Can't Extract Specific Value (variable) from an Array in PERL

Question

DATA FILE temp.csv

1,2015-08-20,00:00:00,89,1007.48,295.551,296.66,

2,2015-08-20,03:00:00,85,1006.49,295.947,296.99,

3,2015-08-20,06:00:00,86,1006.05,295.05,296.02,

4,2015-08-20,09:00:00,85,1005.87,296.026,296.93,

5,2015-08-20,12:00:00,77,1004.96,298.034,298.87,

I want to extract (column 6) value if (column 3 = 09:00:00)

push to $result variable. This needs to be the specific value/variable not an array. I need to do additional calculations with that value and move it to a database. Here is the code I used but I can't extract the specific value to a variable.

 my @temp_9 = map {
 chomp;
 my @t_fh_9 = split /,/;
 sprintf "%.0f", $t_fh_9[6] if $t_fh_9[2] eq '09:00:00';
 } <$ft1>;

When I attempt to subtract or do other math the numbers are nonsensical.

Answer 1

map always has a one to one mapping, if you put 10 things in, you will get a list of 10 things out. What you want is a while loop which selectively pushes onto an array.

use strict;
use warnings;
use v5.10;

my @nines;
while(<DATA>) {
    chomp;
    my @row = split /,/;
    next unless $row[2] eq '09:00:00';
    push @nines, sprintf("%.0f", $row[6]);
}

say join ", ", @nines;

__DATA__
1,2015-08-20,00:00:00,89,1007.48,295.551,296.66,
2,2015-08-20,03:00:00,85,1006.49,295.947,296.99,
3,2015-08-20,06:00:00,86,1006.05,295.05,296.02,
4,2015-08-20,09:00:00,85,1005.87,296.026,296.93,
5,2015-08-20,12:00:00,77,1004.96,298.034,298.87,

From your variable names like @temp_9 and @t_fh_9, I suspect you're hard coding the variable name with the particular time of day. This will lead to a lot of code duplication. Instead, write a little function that takes the time of day you're looking for.

sub extract_column_for_time_of_day {
    my($fh, $column_number, $time_of_day) = @_;

    my @extracts;
    while(<$fh>) {
        chomp;
        my @row = split /,/;
        next unless $row[2] eq $time_of_day;
        push @extracts, sprintf("%.0f", $row[$column_number]);
    }

    return @extracts;
}

say join ", ", extract_column_for_time_of_day(\*DATA, 6, '09:00:00');

Finally, I'm guessing you're going to get data for various times. This will lead to a pile of variables which will be awkward to pass around as a unit. Instead of having a variable for each time, put each list into a hash.

my $time = '09:00:00'; 
$extracts{$time} = [extract_column_for_time_of_day(\*DATA, 6, $time)];

Answer 2

It seems one of the sources of confusion is extracting an element from an array. An array is zero or more scalar elements - you can't just assign one to the other, because .... well, what should happen if there isn't just one element (which is the usual case).

Given an array, we can:

• pop @array will return the last element (and remove it from the array) so you could my $result = pop @array;
• [0] is the first element of the array, so we can my $result = $array[0];
• Or we can assign one array to another: my ( $result ) = @array; - because on the left hand side we have an array now, and it's a single element - the first element of @array goes into $result. (The rest isn't used in this scenario - but you could do my ( $result, @anything_else ) = @array;

So in your example - if what you're trying to do is retrieve a value matching a criteria - the normal tool for the job would be grep - which filters an array by applying a conditional test to each element.

So:

#!/usr/bin/env perl
use strict;
use warnings;
my @lines = grep { (split /,/)[2] eq "12:00:00" } <DATA>;
print "@lines";
print $lines[0];

__DATA__
1,2015-08-20,00:00:00,89,1007.48,295.551,296.66,
2,2015-08-20,03:00:00,85,1006.49,295.947,296.99,
3,2015-08-20,06:00:00,86,1006.05,295.05,296.02,
4,2015-08-20,09:00:00,85,1005.87,296.026,296.93,
5,2015-08-20,12:00:00,77,1004.96,298.034,298.87

Which we can reduce to:

my ( $firstresult ) = grep { (split /,/)[2] eq "12:00:00" } <DATA>;
print $firstresult;

But as we want to want to transform our array - map is the tool for the job.

my ( $result ) = map { (split /,/)[6] - 273.15 } grep { (split /,/)[2] eq "12:00:00" } <DATA>;
print $result;

First we:

• use grep to extract the matching elements. (one in this case, but doesn't necessarily have to be!)
• use map to transform the list, so that that we turn each element into just it's 6th field, and subtract 273.15
• assign the whole lot to a list containing a single element - in effect just taking the first result, and throwing the rest away.

Or perhaps:

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

my ($result) = map {
          ( split /,/ )[2] eq "12:00:00"
        ? ( split /,/ )[6] - 273.15
        : ()
} <DATA>;
print $result;

But personally, I think that's getting a bit complicated and may be hard to understand. map is a powerful function, but can lead to code that's hard to read for future maintenance programmers.

So I would suggest instead:

my $result;
while (<DATA>) {
    my @fields = split /,/;
    if ( $fields[2] eq "12:00:00" ) {
        $result = $fields[6] - 273.15;
        last;
    }
}

print $result;

Iterate your data, split - and test - each line, and when you find one that matches the criteria - set $result and bail out of the loop.

Answer 3

The map function typically returns a list with the same number of elements as the source, unless you return an empty or multi-element list. In fact, one of the first examples in the map perl doc's shows how to do this, once again proving you should always check the docs first.

my @temp_9 = map {
    chomp;
    my @t_fh_9 = split /,/;
    $t_fh_9[2] eq '09:00:00' ? ( $t_fh_9[6] ) : ();
} <$ft1>;