Initialization of hash element....

Chon-Ji has asked for the wisdom of the Perl Monks concerning the following question:

Am I correct to assume that there is no need to intialize a hash element to 0 or 1, when my purpose is just to increment it? I tried these two codes, and both of them seem to have produced the same result. Because of this I thought that if a hash element has no value and it is used on an arithmetic operation then its default value is zero. Am I correct? code1

if(exists $myhash{$key})
{
 $myhash{$key} = $myhash{$key} + 1;
}
else
{
 $myhash{$key} = 1;
}
[download]

code2

$myhash{$key} = $myhash{$key} + 1;
[download]

Comment on Initialization of hash element.... Select or Download Code

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Re: Initialization of hash element.... by GrandFather (Saint) on Jun 01, 2007 at 02:34 UTC
Yes, undef gets treated as 0. Actually you can use += to make it even more succinct: `$myhash{$key} += 1;` [download] DWIM is Perl's answer to Gödel	[reply] [d/l]
Re^2: Initialization of hash element.... by blazar (Canon) on Jun 01, 2007 at 11:51 UTC
`$myhash{$key} += 1;` [download] What's of `$myhash{$key}++; # ? ;-)` [download]	[reply] [d/l] [select]
Re: Initialization of hash element.... by Herkum (Parson) on Jun 01, 2007 at 03:07 UTC
Perl will attempts to do what it thinks were you trying to accomplish. In other words, if you add to 1 to a string, it will treat the hash element as a digit. If you try to concatenate a string it would treat your hash element as a string. It tends to throw people who are used to programming with Java or C#.	[reply]
Re: Initialization of hash element.... by blazar (Canon) on Jun 01, 2007 at 11:58 UTC
Because of this I thought that if a hash element has no value and it is used on an arithmetic operation then its default value is zero. Am I correct? Yes, and this has not strictly to do with hash elements, but with any variable, be it a simple scalar or an element of an aggregate. One caveat though: under warnings, you will get a warning if you use such a value in a generic expression, but you won't if you use mutators instead: `tilde:~ [13:53:30]$ perl -we '$a=$a+1' Use of uninitialized value in addition (+) at -e line 1. tilde:~ [13:53:50]$ perl -we '$a+=1' Name "main::a" used only once: possible typo at -e line 1. tilde:~ [13:53:58]$ perl -we '$a++' Name "main::a" used only once: possible typo at -e line 1.` [download]	[reply] [d/l]
Re^2: Initialization of hash element.... by ysth (Canon) on Jun 01, 2007 at 18:30 UTC
`$ perl -we '$a=0' Name "main::a" used only once: possible typo at -e line 1. Use of uninitialized value in multiplication () at -e line 1.` [download] Only certain mutators suppress the warning - those that most make sense with a 0 / "" default. These are: ++ and -- (either pre or post), +=, -=, .=, \|=, ^=, &&=, \|\|=. But last I checked, some of those erroneously warn when used on a tied variable.	[reply] [d/l]