Beginner's Introduction to Perl - Part 2

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Table of Contents

Part 1 of this series
Part 3 of this series
Part 4 of this series
Part 5 of this series
Part 6 of this series

Comparison operators
while and until
String comparisons
More fun with strings
Filehandles
Writing files
Live free or die!
Subs
Putting it all together
Play around!

In our last article, we talked about the core elements of Perl: variables (scalars, arrays, and hashes), math operators and some basic flow control (the for statement). Now it's time to interact with the world.

In this installment, we're going to discuss how to slice and dice strings, how to play with files and how to define your own functions. But first, we'll discuss one more core concept of the Perl language: conditions and comparisons.

Comparison operators

There's one important element of Perl that we skipped in the last article: comparison operators. Like all good programming languages, Perl allows you ask questions such as ``Is this number greater than that number?'' or ``Are these two strings the same?'' and do different things depending on the answer.

When you're dealing with numbers, Perl has four important operators: <, >, == and !=. These are the ``less than,'' ``greater than,'' ``equal to'' and ``not equal to'' operators. (You can also use <=, ``less than or equal to,'' and >=, ``greater than or equal to.)

You can use these operators along with one of Perl's conditional keywords, such as if and unless. Both of these keywords take a condition that Perl will test, and a block of code in curly brackets that Perl will run if the test works. These two words work just like their English equivalents - an if test succeeds if the condition turns out to be true, and an unless test succeeds if the condition turns out to be false:

    if ($year_according_to_computer == 1900) {
        print "Y2K has doomed us all!  Everyone to the compound.\n";
    }

    unless ($bank_account > 0) {
        print "I'm broke!\n";
    }

Be careful of the difference between = and ==! One equals sign means ``assignment'', two means ``comparison for equality''. This is a common, evil bug:

    if ($a = 5) {
        print "This works - but doesn't do what you want!\n";
    }

Instead of testing whether $a is equal to five, you've made $a equal to five and clobbered its old value. (In a later article, we'll discuss a way to make sure this bug won't occur in running code.)

Both if and unless can be followed by an else statement and code block, which executes if your test failed. You can also use elsif to chain together a bunch of if statements:

    if ($a == 5) {
        print "It's five!\n";
    } elsif ($a == 6) {
        print "It's six!\n";
    } else {
        print "It's something else.\n";
    }

    unless ($pie eq 'apple') {
        print "Ew, I don't like $pie flavored pie.\n";
    } else {
        print "Apple!  My favorite!\n";
    }

while and until

Two slightly more complex keywords are while and until. They both take a condition and a block of code, just like if and unless, but they act like loops similar to for. Perl tests the condition, runs the block of code and runs it over and over again for as long as the condition is true (for a while loop) or false (for a until loop).

Take a look at the following code and try to guess what it will do before reading further:

   $a = 0;

   while ($a != 3) {
       $a++;
       print "Counting up to $a...\n";
   }

   until ($a == 0) {
       $a--;
       print "Counting down to $a...\n";
   }

Here's what you see when you run this program:

    Counting up to 1...
    Counting up to 2...
    Counting up to 3...
    Counting down to 2...
    Counting down to 1...
    Counting down to 0...

String comparisons

So that's how you compare numbers. Now, what about strings? The most common string comparison operator is eq, which tests for string equality - that is, whether two strings have the same value.

Remember the pain that is caused when you mix up = and ==? Well, you can also mix up == and eq. This is one of the few cases where it does matter whether Perl is treating a value as a string or a number. Try this code:

    $yes_no = "no";
    if ($yes_no == "yes") {
        print "You said yes!\n";
    }

Why does this code think you said yes? Remember that Perl automatically converts strings to numbers whenever it's necessary; the == operator implies that you're using numbers, so Perl converts the value of $yes_no (``no'') to the number 0, and ``yes'' to the number 0 as well. Since this equality test works (0 is equal to 0), the if block gets run. Change the condition to $yes_no eq "yes", and it'll do what it should.

Things can work the other way, too. The number five is numerically equal to the string " 5 ", so comparing them to == works. But when you compare five and " 5 " with eq, Perl will convert the number to the string "5" first, and then ask whether the two strings have the same value. Since they don't, the eq comparison fails. This code fragment will print Numeric equality!, but not String equality!:

    $a = 5;
    if ($a == " 5 ") { print "Numeric equality!\n"; }
    if ($a eq " 5 ") { print "String equality!\n"; }

More fun with strings

You'll often want to manipulate strings: Break them into smaller pieces, put them together and change their contents. Perl offers three functions that make string manipulation easy and fun: substr(), split() and join().

