sub ncr {
    my( $n, $r ) = @_;
    my( $i, $result );
                                                                                                                                                           
    $result = 1;
    for $i ( 1 .. $r ) {
        $result *= $n - $r + $i;
        $result /= $i;
    }
    return $result;
}
                                                                                                                                                           
print "ncr( 6, 3 ) = ", ncr( 6, 3), "\n";
# ncr( 6, 3 ) = 20