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in reply to Re: Re:{2} Getting impossible things right (behaviour of keys)
in thread Getting impossible things right (behaviour of keys)

I feel ashamed...

but I donīt understand this code. I would like to before I try if it works or not. (call me a theory fetish monk)

First I donīt understand why you use cmp for the sort.
Then (or because of this) I donīt see how we can be sure that the longest suffixes are examined first.

I could understand what ($name =~ s/($pattern)$/$sufdata{$1}/o) does. And in fact it is elegant to use regexp instead of an own iteration, just the $pattern creation isnīt clear.

Ciao

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Re: Re: Re: Re:{2} Getting impossible things right (behaviour of keys)
by blakem (Monsignor) on Oct 24, 2001 at 22:25 UTC
    OK, I'll take you through it... it looks like I misled several others with my late night post as well.

    First, lets look at a slightly newer version of the code: 

    1: my $pattern = join('|',(keys %sufdata));
2: if ($name =~ s/($pattern)$/$sufdata{$1}/o) {
3:   print "Suffix $1 -> $name\n";
4:   return;
5: }
6: print " no rule apply -> $name\n";

    [download]
    Our main goal is to construct a pattern that matches a piece at the end of the string. This match needs to be one of the keys in %sufdata, specifically the longest such key that matches.

    So, start with line #1. 

    my $pattern = join('|',(keys %sufdata));

    [download]
    Here we create a list of keys in %sufdata, then join them together with '|' symbols. $pattern is now a string that looks like "a|s|k|ar|ic|ec|ek|er|vec" though the order of the keys is unknown (and unimportant in this specific case).

    In line #2: 

    if ($name =~ s/($pattern)$/$sufdata{$1}/o) {

    [download]
    If we expand $pattern oursleves, we get: 
    if ($name =~ s/(a|s|k|ar|ic|ec|ek|er|vec)$/$sufdata{$1}/o) {

    [download]
    We check for a match, and replace the first one we find with the corresponding value from %sufdata.

    Now here's the trick... the regex engine works from left to right, checking if *any* of those keys match at each position along the way. Since we are anchored at the end, and we use the first one we find, this effectively becomes a search for the longest matching suffix.

    So, because of the "left most matching" behavior of the regex engine, we can feel assured that the first match, will actually be the longest one in this particular instance.

    Lines 3-6 are left as an excercise for the reader. ;-)

    -Blake

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