Category Archives: perl

Mar 12 2012
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perl

Reading a (text) file line by line with Perl 

#!/usr/bin/perl
 
# Get the filename from the user via command line
$filename = $ARGV[0];
 
# Here we use the open command that reads a file (second parameter)
# and link it to a "handle" (first parameters, conventionally uppercase).
# "||" means "or", that is if open returns false... exit with an error message
open(FILE, "$filename") || die " FATAL ERROR:\n Unable to open \"$file\".\n";
 
# Now we retrieve line by line calling the file handle with < and >.
# Line content is stored in the special scalar "$_"
while (<FILE>) {
  $linecounter++;
  print "$linecounter: $_";
}

to use this script we need to add a parameter to the command line: the filename! eg:

perl script.pl filename.txt

You can find a detailed tutorial on the open command here.

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Mar 11 2012
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perl

Perl variables

In any programming language we need to store information, an to retrieve it when needed. Perl has three types of variables:

  • scalar – they can store numbers or text (strings).
  • array – they are ordered lists of scalar.
  • hashes – they are unordered lists of values with a “label” called name.

Scalar are identified by the ‘$’ character. You can assign a value to a scalar with the ‘=’:

$age = 15;
$name = 'Larry';
$age = $age+1;        # an more compact alternative is $age++, if you have to add 1
$combine1 = "$name is $age";
$combine2 = '$name is $age';
print "Double quotes:  $combine1\n";
print "Sinngle quotes: $combine2\n;"

This small script has two assignation, a number and a string. Then we increment the $age variable by 1.
Note that for numbers we don’t use any quote, while for string we use single quotes or double quotes. The difference – as mentioned – is that double quotes are interpreted and substitute $name with the content of the variable itself.

Arrays are list of strings. To create a new array the syntax is a list of comma separated scalars enclosed by round brackets:

@array = ('first', 'second', 15, 16);

each element of the array has an index starting from 0, thus the last element of the array in the example has index 3. As each element of the array is a scalar, we refer to it with the “$” sign, specifying the index:

$firstElement = $array[0];

A special variable is created by Perl and contains the index of the last element:

$lastindex = $#array;

Thus:

$lastElement = $array[$#array];

Finally hashes are unordered lists of couples of scalar: one is the key and has an associated value.  We’ll see hashes in detail in lesson2.

(in Italian) c’è un buon tutorial su array ed hash in questo sito, valido anche per altri argomenti ovviamente… vi consiglio il corso completo come lettura serale ;).

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Mar 10 2012
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perl, tutorial

Perl: Hello World!

Here you are the code of our very first script we made in lesson1:

#!/usr/bin/perl
 
$yourname = 'George';
print "Hello world!\n";
print "and hello $yourname too!\n";

Remember that if you want to add comments they should be preceded by the # character. The very first line is an instruction for the shell rather than for Perl, but at the moment just remember to add it.

Each instruction has to end with a “;”.

\n is the special way to add a new-line.

Do you remember the difference between double quotes and single quotes?

 

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