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 Brett Trost 01-22-2004 03:22 AM

Fibonacci problem

OK, I wanted to make a recursive fibonacci method in Perl, and I can't
understand why it is not working, especially since I wrote the exact
same thing in Java and it works. Here's the perl code:

#!/usr/bin/perl
sub fib {
\$num = shift;
return 0 if \$num == 1;
return 1 if \$num == 2;
return fib(\$num - 1) + fib(\$num - 2);
}

print fib(\$ARGV[0]);

And here's the Java code:

class fib
{
public static void main(String[] args)
{
System.out.println(fib(Integer.parseInt(args[0])));
}

static int fib(int x) {
if (x == 1) return 0;
if (x == 2) return 1;
return fib(x - 1) + fib(x - 2);
}
}

The perl one keeps on running forever. If I print out \$num, it goes
rapidly into the negatives. Why is this, when the base case should
take care of that, and the Java code has the exact same logic and
works?! It must be an error with my Perl, but I can't find it.
Thanks for any help.

 Jürgen Exner 01-22-2004 04:13 AM

Re: Fibonacci problem

Brett Trost wrote:
> OK, I wanted to make a recursive fibonacci method in Perl, and I can't
> understand why it is not working, especially since I wrote the exact
> same thing in Java and it works. Here's the perl code:
>
> #!/usr/bin/perl

There is a "use strict;" missing here ...

> sub fib {
> \$num = shift;

....which would have told you that \$num requires a declaration.
You are using the _global_ variable \$num, not a variable that is local to
the sub.

> return 0 if \$num == 1;
> return 1 if \$num == 2;
> return fib(\$num - 1) + fib(\$num - 2);
> }

jue

 lesley_b_linux@yahoo.co.yuk 01-22-2004 02:04 PM

Re: Fibonacci problem

btrost@shaw.ca (Brett Trost) writes:

> OK, I wanted to make a recursive fibonacci method in Perl, and I can't
> understand why it is not working, especially since I wrote the exact
> same thing in Java and it works. Here's the perl code:
>
> #!/usr/bin/perl
> sub fib {
> \$num = shift;
> return 0 if \$num == 1;
> return 1 if \$num == 2;
> return fib(\$num - 1) + fib(\$num - 2);
> }
>
> print fib(\$ARGV[0]);
>

<snip>

> The perl one keeps on running forever. If I print out \$num, it goes
> rapidly into the negatives. Why is this, when the base case should
> take care of that, and the Java code has the exact same logic and
> works?! It must be an error with my Perl, but I can't find it.
> Thanks for any help.

I put in

#! /usr/bin/perl -w

at the start and debugged ....
../test.pl 8
Deep recursion on subroutine "main::fib" at ./test.pl line 7.

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

and using

my \$num = shift;

gives the right results.

hth

Lesley

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