help w/hash arrays

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

Trying to get facilty with hash arrays and I am having a nasty time. Please see the snippet below, Btw, I'm working out of A.L. Johnson's "Elements of Programming with Perl" pp.146. Could a you beakdown/explain the code, I'll tell you specifically where I'm stuck after the snippet:

students.dat
------------
Bill Jones : 2 : 35
Anne Smith : 3 : 41

$students {$fields[0]}->[$fields[1]]=$fields[2]
[download]

So looks like, $student is an anonyous hash reference. $fields[0] is the key (ie. key is student name). Taken in total the left side of the arrow is an anonymous array reference. It's the right side of the arrow I don't understand, what's the "=" doing.....Heck, I don't understand. Could a monk gently break it down for me? Help appreciated.

Lee

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Re: help w/hash arrays by NetWallah (Canon) on May 23, 2004 at 20:02 UTC
Things would be easier to understand if you replaced the @fields array elements with equivalent scalar values. Assume that the following has been entered: `my ($x,$y,$z) = @fields; # to simplify matters # Now, your statement can be written as : $students{$x}->[$y] = $z;` [download] %students is NOT an anonymous hash ref - it has a name - students. It is not a ref (it is a hash), and its values are references to anonymous arrays. So, if you asked for $students{$x} , you would get a refernence to an anonymous array. You de-reference this with the optional "->" operator, then index the array with [$y]. Finally, you assign $z to the indexed array. Hope this lifts the fog somewhat. Offense, like beauty, is in the eye of the beholder, and a fantasy. By guaranteeing freedom of expression, the First Amendment also guarntees offense.	[reply] [d/l]
Re: help w/hash arrays by Zaxo (Archbishop) on May 23, 2004 at 19:52 UTC
You're looking at it with the wrong precedence in mind. That is just assignment with the lhs `$students {$fields[0]}->[$fields[1]` and the rhs `$fields[2]`. The dereference arrow is optional, so that could as well be written `$students{$fields[0]}[$fields[1]] = $fields[2];` which is probably more readable. After Compline, Zaxo	[reply] [d/l] [select]
Re: help w/hash arrays by TGI (Parson) on May 24, 2004 at 00:09 UTC
Others have given some good explainations, no need to repeat them. But here are two tips: Data::Dumper is your friend, and you can use print statements to help tease apart these statements. Data:Dumper is wonderful for figuring out data structures. Just add a `use Data::Dumper;` [download] at the start of your code, and then add a line like: `print Dumper(\%students);` [download] You'll get a nice printout of the data structure and its contents. By the way, the backslash returns a reference to the variable it precedes. You can also try printing things to find out what they are. If you print a reference, you'll get something like `SCALAR(0xda28)` with a datatype and an address/uid. TGI says moo	[reply] [d/l] [select]