If you want to retrieve part of a string (say, the first four characters or a 10-character chunk from the middle), use the substr() function. It takes either two or three parameters: the string you want to look at, the character position to start at (the first character is position 0) and the number of characters to retrieve. If you leave out the number of characters, you'll retrieve everything up to the end of the string.

    $a = "Welcome to Perl!\n";
    print substr($a, 0, 7);     # "Welcome"
    print substr($a, 7);        # " to Perl!\n"

A neat and often-overlooked thing about substr() is that you can use a negative character position. This will retrieve a substring that begins with many characters from the end of the string.

     $a = "Welcome to Perl!\n";
     print substr($a, -6, 4);      # "Perl"

(Remember that inside double quotes, \n represents the single new-line character.)

You can also manipulate the string by using substr() to assign a new value to part of it. One useful trick is using a length of zero to insert characters into a string:

    $a = "Welcome to Java!\n";
    substr($a, 11, 4) = "Perl";   # $a is now "Welcome to Perl!\n";
    substr($a, 7, 3) = "";        #       ... "Welcome Perl!\n";
    substr($a, 0, 0) = "Hello. "; #       ... "Hello. Welcome Perl!\n";

Next, let's look at split(). This function breaks apart a string and returns a list of the pieces. split() generally takes two parameters: a regular expression to split the string with and the string you want to split. (We'll discuss regular expressions in more detail in the next article; for the moment, we're only going to use a space. Note the special syntax for a regular expression: / /.) The characters you split won't show up in any of the list elements.

    $a = "Hello. Welcome Perl!\n";
    @a = split(/ /, $a);   # Three items: "Hello.", "Welcome", "Perl!\n"

You can also specify a third parameter: the maximum number of items to put in your list. The splitting will stop as soon as your list contains that many items:

    $a = "Hello. Welcome Perl!\n";
    @a = split(/ /, $a, 2);   # Two items: "Hello.", "Welcome Perl!\n";

Of course, what you can split, you can also join(). The join() function takes a list of strings and attaches them together with a specified string between each element, which may be an empty string:

    @a = ("Hello.", "Welcome", "Perl!\n");
    $a = join(' ', @a);       # "Hello. Welcome Perl!\n";
    $b = join(' and ', @a);   # "Hello. and Welcome and Perl!\n";
    $c = join('', @a);        # "Hello.WelcomePerl!\n";

Filehandles

Enough about strings. Let's look at files - after all, what good is string manipulation if you can't do it where it counts?

To read from or write to a file, you have to open it. When you open a file, Perl asks the operating system if the file can be accessed - does the file exist if you're trying to read it (or can it be created if you're trying to create a new file), and do you have the necessary file permissions to do what you want? If you're allowed to use the file, the operating system will prepare it for you, and Perl will give you a filehandle.

You ask Perl to create a filehandle for you by using the open() function, which takes two arguments: the filehandle you want to create and the file you want to work with. First, we'll concentrate on reading files. The following statement opens the file log.txt using the filehandle LOGFILE:

    open (LOGFILE, "log.txt");

Opening a file involves several behind-the-scenes tasks that Perl and the operating system undertake together, such as checking that the file you want to open actually exists (or creating it if you're trying to create a new file) and making sure you're allowed to manipulate the file (do you have the necessary file permissions, for instance). Perl will do all of this for you, so in general you don't need to worry about it.

Once you've opened a file to read, you can retrieve lines from it by using the <> construct. Inside the angle brackets, place the name of your filehandle. What is returned by this depends on what you want to get: in a scalar context (a more technical way of saying ``if you're assigning it to a scalar''), you retrieve the next line from the file, but if you're looking for a list, you get a list of all the remaining lines in the file. (One common trick is to use for $lines (<FH>) to retrieve all the lines from a file - the for means you're asking a list.)

You can, of course, close a filehandle that you've opened. You don't always have to do this, because Perl is clever enough to close a filehandle when your program ends or when you try to reuse an existing filehandle. It's a good idea, though, to use the close statement. Not only will it make your code more readable, but your operating system has built-in limits on the number of files that can be open at once, and each open filehandle will take up valuable memory.

Here's a simple program that will display the contents of the file log.txt, and assumes that the first line of the file is its title:

    open (LOGFILE, "log.txt") or die "I couldn't get at log.txt";
    # We'll discuss the "or die" in a moment.

