How to print the previous Key Value in Perl? - perl

Question

In this code, I'm checking if a certain key is present or not.
Here I am checking if key "Uri" present. I am getting output as "3".
use strict;
use warnings;
my %Names = (
Martha =>2,
Vivek =>9,
Jason =>6,
Socrates=>7,
Uri =>3,
Nitin =>1,
Plato =>0,
);
if (exists $Names{Uri} ) {
print "$Names{Uri}\n";
}
foreach my $name (sort {$Names{$a} cmp $Names{$b}} keys %Names)
{
print $name, $Names{$name}."\n";
}
Output
3
Plato 0
Nitin 1
Martha 2
Uri 3
Jason 6
Socrates 7
Vivek 9
But, I want the previous key value present before that key. For example:
If I search for key "Uri" Output should be "2"
If I search for key "Vivek" Output should be "7"
If I search for key "Plato" Output should be "0"
Does anyone know how to do it?

Answer 1

Create a sorted array of the hash values, then search through the array to get the value just lower than the value of your search key.
use strict;
use warnings;
my %Names = (
Martha =>2,
Vivek =>9,
Jason =>6,
Socrates=>7,
Uri =>3,
Nitin =>1,
Plato =>0,
);
my #vals = sort {$a <=> $b} values %Names;
get_prev('Uri');
get_prev('Vivek');
get_prev('Plato');
sub get_prev {
my $k = shift;
if (exists $Names{$k}) {
for (#vals) {
if ($Names{$k} == $vals[$_]) {
my $idx = ($_ == 0) ? 0 : $_ - 1;
print $vals[$idx], "\n";
last;
}
}
}
}
Prints:
2
7
0

Answer 2

If you want to print them all:
my $prev;
for my $name (
sort { $Names{$a} <=> $Names{$b} } # Note to use of <=> for numerical comparisons.
keys(%Names)
) {
say "$name $Names{$prev}" if $prev;
$prev = $name;
}
Similarly, to print just one
my $find = 'Uri';
my $prev;
for my $name (
sort { $Names{$a} <=> $Names{$b} }
keys(%Names)
) {
if ($name eq $find) {
say "$name $Names{$prev}" if $prev;
last;
}
$prev = $name;
}
The above hat would be an expensive way to perform multiple lookups.
For that, we'd build a mapping from names to the previous names.
my %prev_name_lkup;
my $prev;
for my $name (
sort { $Names{$a} <=> $Names{$b} }
keys(%Names)
) {
$prev_name_lkup{$name} = $prev if $prev;
$prev = $name;
}
This could also be done as follows:
my #sorted_names =
sort { $Names{$a} <=> $Names{$b} }
keys(%Names);
my %prev_name_lkup =
map { $sorted_names[$_-1] => $sorted_names[$_] }
1..$#sorted_names;
Either way, the lookups would look like this:
say "Uri $Names{$prev_name_lkup{Uri}}";