Question

#!/usr/bin/perl -w
use strict;

sub fib {
    my($num) = @_;  #give $num to input array
    return(1) if ($num<=1);  #termination condition
    return($num = &fib($num-1) + &fib($num-2));  #should return sum of first "n" terms in the fibonacci sequence
}

print &fib(7)."\n";  #should output 20

This subroutine should be outputting a summation of the first “x” amount of terms, as specified by the argument to the sub. However, it’s one too high. Does this have something to do with the recursion?

Thanks.

Answer 1

The Fibonacci sequence starts with f(0) = 0 and f(1) = 1. After that each Fibonacci number is the sum of the previous two.

Your function uses return (1) if ($num <= 1) which incorrectly evaluates f(0) as 1. If you change this to return $num if $num <= 1 then your sequence will start correctly.

This code outputs the first eleven numbers in the series.

use strict;
use warnings;

sub fib {
  my ($num) = @_;
  if ($num <= 1) {
    return $num;
  }
  else {
    return fib($num-1) + fib($num-2);
  }
}


print join ' ', map fib($_), 0 .. 10;

output

0 1 1 2 3 5 8 13 21 34 55