    $title = <LOGFILE>;
    print "Report Title: $title";
    for $line (<LOGFILE>) {
        print $line;
    }
    close LOGFILE;

Writing files

You also use open() when you are writing to a file. There are two ways to open a file for writing: overwrite and append. When you open a file in overwrite mode, you erase whatever it previously contained. In append mode, you attach your new data to the end of the existing file without erasing anything that was already there.

To indicate that you want a filehandle for writing, you put a single > character before the filename you want to use. This opens the file in overwrite mode. To open it in append mode, use two > characters.

     open (OVERWRITE, ">overwrite.txt") or die "$! error trying to overwrite";
     # The original contents are gone, wave goodbye.

     open (APPEND, ">>append.txt") or die "$! error trying to append";
     # Original contents still there, we're adding to the end of the file

Once our filehandle is open, we can use the humble print statement to write to it. Specify the filehandle you want to write to and a list of values you want to write:

    print OVERWRITE "This is the new content.\n";
    print APPEND "We're adding to the end here.\n", "And here too.\n";

Live free or die!

You noticed that most of our open() statements are followed by or die "some sort of message". This is because we live in an imperfect world, where programs don't always behave exactly the way we want them to. It's always possible for an open() call to fail; maybe you're trying to write to a file that you're not allowed to write, or you're trying to read from a file that doesn't exist. In Perl, you can guard against these problems by using or and and.

A series of statements separated by or will continue until you hit one that works, or returns a true value. This line of code will either succeed at opening OUTPUT in overwrite mode, or cause Perl to quit:

    open (OUTPUT, ">$outfile") or die "Can't write to $outfile: $!";

The die statement ends your program with an error message. The special variable $! contains Perl's explanation of the error. In this case, you might see something like this if you're not allowed to write to the file. Note that you get both the actual error message (``Permission denied'') and the line where it happened:

    Can't write to a2-die.txt: Permission denied at ./a2-die.pl line 1.

Defensive programming like this is useful for making your programs more error-resistant - you don't want to write to a file that you haven't successfully opened!

Here's an example: As part of your job, you write a program that records its results in a file called vitalreport.txt. You use the following code:

    open VITAL, ">vitalreport.txt";

If this open() call fails (for instance, vitalreport.txt is owned by another user who hasn't given you write permission), you'll never know it until someone looks at the file afterward and wonders why the vital report wasn't written. (Just imagine the joy if that ``someone'' is your boss, the day before your annual performance review.) When you use or die, you avoid all this:

    open VITAL, ">vitalreport.txt" or die "Can't write vital report: $!";

Instead of wondering whether your program wrote your vital report, you'll immediately have an error message that both tells you what went wrong and on what line of your program the error occurred.

You can use or for more than just testing file operations:

    ($pie eq 'apple') or ($pie eq 'cherry') or ($pie eq 'blueberry')
        or print "But I wanted apple, cherry, or blueberry!\n";

In this sequence, if you have an appropriate pie, Perl skips the rest of the chain. Once one statement works, the rest are ignored. The and operator does the opposite: It evaluates your chain of statements, but stops when one of them doesn't work.

   open (LOG, "log.file") and print "Logfile is open!\n";

This statement will only show you the words Logfile is open! if the open() succeeds - do you see why?

Subs

So far, our Perl programs have been a bunch of statements in series. This is OK if you're writing very small programs, but as your needs grow, you'll find it's limiting. This is why most modern programming languages allow you to define your own functions; in Perl, we call them subs.

A sub is defined with the sub keyword, and adds a new function to your program's capabilities. When you want to use this new function, you call it by name. For instance, here's a short definition of a sub called boo:

    sub boo {
        print "Boo!\n";
    }

    boo();   # Eek!

(Older versions of Perl required that you precede the name of a sub with the & character when you call it. You no longer have to do this, but if you see code that looks like &boo in other people's Perl, that's why.)

Subs are useful because they allow you to break your program into small, reusable chunks. If you need to analyze a string in four different places in your program, it's much easier to write one &analyze_string sub and call it four times. This way, when you make an improvement to your string-analysis routine, you'll only need to do it in one place, instead of four.

In the same way that Perl's built-in functions can take parameters and can return values, your subs can, too. Whenever you call a sub, any parameters you pass to it are placed in the special array @_. You can also return a single value or a list by using the return keyword.

    sub multiply {
        my (@ops) = @_;
        return $ops[0] * $ops[1];
    }

    for $i (1 .. 10) {
         print "$i squared is ", multiply($i, $i), "\n";
    }

Why did we use the my keyword? That indicates that the variables are private to that sub, so that any existing value for the @ops array we're using elsewhere in our program won't get overwritten. This means that you'll evade a whole class of hard-to-trace bugs in your programs. You don't have to use my, but you also don't have to avoid smashing your thumb when you're hammering nails into a board. They're both just good ideas.

