Exists and arrays

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

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Re: Exists and arrays by AnomalousMonk (Archbishop) on Oct 01, 2009 at 10:01 UTC
Quoth `perldoc -f exists` `exists EXPR Given an expression that specifies a hash element or array element, returns true if the specified element in the hash or array has ever been initialized, even if the corresponding value is undefined. The element is not autovivified if it doesn't exist.` [download] See exists.	[reply] [d/l] [select]
Re: Exists and arrays by ccn (Vicar) on Oct 01, 2009 at 10:04 UTC
from perldoc -f exists: Given an expression that specifies a hash element or array element, returns true if the specified element in the hash or array has ever been initialized, even if the corresponding value is undefined. My perl behaves the same way This is perl, v5.8.8 built for i486-linux-gnu-thread-multi	[reply]
Re: Exists and arrays by johngg (Canon) on Oct 01, 2009 at 13:29 UTC
This code might help to illustrate what is happening. $ perl -le ' > print q{Elem. Exists Defined True Value}; > @ny = ( q{no }, q{yes } ); > $arr[ 9 ] = 9; > $arr[ 0 ] = 0; > $arr[ 1 ] = q{one}; > @arr[ 3 .. 5 ] = ( undef ) x 3; > $arr[ 7 ] = q{}; > print > qq{[ $_ ]: }, > $ny[ exists $arr[ $_ ] ], > $ny[ defined $arr[ $_ ] ], > $arr[ $_ ] ? $ny[ 1 ] : $ny[ 0 ], > $arr[ $_ ] > for 0 .. $#arr;' Elem. Exists Defined True Value [ 0 ]: yes yes no 0 [ 1 ]: yes yes yes one [ 2 ]: no no no [ 3 ]: yes no no [ 4 ]: yes no no [ 5 ]: yes no no [ 6 ]: no no no [ 7 ]: yes yes no [ 8 ]: no no no [ 9 ]: yes yes yes 9 $ [download] I hope this is helpful. Cheers, JohnGG	[reply] [d/l]
Re: Exists and arrays by ikegami (Patriarch) on Oct 01, 2009 at 15:26 UTC
`exists` is useless on arrays. Please use `defined` or check the index against the length of the array.	[reply] [d/l] [select]
Re: Exists and arrays by biohisham (Priest) on Oct 01, 2009 at 11:38 UTC
The element `$dow[5]` exists in the array since you've treated it as an lvalue and assigned to it when you did `$dow[5]=undef` the second time you checked for existence. Why the first time checking for (`exists $dow[5]`) returns you false is obviously because it had nothing assigned to it and that there's no implication if `$dow[6]` does exist that `$dow[5]` has to automatically exist. Being defined is not related to existence, a variable can exist but it doesn't necessarily mean it is defined as is the case when you want to flush out a variable value.Another thing, a return value of undef can indicate many things, as operation failure, end of file...etc Exists checks whether a variable is existing in the array, that is, it has a value associated with it as a result of assignment (i.e. $dow[0]=undef), this is clearly indicated in Perl documentation for Functions, check out both defined and exists functions to decloud the confusion `#!/usr/local/bin/perl use strict; my @dow=(); #array initialization is different from indexes initiali +zation $dow[0]=undef; $dow[6]= "sat"; for(my $i=0; $i <=6;$i++){ if(!(defined($dow[$i]) \|\| exists($dow[$i]))){print "\$dow[$i] +\t not defined\n";} if(exists($dow[$i]) && !defined($dow[$i])){ print "\$dow[$i]\t +exists\t not defined\n"; } if(exists($dow[$i]) && defined($dow[$i])){ print "\$dow[$i]\te +xists\t defined\n" ;} }` [download] Excellence is an Endeavor of Persistence. Chance Favors a Prepared Mind.	[reply] [d/l] [select]
Re: Exists and arrays (Dumper) by toolic (Bishop) on Oct 01, 2009 at 13:19 UTC
Perhaps the Data::Dumper output is a little misleading in this case. I do see an assignment to undef in the source code: `elsif (!defined($val)) { $out .= "undef"; }` [download] Maybe `Dumper` needs to print out something for non-existent array elements. Maybe it loosely uses 'undef' to mean both undefined and non-existent. If you print the Dumper output before and after your `$dow[5] = undef;` assignment, they are the same. You can play around with the source code if you really want to get to the bottom of this mystery.	[reply] [d/l] [select]
Re: Exists and arrays by deepakg (Novice) on Oct 03, 2009 at 18:47 UTC
Thanks everyone for your replies I was assuming that the initialization of elements 0 to 5 was implicitly done by perl. From Learning Perl 5th Ed. If you store an array element that is beyond the end of the array, the array is automatically extended as needed - there's no limit on its length, as long as there's available memory for Perl to use. If Perl needs to create intervening elements, it creates them as undef values. So this led me to expect that exists would work here while defined won't. Of course the dumper output doesn't help the confusion.	[reply]
Re^2: Exists and arrays by JadeNB (Chaplain) on Oct 03, 2009 at 21:18 UTC
If you store an array element that is beyond the end of the array, the array is automatically extended as needed - there's no limit on its length, as long as there's available memory for Perl to use. If Perl needs to create intervening elements, it creates them as undef values. This is just correct enough to be misleading. Assigning to an OOB element of an array `@a` modifies `$#a`, and (I assume) Data::Dumper is using `$#a` to decide how big `@a` is. (See `STORESIZE` in perltie.) It's true that trying to access ‘intervening’ elements will return `undef`, but it's not true that those elements were ‘created’ by the `STORESIZE`.	[reply] [d/l] [select]