Arguments!

Arguments are important for all programs. Arguments for programs have nothing to do with shouting, but are additional bits of information supplied to the program when it is run. Open a new Perl script (nano arguments.pl) and type this;

$n_arguments = @ARGV;

for ($i = 0; $i < $n_arguments; $i = $i + 1)
{
    print "Argument $i equals $ARGV[$i]\n";
}

Run this script by typing

perl arguments.pl here are some arguments

What do you see? Can you work out what happened?

In this case you passed four arguments to your script; here, are, some and arguments. The Perl interpreter read those arguments and placed them into a special variable called ARGV that you can access from your script.

Because there can be more than one argument, the ARGV variable must be capable of holding more than one value (remember that $a holds just a single value). ARGV must be able to hold multiple values. Arrays are variables that can hold multiple values. An array is identified using an at sign @. I remember the difference between a single variable and an array variable by noticing that $ looks like an S (for single variable), while @ looks like an a (for array variable). @ARGV is therefore an array that holds all of the values of the arguments passed to this script.

The size of an array (the number of values it contains) can be found by typing $size_of_array = @array;, so in this case the number of arguments was found by typing $n_arguments = @ARGV;. You can access an individual value within the array using square brackets, e.g. $array[0] is the first value in the array, $array[1] is the second value etc. (Note that we start counting from zero - the first item is at $array[0] not $array[1]) In the case of our script, we loop over each value in the array @ARGV and print out each value (via $ARGV[$i]).

Exercise

Use the knowledge you’ve gained so far to write a Perl script that can print out any times table. Call your script times_table.pl, and have it read two arguments. The first argument should be the times table to print (e.g. the five times table) while the second should be the highest value of the times table to go up to. So

perl times_table.pl 5 12

should print the five times table from 1 times 5 to 12 times 5.

Answer (don’t peek at this unless you are stuck or until you have finished!)

As an extension, can you think of a way to use arrays to print out the times table using words rather than using numbers? To do this you will need to know that you can assign values to an array using the following syntax;

@a = ( 1, 2, 3, 4, 5 );
@b = ( "cat", "dog", "fish", "bird" );
@c = ( "zero", "one", "two", "three" );

Answer (don’t peek at this unless you are stuck or until you have finished!)

Compare with Python

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