You can also use my to set up local variables in a sub without assigning them values right away. This can be useful for loop indexes or temporary variables:

    sub annoy {
        my ($i, $j);
        for $i (1 .. 100) {
            $j .= "Is this annoying yet?\n";
        }
        print $j;
    }

If you don't expressly use the return statement, the sub returns the result of the last statement. This implicit return value can sometimes be useful, but it does reduce your program's readability. Remember that you'll read your code many more times than you write it!

Putting it all together

At the end of the first article we had a simple interest calculator. Now let's make it a bit more interesting by writing our interest table to a file instead of to the screen. We'll also break our code into subs to make it easier to read and maintain.

[Download this program]

        #!/usr/local/bin/perl -w



        # compound_interest_file.pl - the miracle of compound interest, part 2


        # First, we'll set up the variables we want to use.
        $outfile = "interest.txt";  # This is the filename of our report.
        $nest_egg = 10000;          # $nest_egg is our starting amount
        $year = 2000;               # This is the starting year for our table.
        $duration = 10;             # How many years are we saving up?
        $apr = 9.5;                 # This is our annual percentage rate.


        &open_report;
        &print_headers;
        &interest_report($nest_egg, $year, $duration, $apr);
        &report_footer;


        sub open_report {
            open (REPORT, ">$outfile") or die "Can't open report: $!";
        }


        sub print_headers {
            # Print the headers for our report.
            print REPORT "Year", "\t", "Balance", "\t", "Interest", "\t",
                         "New balance", "\n";
        }


        sub calculate_interest {
            # Given a nest egg and an APR, how much interest do we collect?
            my ($nest_egg, $apr) = @_;


            return int (($apr / 100) * $nest_egg * 100) / 100;
        }


        sub interest_report {
            # Get our parameters.  Note that these variables won't clobber the
            # global variables with the same name.
            my ($nest_egg, $year, $duration, $apr) = @_;


            # We have two local variables, so we'll use my to declare them here.
            my ($i, $line);


            # Calculate interest for each year.
            for $i (1 .. $duration) {
                $year++;
                $interest = &calculate_interest($nest_egg, $apr);


                $line = join("\t", $year, $nest_egg, $interest,
                             $nest_egg + $interest) . "\n";


                print REPORT $line;

                $nest_egg += $interest;
            }
        }

        sub report_footer {
            print REPORT "\n Our original assumptions:\n";
            print REPORT "   Nest egg: $nest_egg\n";
            print REPORT "   Number of years: $duration\n";
            print REPORT "   Interest rate: $apr\n";

            close REPORT;
        }

Notice how much clearer the program logic becomes when you break it down into subs. One nice quality of a program written as small, well-named subs is that it almost becomes self-documenting. Take a look at these four lines from our program:

     open_report;
     print_headers;
     interest_report($nest_egg, $year, $duration, $apr);
     report_footer;

Code like this is invaluable when you come back to it six months later and need to figure out what it does - would you rather spend your time reading the entire program trying to figure it out or read four lines that tell you the program 1) opens a report file, 2) prints some headers, 3) generates an interest report, and 4) prints a report footer?

You'll also notice we use my to set up local variables in the interest_report and calculate_interest subs. The value of $nest_egg in the main program never changes. This is useful at the end of the report, when we output a footer containing our original assumptions. Since we never specified a local $nest_egg in report_footer, we use the global value.

Play around!

In this article, we've looked at files (filehandles, open(), close(), and <>), string manipulation (substr(), split() and join()) and subs. Here's a pair of exercises - again, one simple and one complex:

  • You have a file called dictionary.txt that contains dictionary definitions, one per line, in the format ``word space definition''. (Here's a sample.) Write a program that will look up a word from the command line. (Hints: @ARGV is a special array that contains your command line arguments and you'll need to use the three-argument form of split().) Try to enhance it so that your dictionary can also contain words with multiple definitions in the format ``word space definition:alternate definition:alternate definition, etc...''.

  • Write an analyzer for your Apache logs. You can find a brief description of the common log format at http://www.w3.org/Daemon/User/Config/Logging.html. Your analyzer should count the total number of requests for each URL, the total number of results for each status code and the total number of bytes output.

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