Perl parsing the csv file - perl

Question

I am just trying to read .csv file first time.I have gone through the below link :
http://metacpan.org/pod/Text::CSV_XS#Reading-a-CSV-file-line-by-line:
I have few doubt, well if you want, u can tell me this are silly question but i don't know, why i am not able to figure it out that how exactly perl is reading csv file :(
So, my doubt is:
First Question
What is the difference between reading the csv file line by line and parsing the file.
I have simple program where i am reading the csv file line by line.
Below is my program:
#!/usr/bin/perl -w
use strict;
use Text::CSV;
use Data::Dumper;
my $csv=Text::CSV->new( );
my $my_file="test.csv";
open(my $fl,"<",$my_file) or die"can not open the file $!";
#print "$ref_list\n";
while(my $ref_list=$csv->getline($fl))
{
print "$ref_list->[0]\n";
}
Below is the data in csv file :
"Emp_id","Emp_name","Location","Company"
102713,"raj","Banglore","abc"
403891,"Rakesh","Pune","Infy"
530201,"Kiran","Hyd","TCS"
503110,"raj","Noida","HCL"
Second Question:
If I want to get specific Emp_id along with Location then how can i proceed.
Third Question :
If I want only 102713 ,530201,503110 Emp record i.e name,location,compnay name then what should i do ?
Thanks

Answer 1

A CSV file is a good representation of tabular data in a text format, but it is unsuitable for an in-memory represenation. Because of that, we have to create an adequate representation. One such representation would be a hash:
my $hashref = {
Emp_Id => ...,
Emp_name => ...,
Location => ...,
Company => ...,
};
If the header row is in the array #header, we can create this hash with:
my #header = ...;
my #row = #{$csv->getline($fl)}; # turn the arrayref into an array
my $hashref = {};
for my $i (0..$#header) {
$hashref->{$header[$i]} = $row[$i];
}
# The $hashref now looks as described above
We can then create lookup hashes that use the id values as keys. So %lookup looks like this:
my %lookup = (
102713 => $hashref_to_first_line,
...,
);
We populate it by doing
$lookup{$row[0]} = $hashref;
after the above loop. We can then access a certain hashref with
my $a_certain_id_hashref = $lookup{102713};
or access certain elements directly with
my $a_certain_id_location = $lookup{102713}{Location};
If the key does not exist, these lookups should return undef.
If the CSV file is too big, this might cause perl to run out of memory. In that case, the hashes should be tied to files, but that is a different topic completely.

Answer 2

Here's another option that addresses your second question and part of your third question:
use Modern::Perl;
use Text::CSV;
my #empID = qw/ 102713 530201 503110 /;
my $csv = Text::CSV->new( { binary => 1 } )
or die 'Cannot use CSV: ' . Text::CSV->error_diag();
my $my_file = "test.csv";
open my $fl, '<', $my_file or die "can not open the file $!";
while ( my $ref_list = $csv->getline($fl) ) {
if ( $ref_list->[0] ~~ #empID ) {
say "Emp_id: $ref_list->[0] is Location: $ref_list->[2]";
}
}
$csv->eof or $csv->error_diag();
close $fl;
Output:
Emp_id: 102713 is Location: Banglore
Emp_id: 530201 is Location: Hyd
Emp_id: 503110 is Location: Noida
The array #empID contains the ID(s) you're interested in. In the while loop, each Emp_id is checked using the smart match operator (Perl v5.10+) to see if it's in the list of IDs. If so, the Emp_id and its corresponding Location is printed.