perl: give me suggestion

quantt · 05-19-2009, 06:06 AM

hello,
To suggest me whatever so as to do it quite well.

Code:

my %map(
'chips' -> 'food',
'hamburger' -> 'food',
'water' -> 'drink',
'beer -> 'drink'
);
foreach (keys(%map)){
print $_; 
}

would be printed...
food
food
drink
drink
but, how printing once only of food or drink, for inctance...
food
drink

ghostdog74 · 05-19-2009, 06:15 AM

you have some Perl experience judging from your previous posts and i really doubt you can't do it. Have you tried running the code? seems not i guess. I suggest you run the code and see what it says.

Telemachos · 05-19-2009, 06:57 AM

Quote:

Originally Posted by quantt

hello,
To suggest me whatever so as to do it quite well.

Code:

my %map(
'chips' -> 'food',
'hamburger' -> 'food',
'water' -> 'drink',
'beer -> 'drink'
);
foreach (keys(%map)){
print $_; 
}

would be printed...
food
food
drink
drink
but, how printing once only of food or drink, for inctance...
food
drink

First, you're doing an assignment, so you need my %map =. Second, you want '=>' not '->' to associate hash items. Third, you're missing a single-quote mark after 'beer, which is going to confuse the hell out of the Perl interpreter. Next, unless you add an explicit "\n", you won't get a newline in your print. Finally, as things stand you are running through all the keys in your hash and trying to print them. The keys are the items on the left hand side of each hash couplet. So what you will get printed, if you fix the code you're starting with, is "chips, hamburger, water, beer" (on new lines if you add those).

What are you really trying to do? It might help us to give you advice.

Oh, and you should also always use strict and warnings in your code. They help you find little problems and save you from big ones.

Sample code:

Code:

#!/usr/bin/env perl
use strict;
use warnings;


my %map = (
  'chips' => 'food',
  'hamburger' => 'food',
  'water' => 'drink',
  'beer '=> 'drink',      # The last comma is optional, but it's
                          # a good idea since you might add items later
);

foreach ( keys (%map) ){
  print $_, "\n"; 
}

If you're looking for a good initial book about Perl, Beginning Perl is available freely online now (since it's out of print). It's a bit out of date, and so lacks some newer features, but for the basics of the language it's still very good.

quantt · 05-19-2009, 08:57 AM

ok, that printing
food
food
drink
drink
Each hash should be printed once only.
food
drink

Code:

foreach  (  keys (%map) ){
  print $map{$_}, "\n";
}

kellinwood · 05-19-2009, 09:11 AM

I think the idea you need is to take the values from this hash and put them into a second hash as the keys of the second hash. Then in a second loop, print the keys. Since the keys in a hash must be unique, this will reduce each unique value to a single instance and result in only one line per unique value.

Telemachos · 05-19-2009, 02:12 PM

Quote:

Originally Posted by quantt

ok, that printing
food
food
drink
drink
Each hash should be printed once only.
food
drink

Code:

foreach  (  keys (%map) ){
  print $map{$_}, "\n";
}

No, the code you printed should do exactly what it does do. It prints the value of each item in the hash. Hash keys must be unique (a second, third, fourth, etc. key identical to an earlier one will simply overwrite the earlier one), but there's no such restriction on hash values. So if you want 'food' and 'drink' to be unique, you have to do some extra work.

As Kellinwood says, you could create a second hash where the keys are the values of the previous hash. Or you can use Perl's grep function to test for values that have already been seen. See perldoc -q duplicate for more on this:

Code:

#!/usr/bin/env perl
use strict;
use warnings;


my %map = (
  'chips' => 'food',
  'hamburger' => 'food',
  'water' => 'drink',
  'beer '=> 'drink',      # The last comma is optional, but it's
                          # a good idea since you might add items later
);

my %seen;
my @unique_values = grep { ! $seen{$_}++ } values %map;

print "$_\n" foreach @unique_values;