It’s my bi-monthly perl blog entry, to earn my place on planet.perl.org! ;)
Here’s an interesting “gotcha”. Take this code:
perl -e '%t=map{$_=>1}qw/1 2 3/;
while(($k,$v)=each %t){print "1: $k\n"; last;}
while(($k,$v)=each %t){print "2: $k\n";}'
In other words, iterate through all the key-value pairs in %t once,
then do it again — but exit early in the first loop.
You would expect to get something like this output:
1: 1
2: 1
2: 3
2: 2
instead, you see:
1: 1
2: 3
2: 2
The “1″ entry in the second loop is AWOL. Here’s why — as “perldoc -f each” notes:
There is a single iterator for each hash, shared by all “each”, “keys”, and
“values” function calls in the program
That’s all “each” calls, throughout the entire codebase, possibly in a different class entirely. Argh.
The workaround: reset the iterator using “keys” between calls to “each”:
perl -e '%t=map{$_=>1}qw/1 2 3/;
while(($k,$v)=each %t){print "1: $k\n"; last;}
keys %t;
while(($k,$v)=each %t){print "2: $k\n";}'
This got us in SpamAssassin — bug 4829.
To be honest, having to call “keys” after the loop is kludgy — as you can see if you check the patch in bug 4829 there, we had to change from a “return inside loop” pattern to a “set variable and exit loop, reset state, then return” pattern. It’d be nice to have a scoped version of each(), instead of this global scope, so that this would work:
perl -e '%t=map{$_=>1}qw/1 2 3/;
{ while(($k,$v)=scoped_each %t){print "1: $k\n"; last;} }
# that each() iterator is now out of scope, so GC'd;
# the next call uses a new iterator, starting from scratch
{ while(($k,$v)=scoped_each %t){print "2: $k\n";} }'
Scoping, of course, has the benefit of allowing “return early” patterns to work; in my opinion, those are clearer — at the least because they require less lines of code ;)
Tags: bugs, coding, each, perl, software
