').reverse.join(' ')

And Rust is… the only one of these languages that doesn't special-case split with a single space parameter to elide multiple spaces. Hey ho. But at least I have a grep-equivalent. I have to reverse in place as I do with Python.

fn rw(n: &str) -> String {
    let mut nr: Vec<&str>=n.split(' ').filter(|x| x.len()>0).collect();
    let k=nr.join(" ");
    return k;

TASK #2 › Edit Distance

You are given two strings $S1 and $S2.

Write a script to find out the minimum operations required to convert $S1 into $S2. The operations can be insert, remove or replace a character. Please check out Wikipedia page for more information.

So this is Levenshtein distance (I do not think I have ever, in my computing career, had an actual use for this as distinct from the simpler "how many characters differ"), and therefore I shall stand on the feet of giants and implement Wagner-Fischer. There are lots of ways of making this less demanding of time and storage, which I do not use here. Here's the Raku version; they pretty much all look the same.

sub ed($s,$t) {
  my @ss=(0,$s.comb).flat;
  my @tt=(0,$t.comb).flat;
  my @d;
  for 0..@ss.end {
    push @d,[(0) xx (@tt.elems)];
  map {@d[$_][0]=$_}, 1..@ss.end;
  map {@d[0][$_]=$_}, 1..@tt.end;
  for 1..@tt.end -> $j {
    for 1..@ss.end -> $i {
      my $sc=0;
      if (@ss[$i] ne @tt[$j]) {
  return @d[@ss.end][@tt.end];

(Except that, I find, Rust does have a built-in min function… but it takes only two parameters, or an iterator, not an arbitrary list of things. Feels wrong to turn this list into an iterator…)


Full code on github.

See also:
Perl Weekly Challenge 90: Ethiopian DNA

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