Re: qw vs. q

(John Smith <wleung7@xxxxxxxxx> uttered:)
I found the following example on CPAN:

my $key = 'welcome';
my %data = (
'this' => qw(that),
'tom' => qw(and jerry),
'welcome' => q(Hello World),
'zip' => q(welcome),
);
my @data = keys %data;

First of all, bear in mind that that's a contrived example from
perldebtut, the debugging tutorial, and it has errors. Run it with
strictures and warnings (use strict; use warnings;) and you get:

Odd number of elements in hash assignment at ./foo.pl line 9.

....because what that code actually means is

my %data = (
'this', 'that',
'tom', 'and', 'jerry',
'welcome', 'Hello World',
'zip', 'welcome',
);


my question is:
1) what is the difference between qw and q?

qw is the word list quoting operator. Instead of giving you back
one string like '' and "" do, it returns a list of the whitespace-
separated `words' inside.

q{foo} is just a more general way of saying 'foo' -- it works exactly
like singlequotes. Similarly, qq{bar} is just a more general way of
saying "bar" -- qq works just like doublequotes. See the "Quoting and
Quote-like operators" section of perlop:

perldoc perlop

To wit:

qw(that) # gives you one item: 'that'
qw(and jerry) # gives you TWO items: 'and', 'jerry'
q(Hello World) # gives you ONE item: 'Hello world'
q(welcome) # gives you one item: 'welcome'



2) why is qw used in first 2 key-element; and q used in last 2
key-element?

Using qw to assign specifically to hash *values* like your example did
doesn't make sense. Hash values have to be single values, and qw is
generally used when you're looking for a list. Using qw to assign to
a whole hash, on the other hand, is a common idiom:

my %pt_number = qw{
1 um 2 dois 3 tres 4 quatro 5 cinco
6 seis 7 sete 8 oito 9 nove 10 dez
}




Cheers,
Rick
--
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.