Re: why use a hash instead of an array

[2teez]

Hi DarrenSol
Wonderful answers has been given, but maybe this little example / question could also "say" something.
If I give the following:

teacher students professor students
school class teacher desk table
chalk borad class school teacher

And I ask that for these:
1. print out all the words, without repeating any, 2. print out all the repeated words only, 3. print out the number of times each words is seen and probably an illustration this with an histogram.
How would you do these.

Let me answer QnS 1. and 2., allow you to figure question 3 and try your hands on other solution WITHOUT USING HASH

use strict;
use warnings;

my %hash;

while(<DATA>){
chomp;
  $hash{$_}++ for split;  
}

print join $/ => sort keys %hash; 
print $/,$/, join $/ => grep {$_ if $hash{$_} > 1} sort keys %hash;
__DATA__
teacher students professor students
school class teacher desk table
chalk borad class school teacher

Solution:

1. borad
   chalk
   class
   desk
   professor
   school
   students
   table
   teacher
   

2. class
   school
   students
   teacher
   

That brings me to a statement I read in Programming Perl a while a go that 'Until you start thinking in terms of hashes, you aren’t really thinking in Perl.'

[5mi11er]

For a person just learning perl, your example code is quite obtuse. You should at least attempt to explain how your code actually works.

I'd do it for you, but need to leave work now...

Update: since no one else has yet done so, and I'm back at work now, I'll try:

So, the $/ floating all over the above post is simply a special variable holding, by default, a "new line" character, ie. "\n". The first section

while(<DATA>){
chomp;
  $hash{$_}++ for split;  
}

simply reads in the data, and for every word in the DATA section, increments the hash table entry for that word. It would be more clear to beginners if it were written thusly:

while(<DATA>){  #get a line of text from the DATA area, put it in spec
+ial variable $_
    chomp;      #remove the "new line" character from the $_ variable
    #create a word array from the $_ variable
    #split with no options splits on whitespace by default
    @words = split;
    foreach $word (@words) {
        $hash{$word}++;  #increment the value pointed to by the $hash{
+$word} "key"
    }  
}

Once you have the data read in, you'll have the following data structure:

board => 1
chalk => 1
class => 2
desk => 1
professor => 1
school => 2
students => 2
table => 1
teacher => 3

2teez then uses more advanced perl to essentially do the following:

foreach $word (sort keys %hash) {
    print "$word\n";
}
print "\n";
foreach $word (sort keys %hash) {
    if ($hash{$word} > 1) {
        print "$word\n";
    }
}

From these two beginner friendly examples, it should be pretty easy to do 2teez's 3rd challenge which is to print out the data structure I gave above.

-Scott

[Lotus1]

grep {$_ if $hash{$_} > 1} sort keys %hash

The '$_ if' is redundant since that is what grep does by default, return $_ if the clause is true. The following works the same way:

grep { $hash{$_} > 1 } sort keys %hash

[LanX]

actually not quite =)

grep { $hash{$_} > 1 } is equivalent to map {$_ if $hash{$_} > 1}

grep {$_ if $hash{$_} > 1} only greps if the key $_ is also true.

Which I agree is very odd code!

[Lotus1]

It looks like we were both wrong. None of them are equivalent. ;)

use strict;
use warnings;
use Data::Dumper;

my %hash;

while(<DATA>){
#chomp;
  $hash{$_}++ for split;  
}

$hash{''}=2;

print Dumper(\%hash);
print '*'x55,"\n";

my @greparr=  grep {$hash{$_} > 1} sort keys %hash;
print "greparr has ", scalar @greparr, " elements.\n";

my @maparr = map {$_ if $hash{$_} > 1} sort keys %hash;
print "maparr has ", scalar @maparr, " elements.\n";

my @grep2 = grep {$_ if $hash{$_} > 1} sort keys %hash;
print "grep2 has ", scalar @grep2, " elements.\n";

print '*'x55,"\n";
print Dumper(\@greparr);
print '*'x55,"\n";
print Dumper(\@maparr);
print '*'x55,"\n";
print Dumper(\@grep2);
print '*'x55,"\n";

__DATA__
0 0
teacher students
teacher students
nope

Output:

$VAR1 = {
          '' => 2,
          '0' => 2,
          'nope' => 1,
          'students' => 2,
          'teacher' => 2
        };
*******************************************************
greparr has 4 elements.
maparr has 5 elements.
grep2 has 2 elements.
*******************************************************
$VAR1 = [
          '',
          '0',
          'students',
          'teacher'
        ];
*******************************************************
$VAR1 = [
          '',
          '0',
          '',
          'students',
          'teacher'
        ];
*******************************************************
$VAR1 = [
          'students',
          'teacher'
        ];
*******************************************************