Tinkering with Perl

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Friends and pets

Here, we will have a program that asks you about you friends and pets, and remembers them for as long as it's running. Let me give a listing. I am trying to document it well by putting in comments. Can you tell what it does? If you can't, look up the parts you can't understand.

#!/usr/bin/perl -w
#
# Friends and pets -- a program to keep track of your friends and their pets.
#
# Last modified 12-29-01, by Jonathan Hayward
#
# While you say you want to continue, it asks you for another friend's name,
# and then asks for the pet's name.  Then, it tells you all of the friends it's
# been told, and what their pets are.
#
# This is done with a while loop.
# The friends are stored in a list, and
# the pets are stored in a hash.
#
# We also use a couple of functions.
#

#
# The stuff it does in the beginning, called initialization.
#

# List the subroutines and functions.
sub Confirm();

#
# Confirm is a function that gets a yes or no answer from the user.
#
# Note that this function uses another function -- ReadLine.  It is
# possible, and indeed very useful, to have one subroutine or function use
# other subroutines or functions.
#

sub Confirm()
    {
    $UserAnswer = &ReadLine();
    if ((not($UserAnswer eq "n")) and # If it's not a yes or a no,
      (not($UserAnswer eq "y")) and
      (not($UserAnswer eq "N")) and
      (not($UserAnswer eq "Y")))
        {
        while ((not($UserAnswer eq "n")) and # Keep going until we get a yes
          (not($UserAnswer eq "y")) and      # or a no.
          (not($UserAnswer eq "N")) and
          (not($UserAnswer eq "Y")))
            {
            print "Please answer \"y\", for yes, or \"n\", for no.\n";
	    $UserAnswer = &ReadLine();
            }
        }
    # If we've gotten here, the user has given a yes or a no answer.
    return (($UserAnswer eq "Y") or ($UserAnswer eq "y"));
    }

# Make the list of friends and hash of pets empty, so that they don't contain
# anything.
@Friends = ();
%Pets = ();

# Have scalars that we can use for true and false conditional clauses.
$True = 1;
$False = 0;

# Welcome the user to the program.
print "Welcome to the friends and pets program.\n";

$ShouldContinue = $True;

#
# The main loop.  This is where the meat of the program is.
#

while ($ShouldContinue)
    {

    # Read in the friend's name.
    print "Please enter the name of your friend:\n";
    $NewFriend = &ReadLine();

    # Add the new friend to the list.
    @Friends = (@Friends, $NewFriend);

    # Read in the friend's pet.
    print "Please enter the kind of pet $NewFriend has:\n";
    $NewPet = &ReadLine();

    # Add the new friend's pet to the list of hashes.
    $Pets{$NewFriend} = $NewPet;

    # Print a blank line, so that the output doesn't look too crowded:
    print "\n";

    # Now, recite the friends and their pets.
    foreach $CurrentFriend (@Friends)
	{
	$CurrentPet = $Pets{$CurrentFriend};
	print $CurrentFriend ."'s pet is a $CurrentPet.\n";
	}

    # Finally, ask the user if he wants to continue.
    print "Do you want to continue (y/n)?\n";
    $ShouldContinue = &Confirm();

    # And we reach the end of the loop.
    }


# If we get here in the program, the user does not want to continue.
# So, we say "Bye!", and leave.

print "Bye!\n";
exit 0;

#
# The program will never get here by itself, because it is after the exit
# statement.  But we can still put procedures and functions here.  We will put
# two functions here:
#

#
# ReadLine is a function that reads a line in, and gets rid of the trailing
# newline.  This does input exactly as specified earlier.
#

sub ReadLine()
    {
    $UserInput = <>;
    chomp $UserInput;
    return $UserInput;
    }

A sample output for this program might be:

Welcome to the friends and pets program.
Please enter the name of your friend:
Fred
Please enter the kind of pet Fred has:
furball

Fred's pet is a furball.
Do you want to continue (y/n)?
y
Please enter the name of your friend:
David
Please enter the kind of pet David has:
dog

Fred's pet is a furball.
David's pet is a dog.
Do you want to continue (y/n)?
n
Bye!

Tinkering with Perl

Friends and pets

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