Fibonacci numbers becoming negative after a certain term

Question

I wrote this program in Fortran to display the Fibonacci numbers up to the x-th term:

program fibonacci
implicit none
integer :: x,p,c,i,t !initializes limit         


        

Answer 1

I'm no fortran expert, but I did take a class once...

Apparently you're using a four byte integer (http://www-classes.usc.edu/engr/ce/108/text/fbk01.htm) for these. After, 1836311903 you're exceeding the maximum integer value (2147483647) and the calculations are overflowing.

You have two options for calculating fibonacci numbers with more precision. First, you can find a system/fortran compiler combination that support 8 or 16 byte integers. Support in gfortran at least, seems to by system specific. Another option is to use a multiple precision library like gmp.