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Reading two lines at a time

blackeagle225
Novice

Jun 22, 2011, 12:11 PM

Post #1 of 8 (10581 views)

Reading two lines at a time	Can't Post

Hello,

I have a data set, with 4 columns of data. I am trying to compare the element in the first row, column 4 with the second row, column 4 element. I am having a hard time, and would appreciate being pointed in the right directions. Thanks in advance.

miller
User

Jun 22, 2011, 1:48 PM

Post #2 of 8 (10580 views)

Re: [blackeagle225] Reading two lines at a time [In reply to]	Can't Post

What have you tried thus far?

- Miller

blackeagle225
Novice

Jun 22, 2011, 1:59 PM

Post #3 of 8 (10579 views)

Re: [miller] Reading two lines at a time [In reply to]	Can't Post

Hi Miller,

I tried a ton of different things, including seek, and loops within loops. I finally did figure it out. I just did

while (<FILE>) {
$CurrentLine = $NextLine;
$NextLine = $_;

....

It does require going through the loop one time to set both variables. Seems to be working now though. Thanks for your help anyway.

miller
User

Jun 22, 2011, 2:23 PM

Post #4 of 8 (10577 views)

Re: [blackeagle225] Reading two lines at a time [In reply to]	Can't Post

Cool, glad you figured it out. I'd probably do something like the following:

Code

my $lastline = <FILE>; 
 
while (<FILE>) { 
	# Work with $_ and $lastline 
	... 
	 
	$lastline = $_; 
}

(This post was edited by miller on Jun 22, 2011, 3:09 PM)

FishMonger
Veteran / Moderator

Jun 23, 2011, 8:00 AM

Post #5 of 8 (10560 views)

Re: [blackeagle225] Reading two lines at a time [In reply to]	Can't Post

I'd do it like this:

Code

while ( my $line1 = <$fh> ) { 
    my $line2 = <$fh>; 
 
}

My reasons: 1) it uses a lexical var for the filehandle, 2) it declares both vars in the smallest scope that the require, and 3) it doesn't needlessly use $_.

(This post was edited by FishMonger on Jun 23, 2011, 8:05 AM)

miller
User

Jun 23, 2011, 12:03 PM

Post #6 of 8 (10551 views)

Re: [FishMonger] Reading two lines at a time [In reply to]	Can't Post

Agreed FishMonger,

However, does he want pairwise reading, or want to be doing logic on each adjacent line?

If based off his thread title, then yours would be the correct solution. But I think he actually wants to compare each line with it's adjacent, and your method would skip even pairs.

- Miller

rovf
Veteran

Jun 24, 2011, 5:12 AM

Post #7 of 8 (10539 views)

Re: [blackeagle225] Reading two lines at a time [In reply to]	Can't Post

Since your implicitly require, that the input file contains an even number of lines, I would make this fact more explicit, even if the code looks a bit more complicated:

Code

while(1) { 
  my $odd_line=<FILE>; 
  last unless defined $odd_line; 
  my $even_line=<FILE>; 
  die "This is odd: File has an odd number of lines\n" unless defined $even_line; 
  # process your pair of lines here 
}

blackeagle225
Novice

Jun 24, 2011, 6:05 AM

Post #8 of 8 (10535 views)

Re: [miller] Reading two lines at a time [In reply to]	Can't Post

Yeah, thanks Miller for understanding what I meant. Obviously still very green. I wanted to read a line, and read the next line and compare the two against one another. So far the program seems fine, but I appreciate all the help.

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