why such an error?

lightoverhead has asked for the wisdom of the Perl Monks concerning the following question:

I have written a script to grap the duplicate row of a file.

1 #!/usr/bin/perl -w
  2 use strict;
  3 my %dup;
  4 while(<>){
  5 my @row=split(/\s+/,$_);
  6 $dup{$row[3]}=$dup{$row[3]}+1; #if change to $dup{$row[3]}++, it w
+ill run smoothly no error report
  7 }
  8
  9 foreach my $id (keys %dup){
 10 if ($dup{$id}>1) {
 11 print "$id\t$dup{$id}\n";
 12 }
 13 }
[download]

it report the error: "Use of uninitialized value in addition (+) at dup.pl line 6.." If I change that line code to "$dup{$row3}++", then it's all right. Who can tell me why this happened? Thanks.

Comment on why such an error? Download Code

Replies are listed 'Best First'.
Re: why such an error? by FunkyMonk (Bishop) on Sep 25, 2008 at 19:07 UTC
++ is magic in this regard. See Auto increment and decrement in perlop: undef is always treated as numeric, and in particular is changed to 0 before incrementing (so that a post-increment of an undef value will return 0 rather than undef).	[reply]
Re^2: why such an error? by lightoverhead (Pilgrim) on Sep 25, 2008 at 20:30 UTC
Thank you for your answer. Other than using the magic "++". Could we have another method to define the `$dup{$row[3]}`? Thanks.	[reply] [d/l]
Re^3: why such an error? by GrandFather (Saint) on Sep 25, 2008 at 23:24 UTC
There are a number of ways you could do it: `$dup{$row[3]} = $dup{$row[3]} + 1; # Has trouble with undef $dup{$row[3]} += 1; ++$dup{$row[3]}; $dup{$row[3]}++;` [download] += and ++ are magical to the extent that they can update an undef value without complaint. That is often very useful, especially where a hash or array element is autovivified. For example, if you are using a hash to count the number of occurrences of words in a string you could: `++$wordCount{$_} for split ' ', $string;` [download] which generates an entry in %wordHash for each word and increments the count - very clear and concise. Perl reduces RSI - it saves typing	[reply] [d/l] [select]
Re^3: why such an error? by rovf (Priest) on Sep 26, 2008 at 08:59 UTC
You seem to have an inclination for dyadic plus ;-) `$dup{$row[3]} = ($dup{$row[3]}\|\|0) + 1;` [download] -- Ronald Fischer <ynnor@mm.st>	[reply] [d/l]