Re^2: Make calculation with values from hash

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Re^3: Make calculation with values from hash by johngg (Canon) on Jan 02, 2007 at 20:54 UTC
Yes, you have that correctly. Put simply, if you read from a filehandle in a while loop without explicitly assigning what's read to a different variable, it goes into `$_`, i.e. `while (<$inFH>) { # $_ gets each line in here }` whereas `while (my $line = <$inFH>) { # $line gets each line in here }`. I think that only holds in a `while` loop though. If you are reading a single line outside of a loop you have to assign to a variable. If I do the following, thinking to print just the first line of the data file `use strict; use warnings; my $inFile = q{spw592594.dat}; open my $inFH, q{<}, $inFile or die qq{open: $inFile: $!\n}; <$inFH>; chomp; print qq{-->$_<--\n}; close $inFH or die qq{close: $inFile: $!\n};` [download] I get `Use of uninitialized value in scalar chomp at ./spw592594B line 11, <$ +inFH> line 1. Use of uninitialized value in concatenation (.) or string at ./spw5925 +94B line 12, <$inFH> line 1. --><--` [download] because the line is read then silently thrown away because I have not assigned it to any variable. I have to assign to `$_` or some other variable to be able to use it, like this `use strict; use warnings; my $inFile = q{spw592594.dat}; open my $inFH, q{<}, $inFile or die qq{open: $inFile: $!\n}; $_ = <$inFH>; chomp; print qq{-->$_<--\n}; close $inFH or die qq{close: $inFile: $!\n};` [download] which produces `-->1st line<--` [download] I hope this makes it clear what is happening. Cheers, JohnGG	[reply] [d/l] [select]

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Re^3: Make calculation with values from hash
by johngg (Canon) on Jan 02, 2007 at 20:54 UTC

$_

while (<$inFH>) { # $_ gets each line in here }

while (my $line = <$inFH>) { # $line gets each line in here }

I think that only holds in a while loop though. If you are reading a single line outside of a loop you have to assign to a variable. If I do the following, thinking to print just the first line of the data file

use strict;
use warnings;

my $inFile = q{spw592594.dat};
open my $inFH, q{<}, $inFile
   or die qq{open: $inFile: $!\n};

<$inFH>;
chomp;
print qq{-->$_<--\n};

close $inFH
   or die qq{close: $inFile: $!\n};
[download]

I get

Use of uninitialized value in scalar chomp at ./spw592594B line 11, <$
+inFH> line 1.
Use of uninitialized value in concatenation (.) or string at ./spw5925
+94B line 12, <$inFH> line 1.
--><--
[download]

because the line is read then silently thrown away because I have not assigned it to any variable. I have to assign to $_ or some other variable to be able to use it, like this

use strict;
use warnings;

my $inFile = q{spw592594.dat};
open my $inFH, q{<}, $inFile
   or die qq{open: $inFile: $!\n};

$_ = <$inFH>;
chomp;
print qq{-->$_<--\n};

close $inFH
   or die qq{close: $inFile: $!\n};
[download]

which produces

-->1st line<--
[download]

I hope this makes it clear what is happening.

Cheers,

JohnGG

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