Does split modify $_ ?

[rebound]

Hi there,

Please see this code:

use strict;
use warnings;

open(IN, 'myfile.txt') or die "Open failed..$!\n";
while(my @arr = split /:/, <IN>)
{
#print "* $_ *\n";
if (/^#/) { print "Comment found\n"; }
}
close(IN);

I attempted to read the filename (line-by-line), split it using ':' as
seperator and stored the contents in an array. My IF check fails
despite the file having only perl comments.
$cat myfile.txt
#Simple comment
#another comment
##end

I think that $_ is being set to undef by split.
The split documentation does not say that $_ is modified by split.

Can someone please explain why $_ is being modified here.

TIA.
Kasp

 

[Arndt Jonasson]

use strict;
use warnings;

open(IN, 'myfile.txt') or die "Open failed..$!\n";
while(my @arr = split /:/, <IN>)
{
#print "* $_ *\n";
if (/^#/) { print "Comment found\n"; }
}
close(IN);

I attempted to read the filename (line-by-line), split it using ':' as
seperator and stored the contents in an array. My IF check fails
despite the file having only perl comments.
$cat myfile.txt
#Simple comment
#another comment
##end

I think that $_ is being set to undef by split.
The split documentation does not say that $_ is modified by split.

Can someone please explain why $_ is being modified here.

$_ is not being modified by 'split'; it's in fact never set at all.
You 'while' statement does not both iterate over the lines in the file
and over the items in the split list; it only does the former. You need
another iteration level in your code:

while(my @arr = split /:/, <IN>) {
for (@arr) {
#print "* $_ *\n";
if (/^#/) { print "Comment found\n"; }
}
}

(There may be more to say about this code - I'm new myself - but this
works.)

It's the file you want to read, by the way, not the filename.

I see that I'm assuming that your code was nearly correct already, but
do you want '#' to start a comment within each ':'-delimited field, or
only at the beginning of a line? The latter is more usual. If only at
the beginning of a line, it doesn't make sense to check for that while
iterating over the result of 'split'. I leave the necessary changes as
an exercise.

 

[Kasp]

The <IN> in while loop will modify the $_ to contain the line read.

You can try this:
<IN>
print $_; #or simply print;

I don't understand why split sets $_ to undef...and how can I avoid
doing so?

Cheers
Kasp

--Posted via Google newsgroup...Sorry, folks.

 

[Anno Siegel]

The <IN> in while loop will modify the $_ to contain the line read.

You can try this:
<IN>
print $_; #or simply print;

Did *you* try this?

A lone <$IN> does *not* set $_ to anything. You are working on wrong
assumptions. Read up on "while" in perlsyn to see when it does what
to $_.

I don't understand why split sets $_ to undef...and how can I avoid
doing so?

Split has nothing to do with it, $_ has never been set.

Anno

 

[Joe Smith]

The <IN> in while loop will modify the $_ to contain the line read.

For a simple while() conditional, yes.
For a nontrivial while() conditional, no.

while(<IN>){} is the same as while(defined($_=<IN>)){};

You can try this:
<IN>
print $_; #or simply print;

Oh, really? Did you actually try it?
$_ = 'original unchanged value';
<IN>;
print $_;
will not put a line of input into $_.

I don't understand why split sets $_ to undef...and how can I avoid
doing so?

If you never put anything into $_, then of course it remains undef.
You avoid this by writing code that actually puts data into $_.

-Joe