File handling in Perl through a simple sort example

When it comes to text parsing, I always stick with Perl for a simple and fast implementation. Often referred as “the Swiss Army chainsaw of scripting languages”, Perl can be very handy to interpret plain text, logs and even perform model transformations.

The common usage often requires reading an writing to files, which is merely a recipe widespread around the internet. So, as you can guess, the content below is not new at all, but my intention is to keep a reference of syntax and common features for my own future usage.

The example below is quite simple: a Perl script process an input file which contains a sequence of integers CSV format, and writes an output file with the ordered sequence of those numbers.

So, starting from the input parameters of the Perl script and the creation of a handler:

# checking for two parameters: input and output files
if ($#ARGV != 1) {
	print "usage: ./SortNumbers <input_file> <out_file>\n";
	exit;
}

# getting input parameters into variables
my $input_file         = shift(@ARGV);
my $output_file        = shift(@ARGV);

# filenames can be easily printed as follows
print "Input file is: $input_file\n";
print "Output file is: $output_file\n";

# trying to open the input file
# if the input file cannot be read, the application exits
open FILE, "< $input_file" or die $!;

# reading file (multiple lines allowed)
my $input_data;
while (<FILE>) {
	$input_data.=$_;
}

# close input reader
close FILE;

Input parameters are contained in the @ARGV array. So, the first line checks if the index of the last position of this array ($#ARGV) is equals to 1, that is, the array @ARGV has exactly two values. The shift(@array) function then pulls the first value out of the array.

Afterwards, the script tries to open the file handler FILE for the given filename in $input_file. If the attempt fails, the script exits with an errorcode and message. Otherwise, the content of the input file is concatenated into $input_data variable before the file handler is closed.

The block below starts with a function call &sort_input() which will be briefly discussed at the end.

# call a subroutine to validate the input, split and sort
my $output_data = &sort_input($input_data);

# open output file
open OUTPUT, "> $output_file";

# write to output file
print OUTPUT "$output_data\n";

# close output writer
close OUTPUT;

As seen above, the process of writing to file is very similar to reading. The operator > indicates that the content will be overwritten into the output file, while >> would concatenate instead.

Finally the subroutine sort_input() is shown below:

# subroutine to validate the input, split and sort values
sub sort_input {

	my $input_data = $_[0];

	# remove endlines and carriage return
	$input_data =~ tr/\r//d;
	$input_data =~ tr/\n//d;

	# check if the input data contains only numbers and single commas
	# if not, the application exits
	if ($input_data !~ /^\d+(,\d+)*$/) {
		print "Error: invalid input file.\n";
		exit;
	}

	# split the input where there are commas and store into array
	my @input_array = split(',', $input_data);

	# sort values from the array
	my @sorted_array = sort {$a <=> $b} @input_array;

	# join values placing comma in between them
	return join(",", @sorted_array);
}

@_ hold the parameters of the sort_input function. After, the function shift(@_) could be used instead of $_[0] to get the first value from the @_ array. They are just different ways of obtaining values from an array, being the first one destructive while the last one keeps the original array.

The content of the $input_data is validated throught the regular expression /^\d+(,\d+)*$/: the value should contain only integers with a single comma in between.

The function split() is self-explanatory as well as its counter-operation join(). The sort() function sorts the elements of a given array alphabetically. Therefore, the operator <=> forces the sort function to perform a numerical comparison. If this operator is omitted, the sort function would place 12345 before 23, for instance.

The complete code above can be placed in a single .pl file. An example of execution is shown below:

$ echo "1,23,44,567,32,12,1,7,90,123,1024" > input.csv

$ ./SortNumbers.pl input.csv output.csv
Input file is: input.csv
Output file is: output.csv

$ cat output.csv
1,1,7,12,23,32,44,90,123,567,1024

More complete examples of Perl scripts can be found in my repository.