# exercise 5.1 by erikt(at)science.uva.nl

SOFTWARE

use strict;

# read list numbers from STDIN, one line by one
sub readNumbers {
   my @numbers = ();

   print "Please enter some numbers and finish with an empty line:\n> ";
   while (<STDIN>) {
      my $line = $_;
      chomp($line);
      if ($line eq "") { last; }
      elsif ($line !~ /^\d+$/) { die "illegal input: $line\n"; }
      push(@numbers,$line);
      print "> ";
   }
   return(@numbers);
}

# sort a list with numbers
sub sortList {
   my @numbers = @_;
   my $sorted = 0;
   my $i;

   while (not $sorted) {
      # bubble sort
      $sorted = 1;
      for ($i=0;$i<$#numbers;$i++) {
         if ($numbers[$i] > $numbers[$i+1]) {
            ($numbers[$i],$numbers[$i+1]) = ($numbers[$i+1],$numbers[$i]);
            $sorted = 0;
         }
      }
   }
   return(@numbers);
} 

sub computeMedian {
   my @numbers = @_;

   # return undef for empty list
   if (not @numbers) { return(undef); }
   # sort the numbers: easiest solution: @numbers = sort {$a<=>$b} @numbers
   @numbers = &sortList(@numbers);
   # return the middle number if @numbers has an odd number of elements
   if ($#numbers%2 == 0) { return($numbers[$#numbers/2]); }
   # else return the average of the two center numbers
   else { return(($numbers[($#numbers-1)/2]+$numbers[1+($#numbers-1)/2])/2); }
}

# main part of the code
my @numbers = &readNumbers();
my $median = &computeMedian(@numbers);
if (defined $median) {
   print "The median number of these ",$#numbers+1," numbers is: ",$median,"\n";
}


# TEST

erikt@stuwww:~ perl -w 51.pl
Please enter some numbers and finish with an empty line:
> 3
> 8
> 0
> 2
> 
The median number of these 4 numbers is: 2.5
erikt@stuwww:~ perl -w 51.pl
Please enter some numbers and finish with an empty line:
> 9
> 10
> 2
> 
The median number of these 3 numbers is: 9