# 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): t=n.split() t.reverse() 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(); nr.reverse(); 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]) { \$sc=1; } @d[\$i][\$j]=min( @d[\$i-1][\$j]+1, @d[\$i][\$j-1]+1, @d[\$i-1][\$j-1]+\$sc, ); } } 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…) `````` d[i][j]=min( d[i-1][j]+1, min( d[i][j-1]+1, d[i-1][j-1]+sc ) ); `````` 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. 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