Learning Perl with Testing

If you want to learn Perl, you will find many tutorials which teach you how to program. And then, when you want to learn testing, you will find other tutorials, which teach you only how to test.

Eventually, you will want to do test driven programming where you write code and tests and the same time.

Much better to learn to code and test at the same time right from the beginning.

Here is an example of how to do it in Perl.

binary.pl

#!/usr/bin/perl
# Recursion - convert decimal number to binary
# from Higher-Order Perl: Transforming Programs with Programs
sub binary {
  my $n = shift;

  return $n if $n == 0 || $n ==1;

  my $k = int($n / 2);
  my $b = $n % 2;

  my $E = binary($k);
  return $E . $b;
}

binary.t

require "binary.pl";
use Test::More;

is( binary(0), 0, "0 : 0" );
is( binary(1), 1, "1 : 1" );
is( binary(2), 10, "2 : 10" );
is( binary(3), 11, "3 : 11" );
is( binary(4), 100, "4 : 100" );
is( binary(5), 101, "5 : 101" );
is( binary(6), 110, "6 : 110" );
is( binary(7), 111, "7 : 111" );
is( binary(37), 100101, "37 : 100101" );

done_testing;

You can run it to see all the tests passing.
ok 1 - 0 : 0
ok 2 - 1 : 1
ok 3 - 2 : 10
ok 4 - 3 : 11
ok 5 - 4 : 100
ok 6 - 5 : 101
ok 7 - 6 : 110
ok 8 - 7 : 111
ok 9 - 37 : 100101
ok 10 - 0 : 0
ok 11 - 1 : 1
ok 12 - 2 : 10
ok 13 - 3 : 11
ok 14 - 4 : 100
ok 15 - 5 : 101
ok 16 - 6 : 110
ok 17 - 7 : 111
ok 18 - 37 : 100101

This tests that it works. Now, we think about how to break it. Lets add tests to see what our function does if we give it a negative number or a string as an input.
Lets assume that the function should return a 0 if the input is incorrect.

is( binary("a"), 0, "a : 0" );
is( binary(-1), 0, "-1 : 0" );

You see that the function fails like this:

not ok 19 - a : 0
# Failed test 'a : 0'
# at C:\Documents and Settings\others\My Documents\perl_stuff\recursion\dec_to
_bin.pl line 53.
# got: 'a'
# expected: '0'
not ok 20 - -1 : 0
# Failed test '-1 : 0'
# at C:\Documents and Settings\others\My Documents\perl_stuff\recursion\dec_to
_bin.pl line 54.
# got: '01'
# expected: '0'
1..20
# Looks like you failed 2 tests of 20.

So, you change your binary function to return a 0 if the input is not a positive integer, and you are done.

sub binary {
my $n = shift;

return 0 unless $n =~ /\d+/ && $n >= 0;
return $n if $n == 0 || $n ==1;

my $k = int($n / 2);
my $b = $n % 2;

my $E = binary($k);
return $E . $b;
}