Selecting successive lines

[zee3b]

Hey guys, I'm working on a text parser. I grab the data from the file and push it into an array and then I foreach it, but I'm only looking for specific successive lines when I'm calculating the average. Here's an example. My data file is of this structure

Jack
Student ID - 12445
Math Score - 45

Jill
Student ID - 234254
Math Score - 90

Jack
Student ID -12445
Math Score2 - 33

Jill 
Student ID - 234254
Math Score2 - 10

So basically as soon as my regex matches the name Jill or Jack. I want it to pick the score from the 3rd line, any command to add into the if loop? e.g

if ($line ~= /Jill/) {
*pick the score from the sucessive second line*

This way I can average the scores for each student for different tests.

output = Jack Average 39

Jill Average 50

[kcott]

G'day zee3b,

You can read that data in paragraph mode (see perlvar for details): basically, this allows you to read each group of three lines as a single record. When you do this, you can grab the name, from line 1, and the score, from the end of line 3, in a single operaton.

#!/usr/bin/env perl -l

use strict;
use warnings;

my %score;

{
    local $/ = "";

    while (<DATA>) {
        /\A(\w+).*?(\d+)\D*\z/ms;
        ++$score{$1}{count};
        $score{$1}{total} += $2;
    }
}

for (sort keys %score) {
    print $_, ' Average ', $score{$_}{total} / $score{$_}{count};
}

__DATA__
Jack
Student ID - 12445
Math Score - 45

Jill
Student ID - 234254
Math Score - 90

Jack
Student ID -12445
Math Score2 - 33

Jill 
Student ID - 234254
Math Score2 - 10

Output:

$ pm_file_parse_avg.pl
Jack Average 39
Jill Average 50

For your real application, you'll probably also want either int for whole number averages, or sprintf to format floating point results.

[marinersk]

You can read that data in paragraph mode (see perlvar for details)

OMG I have been doing this the hard way (with manual parse phase state and similar techniques) for over a decade.

:: facepalm ::

Off to read now. And let the blush drain from my face.

Thanks for the reference!

[frozenwithjoy]

If I were going to do this and wanted to have it expandable, I'd probably plan to make a hash like this (you can simplify it if you don't care about keeping track of student IDs and never have students with the same names):

my %records = (
    12445 => {
        name => 'Jack',
        scores => [ 45, 10 ],
    },
    234254 => {
        name => 'Jill',
        scores => [ 45, 10 ],
    },
);

If the data are consistently in the format shown, the following will make the hash (it can accept scores w/ decimals). It then calculates and reports the mean score.

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';
use List::Util 'sum';

my %records;

while ( my $name = <DATA> ) {
    my ($id)    = <DATA> =~ /(\d+)$/;
    my ($score) = <DATA> =~ /(\d+\.?\d*)$/;
    <DATA>;
    chomp( $name, $id, $score );

    $records{$id}{name} = $name;
    push @{ $records{$id}{scores} }, $score;
}

for ( sort { $a <=> $b } keys %records ) {
    my $name   = $records{$_}{name};
    my @scores = @{ $records{$_}{scores} };
    my $mean   = sum(@scores) / @scores;
    say "$name ($_) has an average score of $mean";
}

__DATA__
Jack
Student ID - 12445
Math Score - 45

Jill
Student ID - 234254
Math Score - 90

Jack
Student ID -12445
Math Score2 - 33

Jill
Student ID - 234254
Math Score2 - 10

OUTPUT:

Jack (12445) has an average score of 39
Jill (234254) has an average score of 50

[davido]

I would read each record complete rather than one line at a time, and to keep it simple, would push scores into an anonymous array per student. Then just iterate over each student and do the math.

use List::Util qw( sum );

my %student;

local $/ = '';

while( <DATA> ) {
  push @{$student{$1}}, $2 and next if m/\A([^\n]+).+?(\d+)\n?\Z/s;
  warn "Invalid record #$.: <<$_>>\n";
}

while( my( $name, $scores ) = each %student ) {
  print "$name: Average ", sum( @{$scores} ) / @$scores, "\n";
}

__DATA__
Jack
Student ID - 12445
Math Score - 45

Jill
Student ID - 234254
Math Score - 90

Jack
Student ID -12445
Math Score2 - 33

Jill 
Student ID - 234254
Math Score2 - 10

[ansh batra]

I have a simple way
keep a flag,when you get the name set the flag as one and if flag is one then get score.


foreach(traverse data) {
    if($data =~ /jack/) {  //put some variable here for name
        $flag=1;
    }
    if($flag ==1 && $data=~ /Math Score2 - /) {
        $score=$'; 
        $flag=0;
    }
}


Since you need to check for multiple names , you may also use
if ( grep( /^$data$/, @array ) )

[sundialsvc4]

The customary approach to doing this sort of thing is the approach taken by the awk utility:  

/regular_expression /
  { code to execute if regex is matched }
... rinse and repeat ...

So, in this file, there would be .. it looks like .. about five different “kinds” of lines, including blank-line, and you have things-to-do with two of them.   For a student_name line, you capture the name and proceed.   For a Score(n) line, you extract the score and do something with it, using the most-recently captured student name.   Perhaps for a blak-line you forget the name.   And so on.

One advantage of this approach is that it is relatively “future-proof.”   You are making fewer assumptions about the data, such as “the third line.”   It is also now much easier for your program to recognize when there is a bug in the program that produced the file, which is another important consideration in production settings.