Why this Perl program is not giving expected output? [closed] - perl

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open ( F1, "file1.txt" );
open ( F2, "+>file2.txt" );
$/ = "\n\n" ;
while (<F1>) {
print F2;
$/ = "\n" ;
#arr = <F2> ;
#found = grep(/^: /, #arr);
if( $#found == -1) {
truncate(F2, $length);
}
$/ = "\n\n" ;
}
Please help me to find the error in this code.
file1.txt:
:a
:b
: x
:y
note::a , :b and : x , :y are separated by "\n" and :b and :x by "\n\n"
Expected contents in file2.txt after execution of program:
: x
:y

Answer 1

There are things wrong with your program, but its difficult to tell what you might be referring to, since you do not actually specify a problem or a question.
open ( F1, "file1.txt" );
open ( F2, "+>file2.txt" );
You should use three argument open, with explicit mode and lexical file handle. Also, you should check the return value of the open to see that it did not fail and why. Also, using better names for file handles does not hurt.
open my $in, "<", "file1.txt" or die "Cannot open file1.txt for reading: $!";
open my $out, "+>", "file2.txt" or die "Cannot open file2.txt for overwrite: $!";
Do note that the file open mode +> will truncate your file when you open it, but allow you to both read and write to/from the file handle. Most of the time, you do not want this.
$/ = "\n\n" ;
while (<F1>) {
Setting the input record separator to $/ will read paragraphs from your input file, in your case (assuming I got the formatting right), that would be
$_ = ":a
:b
";
You then print this value to your output file
print F2; # this means "print F2 $_"
You then change the input record separator again, and read all the lines in your output file:
$/ = "\n" ;
#arr = <F2> ;
But unfortunately, this is wrong, because the position of the file handle will be at the end of file (eof), because this is a file handle your are printing to. So #arr will be empty.
#found = grep(/^: /, #arr);
if( $#found == -1) {
truncate(F2, $length);
}
So this code, with the truncate will always happen. Also, of course, $length is an undefined variable, so it will give you a warning such as Use of uninitialized value $length in truncate at ... unless you have been so foolish as to not use:
use warnings;
I assume that what you are trying to do here is to check the input received before printing it to the output, but you should know that trying to print and truncate afterwards is a horrible idea. Why not check it before printing it instead? That's not only how its done 99.99% of the times, its also the simplest and most logical way to do it.
if (/^: /) {
print $out $_;
}
/^: / is short for $_ =~ /^: /, in case you are uncertain -- it applies a regex match operator to the default input variable $_, which are what you are reading to in the while (<F1>) loop condition, which is short for while ($_ = <F1>)
In your programs you should always use
use strict;
use warnings;
And learn to use them, because they will save you lots of time when debugging, and give you vital information about what your program is doing.
So you get:
use strict;
use warnings;
open my $in, "<", "file1.txt" or die "Cannot open file1.txt for reading: $!";
open my $out, "+>", "file2.txt" or die "Cannot open file2.txt for overwrite: $!";
$/ = "\n\n";
while (<$in>) {
if (/^: /) {
print $out $_;
}
}
Be advised that you can solve this with a simple one-liner program
perl -00 -nle 'print if /^: /' file1.txt > file2.txt