# ETOOBUSY ðŸš€ minimal blogging for the impatient

# PWC096 - Edit Distance

**TL;DR**

On with TASK #2 from the Perl Weekly Challenge #096. Enjoy!

# The challenge

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.

# The questions

To be honest, no questions here. The link to the Wikipedia page basically tells everything we need to know, soâ€¦ weâ€™re ready.

Except thatâ€¦ is it fine to raise an exception if the input is undefined? ðŸ¤“

# The solution

Oh, the joys of reuse! I knew that coding the algorithm in Levenshtein distance - Iterative with two matrix rows would pay off at some time!

So I went to cglib, selected Levenshtein.pm and *presto!* I had
a solution:

```
sub edit_distance ($S1, $S2) { return levenshtein($S1, $S2) }
```

Oh! You wanted to see the solution?!? Rightâ€¦

```
# Wikipedia: .../Levenshtein_distance#Iterative_with_two_matrix_rows
sub levenshtein {
my ($v, $s, $t) = ([0 .. length($_[0])], @_);
for my $i (1 .. length($t)) {
my $w = [$i]; # first "column" of full matrix
for my $j (1 .. length($s)) {
my ($D, $I, $S) = ($v->[$j] + 1, $w->[$j - 1] + 1, $v->[$j - 1]);
$S++ if substr($s, $j - 1, 1) ne substr($t, $i - 1, 1);
my $mDI = $I < $D ? $I : $D; # min($D, $I);
push @$w, ($S < $mDI ? $S : $mDI); # min($S, min($D, $I))
} ## end for my $j (1 .. length(...))
$v = $w; # "swap" and prepare for nest iteration
} ## end for my $i (1 .. length(...))
return $v->[-1];
} ## end sub levenshtein ($s, $t)
```

Itâ€™s basically a direct implementation of the algorithm in the Wikipedia page, so nothing to add to it.

If youâ€™re curious about the whole program, here it is:

```
#!/usr/bin/env perl
use 5.024;
use warnings;
use experimental qw< postderef signatures >;
no warnings qw< experimental::postderef experimental::signatures >;
# Wikipedia: .../Levenshtein_distance#Iterative_with_two_matrix_rows
sub levenshtein {
my ($v, $s, $t) = ([0 .. length($_[0])], @_);
for my $i (1 .. length($t)) {
my $w = [$i]; # first "column" of full matrix
for my $j (1 .. length($s)) {
my ($D, $I, $S) = ($v->[$j] + 1, $w->[$j - 1] + 1, $v->[$j - 1]);
$S++ if substr($s, $j - 1, 1) ne substr($t, $i - 1, 1);
my $mDI = $I < $D ? $I : $D; # min($D, $I);
push @$w, ($S < $mDI ? $S : $mDI); # min($S, min($D, $I))
} ## end for my $j (1 .. length(...))
$v = $w; # "swap" and prepare for nest iteration
} ## end for my $i (1 .. length(...))
return $v->[-1];
} ## end sub levenshtein ($s, $t)
sub edit_distance ($S1, $S2) { return levenshtein($S1, $S2) }
my $first = shift // 'kitten';
my $second = shift // 'sitting';
say edit_distance($first, $second);
```

Stay warm (if youâ€™re in the north emisphere, at least), keep your cool and stay safe!

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