Book Code

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

I am attempting to srudy Perl with the aid of a book titled Professional Perl Programming by Peter Wainwright et al. I decided to type in and run a hash example. The code looked like this:

p.135-136(Professional Perl Programming)

#!/usr/bin/perl
# indexhash.pl
use warnings;
use strict;

#create a hash with integrated index
my %hash = {
    Mouse => {Index => 0, Value => 'Jerry'},
    Cat   => {Index => 1, Value => 'Tom'},
    Dog   => {Index => 2, Value => 'Spike'}
    };

# sort a hash by integrated index
foreach (sort {$hash{$a} {'Index'} cmp $hash{$b} {'Index'}} keys %hash
+)  
    {
    print "$hash{$_} {'Value'} <= $_ \n";
    }
[download]

Perl told me the syntax was ok but when I ran it I got the following error messages:

Reference found where even-sized list expected at indexhash.pl line 7.
Use of uninitialized value in concatenatization (.) or string at index
+hash.pl line 16.
[download]

My question is twofold. First, assuming I can read and type what is the error? Second, does any Perl Monk have an opinion on this book and is it worth using?
This is my first post. I hope this question is suitable.
Thanks in advance,
xenchu

Comment on Book Code Select or Download Code

Replies are listed 'Best First'.
Re: Book Code by liz (Monsignor) on Nov 15, 2003 at 19:57 UTC
I don't know whether this is a typo on your part, or a misprint in the book, but In think you will need to put parentheses instead of curly braces here: `my %hash = ( # note parenthesis open here Mouse => {Index => 0, Value => 'Jerry'}, Cat => {Index => 1, Value => 'Tom'}, Dog => {Index => 2, Value => 'Spike'} ); # note parenthesis close here` [download] The output then becomes: `HASH(0xfd578) {'Value'} <= Mouse HASH(0xfd494) {'Value'} <= Cat HASH(0x10e080) {'Value'} <= Dog` [download] Liz	[reply] [d/l] [select]
Re: Book Code by pg (Canon) on Nov 15, 2003 at 20:04 UTC
It is either a typo on your side, or a printing error in the book. The correct syntax shall be: (it is not {}, but (), as you are defining a hash, not a hash ref) `my %hash = ( Mouse => {Index => 0, Value => 'Jerry'}, Cat => {Index => 1, Value => 'Tom'}, Dog => {Index => 2, Value => 'Spike'} );` [download] If you define a hash ref, not a hash, then you {}, instead of (), the syntax shall be: `my $hash = { Mouse => {Index => 0, Value => 'Jerry'}, Cat => {Index => 1, Value => 'Tom'}, Dog => {Index => 2, Value => 'Spike'} };` [download] But this does not agree with some of the later lines in your script, for example: `foreach (sort {$hash{$a} {'Index'} cmp $hash{$b} {'Index'}} keys %hash +)` [download] Obviously this line expects a hash, not a hash ref. To retrieve elements from hash ref, you say: `$hash->{$a}` [download]	[reply] [d/l] [select]
Re: Book Code by xenchu (Friar) on Nov 15, 2003 at 21:28 UTC
It seems I can't read. Thank you all very much for the help. I am getting new glasses. I appreciate both the effort and the prompt response. xenchu	[reply]
Re: Re: Book Code by mooseboy (Pilgrim) on Nov 16, 2003 at 08:13 UTC
Welcome to the monastery! To answer your question about the Wrox book you're learning from: I've looked through it a few times and wasn't especially impressed. You'll save yourself a lot of headaches if you invest in the Camel book instead, or if that seems a bit heavy going, try the Llama book first. The investment will repay itself many times over, and you'll have a lot more fun with Perl into the bargain ;-)	[reply]
•Re: Re: Re: Book Code by merlyn (Sage) on Nov 16, 2003 at 16:56 UTC
And after the llama book, keep reading into the alpaca book, to learn how to write programs from 100 to 10,000 lines long! -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re: Book Code by Anonymous Monk on Nov 15, 2003 at 19:58 UTC
Change the curlies in `my %hash = { .. };` [download] to parens: `my %hash = ( .. );` [download]	[reply] [d/l] [select]