Coding

Perl on your Mac part 2

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By Jon Gales

If you missed my last lesson read it before you read this one. If not you have a good change of being lost. If you don't feel 100% about last weeks lesson that is ok, you will after today. The last lesson was a quick into to the basics of Perl, scalar variables and the print function. We will be covering them both in more detail as well as standard input and comments.

Scalar variables:

    • Always marked with a $ before ($scalar_variable)
    • Can contain A-Z, a-z, 0-9, and underscores in the name and must start with a letter.
    • Integers and character strings can be assigned as the value
    • Can be assigned different values in the same script such as:

    $one = 1;
    $one = 2; #$one now contains 2

    • Are case sensitive so $A and $a are two different scalars
    • Can be used in conjunction with each other such as:

    $one = 1;
    $two = 2;
    $answer = $one + $two;

    The operators for Scalar's are as follows:

    Add
    =
    		+
    Subtract
    =
    		-
    Divide
    =
    		/
    Multiply
    =
    		*

    To increment a scalar (has to be a number) add ++ or -- after it like:

    $value = 6;
    $value++; #the ++ increases the 6 to a 7
    print $value; #will print 7

    This is the principal that web counters work on. When a visitor visits the page it triggers a Perl script that reads a number from a file increases it by one prints it to the web and then prints it to file for the next time a hit is occurred.

Print function:

The print function is probably one of the most important things you will need to learn in Perl. Thankfully it is simple. Perl doesn't actually print things (on your printer) it outputs them on screen. To print something just put the following:

print "WHAT YOU WANT TO PRINT";

You can print scalars, strings, numbers, or whole files (you will learn this at a later date). Be careful to quote strings of text (Perl will let you get away without quoting scalar variables but not text strings). Also don't forget that ending semicolon.

Standard Input:

We briefly touched this in our last lesson but not enough to thoroughly understand it. The code for standard input looks like this:

$VARIABLE_NAME = <STDIN>;
chomp($VARIABLE_NAME);

That chomp isn't always needed but is a good idea because sometimes input can get an extra line in it and we don't want that. If there is nothing to chomp it does nothing. This code picks up whatever was typed in and dumps it into the $VARIABLE_NAME variable.

Comments:

Comments are pieces of text that are solely meant for human readers of the code. The computer automatically ignores them. In Perl they are marked by a pound sign #. Everything after a pound sign is ignored. You have to comment every line (there are no multi-line comments in Perl like there are in C, PHP, and many other languages). Commenting is a really really really really (did I make my point?) good idea. Even if you code looks good after your 5th Mountain Dew at 3 a.m. I promise you it won't look good when your boss is looking over your shoulder at 9 a.m. The first line of every Perl script is a comment but this one isn't ignored, it tells the comp where to find Perl.

A simple program using: Scalar Variables, Print Function and Standard Input and comments is below:

#!/usr/bin/perl
print "How old are you? "; #prints question
$age = <STDIN>; #puts response into scalar
chomp($age);#get rid of that new line
$age_days = $age * 365; #times age by number of days in a year
print "You are approximately $age_days days old "; #print answer

If you are in X make the code a text file ending in .pl and chmod it to 755. Then you can run it by typing the path to file in the terminal.

 

Homework:
Your homework is to mess with the X terminal. Practice writing stuff in pico and chmoding it and moving around the directory. Here are a few commands to get you started:

• ls lists current directory
• cd changes directory (cd usr moves you to the usr folder, cd / always moves you back to the root directory).

If you are on Classic you don't have any homework except to get OS X as soon as you can. It is the future and kicks butt for Perl!

If you are having trouble or would like to ask me a question please send me mail: jonknee@macmerc.com or chat through AIM: jonknee41 (add to buddy list)   


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