perlchomp

Clarification on chomp

I'm on break from classes right now and decided to spend my time learning Perl. I'm working with Beginning Perl (http://www.perl.org/books/beginning-perl/) and I'm finishing up the exercises at the end of chapter three.

One of the exercises asked that I "Store your important phone numbers in a hash. Write a program to look up numbers by the person's name."

Anyway, I had come up with this: 

#!/usr/bin/perl
use warnings;
use strict;

my %name_number=
(
Me => "XXX XXX XXXX",
Home => "YYY YYY YYYY",
Emergency => "ZZZ ZZZ ZZZZ",
Lookup => "411"
);

print "Enter the name of who you want to call (Me, Home, Emergency, Lookup)", "\n";
my $input = <STDIN>;
print "$input can be reached at $name_number{$input}\n";

And it just wouldn't work. I kept getting this error message:

Use of uninitialized value in concatenation (.) or string at hello.plx line 17, line 1

I tried playing around with the code some more but each "solution" looked more complex than the "solution" that came before it. Finally, I decided to check the answers.

The only difference between my code and the answer was the presence of chomp ($input); after <STDIN>;.

Now, the author has used chomp in previous example but he didn't really cover what chomp was doing. So, I found this answer on www.perlmeme.org:

The chomp() function will remove (usually) any newline character from the end of a string. The reason we say usually is that it actually removes any character that matches the current value of $/ (the input record separator), and $/ defaults to a newline..

Anyway, my questions are:

  1. What newlines are getting removed? Does Perl automatically append a "\n" to the input from <STDIN>? I'm just a little unclear because when I read "it actually removes any character that matches the current value of $/", I can't help but think "I don't remember putting a $/ anywhere in my code."

  2. I'd like to develop best practices as soon as possible - is it best to always include chomp after <STDIN> or are there scenarios where it's unnecessary?

Solution

  • <STDIN> reads to the end of the input string, which contains a newline if you press return to enter it, which you probably do.

    chomp removes the newline at the end of a string. $/ is a variable (as you found, defaulting to newline) that you probably don't have to worry about; it just tells perl what the 'input record separator' is, which I'm assuming means it defines how far <FILEHANDLE> reads. You can pretty much forget about it for now, it seems like an advanced topic. Just pretend chomp chomps off a trailing newline. Honestly, I've never even heard of $/ before.

    As for your other question, it is generally cleaner to always chomp variables and add newlines as needed later, because you don't always know if a variable has a newline or not; by always chomping variables you always get the same behavior. There are scenarios where it is unnecessary, but if you're not sure it can't hurt to chomp it.

    Hope this helps!