Perl 5 Babysteps 02 - User Input

Having a program that displays the exact same message every time you run is nice when it comes to being consistent, but not so entertaining as a program. “What does it do?” “It prints out my name.” “Oh.” Let’s make things a little more interesting. We could change the value of $name in the code, but it might be a little tiresome to do this before showing it to each new person. How about making the program ask for a name? User interaction - a neat idea.

It’s so neat that I’m going to show you two ways to do it.

Traditional

Here’s how I got user input before Perl 5.10. You’ll find a lot of code that looks like this.

=pod

=head1 hello.pl

Displays a warm message.

=cut

# Follow some common-sense guidelines for Perl coding.
use Modern::Perl;

print "What is your name? ";
my $name = <STDIN>;
chomp($name);
say "Hello, $name!";

Why don’t we run this before we start trying to decipher it?

$ perl hello.pl
What is your name? Brian[ENTER]
Hello, Brian!

There is a lot going on in the few lines we added. Let’s look at them one by one.

print "What is your name? ";

print is how we used to get messages displayed on the console. It’s still useful when you want to say something but you don’t want to start a whole new line when it’s done. We use print so that our answer to the question will display on the same line as the question itself. That is a common approach to user prompts.

my $name = <STDIN>;

We’ve got some funny-looking thing instead of “Brian”. What is it? Here’s the short explanation:

<STDIN> gets input from the user that you can save in a variable.

There’s also a long version. Feel free to skip it if you just want to get on with it. The long version works out like this:

The scalar $name is assigned the value of <STDIN>. <...> tells Perl that we want it to read a filehandle and hand the results back to us. A filehandle is a source of information. It could be an open file, but in this case it is STDIN. STDIN is the standard input stream - geek talk for “wherever we expect user input to be coming from.” STDIN usually just means “keys the user enters from the keyboard.” The result of reading from the filehandle - which is STDIN in this case, which is the keys you entered from the keyboard in this case - is stored in $name.

There’s a really long version, but I’m getting bored so we’ll skip it.

chomp($name);

chomp removes the last character from a string if that character is a newline - whatever a newline is defined as on your platform. Why do we need that? Well, when Perl reads with the <...> operator, it gives you everything. That includes the [ENTER] key that you pressed to send your name to the program. What that ENTER actually looks like depends on your platform: but the end result is a line break in the text. Sometimes we like that, but not today. Here’s what you would see if you left out chomp:

$ perl hello.pl
What is your name? Brian[ENTER]
Hello, Brian
!
$

You can see that you’re going to use chomp a lot when getting user input. But what if there were an easier way? There is … sort of.

A new way

The new way looks better to my eyes, but I’m willing to admit that there’s a lot going on for a beginner. You can feel free to ignore this section if you like, and my feelings won’t be hurt at all.

=pod

=head1 hello.pl

Displays a warm message.

=cut

# Follow some common-sense guidelines for Perl coding.
use Modern::Perl;

use Term::UI;
use Term::ReadLine;

my $console = Term::ReadLine->new();
my $name    = $console->get_reply( prompt => "What is your name? " );
say "Hello, $name!";

The program is doing nearly the same thing as before, but the code looks a lot different. We use some new stuff, added a new variable, and have changed the way we get the user’s name. On the other hand, we don’t have to chomp anything.

Looking at the changes

use Term::UI;
use Term::ReadLine;

These lines tell Perl to load a couple of modules for some additional functionality. This is different from when we called use before. With Modern::Perl we were giving Perl a new personality. This time we’re just making some new functions available. Module names are usually capitalized like proper names, and that :: is conventionally used to indicate related modules within a category.

Why do we want those modules? So we can create an object that reads user input.

my $console = Term::ReadLine->new();
my $name    = $console->get_reply( prompt => "What is your name? " );

There are a lot of things to learn in these two lines of code, so please forgive me if I rush through them too quickly.

What’s an object? Oh boy. That is a tricky question for a beginner tutorial. It’s so tricky that I’ve decided to skip it completely. Almost completely. Objects are basically magic scalar variables that hold extra information such as data (the fields) and subroutines (the methods). Intelligent use of objects allow you to quickly write powerful programs while hiding the complexity of what’s going on behind a sweet and smiling face.

Classes define the structure of a particular type of object. You create an object with a constructor method defined in the class. The constructor is usually called new, and you access it with the -> operator. The same operator is used to access the methods of your $console object.

The last source of confusion in this new example is the way we asked $console to present its prompt to the user. The quick way to think of it is that we are giving a dictionary to get_reply. get_reply looks at this dictionary for keywords that are important to it, such as prompt. We use that => operator to tell $console->get_reply that we want the prompt to be “What is your name? "

I’ll get more into dictionaries later, but that’s the basic idea for now. Wait, one more thing. It’s helpful to think of those special keyword lookup collections as dictionaries, but they’re really called hashes.

Enough babbling. Let’s run it.

$ perl hello.pl
What is your name? Brian
Hello, Brian!

Nice, it adds some formatting to the process! Well, it does for me. You might not get the formatting if your console doesn’t support it.

We only added a few lines to our program, but it made a significant difference in the end. Maybe you don’t understand what is going on behind those changes. That’s okay. Perl is a strange language, full of things that are very easy and fairly challenging at the same time. Give yourself time to learn the language and explore its features. Definitely explore the available libraries, because you will be amazed by how much you can improve your programs. And I don’t forget to explore CPAN, the gigantic repository of libraries for Perl.