Perl array element manipulation - arrays

Question

I've been trying and trying with this one, but it just doesn't seem to click.
If I have an array with let's say 6 numbers:
#a = (1,2,3,4,5,6)
How do I get every second index ( 2, 4, 6) in this case?
how do I compute the difference of every two elements, so
the output here would be:
1 1 1 (because 2-1 =1 and 4-3 =1 and so on..)

Answer 1

Note: don't ever use $a or $b, they're special (sort uses them) ... it's generally better to give your variables a descriptive name, name it as to what's in there rather than what type of variable it is.
for ( my $index = 0; $index < scalar( #pairs ); $index += 2 ) {
my $first = $pairs[ $index + 0 ];
my $second = $pairs[ $index + 1 ];
my $pair = $index / 2;
my $difference = $second - $first;
print "the difference of pair $pair is $difference\n";
}

Answer 2

I think you should post your earlier attempts. In my opinion, the best way to learn is to learn from your mistakes, not being presented a correct solution.
For this problem, I think I would use a C-style for-loop for the first part, simply because it is straightforward, and can easily be tweaked if some new requirement comes up.
The second problem can easily be solved using a regular Perl-style for-loop.
use strict;
use warnings; # always use these two pragmas
my #nums = 1..6;
my #idx;
for (my $n = 0; $n <= $#nums; $n += 2) { # loop from 0 to max index, step 2
push #idx, $n; # store number in #idx
}
print "Indexes: #idx\n";
my #diff;
for my $n (0 .. $#nums - 1) { # loop from 0 to max index minus 1
push #diff, $nums[$n + 1] - $nums[$n]; # store diff in #diff
}
print "Diff: #diff\n";
Output:
Indexes: 0 2 4
Diff: 1 1 1 1 1

Answer 3

Try this:
use strict;
use warnings;
my $index = 1;
my #a = (1,2,3,4,5,6);
for (#a) {
if ($index % 2 == 0) {
my $diff = $_ - $a[$index-2];
print $diff;
}
$index++;
}

Answer 4

You likely want to use the new List::Util pair functions.
For your first question:
use List::Util 'pairvalues';
my #seconds = pairvalues #list; # yields (2, 4, 6)
For your second question:
use List::Util 'pairmap';
my #diffs = pairmap { $b-$a } #list; # yields (1, 1, 1)

Answer 5

You can use map:
my #a = 1 .. 6;
print join ' ', 'Every second:', map $a[ 1 + $_ * 2 ], 0 .. $#a / 2;
print "\n";
print join ' ', 'Differences:', map $a[ 1 + $_ * 2 ] - $a[ $_ * 2 ], 0 .. $#a / 2;
print "\n";

Answer 6

First: Don't use variables a and b. $a and $b are special variables used in sorting. Just be a bit more descriptive of your variables (even if it's merely #my_array) and you should be fine.
You can loop through your array any which way you like. However, I prefer to use a while loop instead of the thee part for because the three part for loop is a bit misleading. It is a while loop in disguise and the promised indexing of the loop can be misleading.
#! /usr/bin/env perl
use warnings;
use strict;
use feature qw(say);
my #array = qw( 1 2 3 4 5 6 );
my $index = 1; # Remember Perl indexes start at zero!
while ( $index <= $#array ) {
say "Item is $array[$index]";
say "The difference is " . ($array[$index] - $array[$index-1]);
$index += 2;
}
You said every second element. Indexes of arrays start at 0, so you want the odd number elements. Most of the answers use map which is a very nice little command, but does an awful lot in a single line which can make it confusing for a beginner. Plus, I don't think the Perldoc on it is very clear. There should be more simple examples.
The say is a newer version of print. However say always adds a \n at the end. You should always use strict; and use warnings;. These will catch about 90% of your programming bugs.
The qw( ... ) is a quick way to make an array. Each word becomes an array element. You don't need quotes or commas.

Answer 7

#!/usr/bin/perl
use strict;
use warnings;
my #ar = (1, 2, 3, 4, 5, 6);
# 1. How do I get every second index ( 2, 4, 6) in this case?
my #even = map { $_ & 1 ? $ar[$_] : () } 0 .. $#ar;
# 2. how do I compute the difference of every two elements?
my (#c, #diff) = #ar;
push #diff, -1 * (shift(#c) - shift(#c)) while #c;
use Data::Dumper;
print Dumper \#even;
print Dumper \#diff;
1;
__END__
$VAR1 = [
2,
4,
6
];
$VAR1 = [
1,
1,
1
];