Question

So I have the code

#include <iostream>
using namespace::std;
int main() {
    int a = 1;
    int b = 1;
    while (a < 100) {
        a = a + b; 
        b = a + b;
        cout << a << endl;
        cout << b << endl;
    }
}

What it does is print the fibonacci sequence up to 100. But, when I make “a” bigger, say, (OVER 9000!!!) 10 billion, it just prints out random numbers it seems. Why does it do this?

Answer 1

This probably happens when you exceed the range of an integer (probably 2,147,483,647 on Windows 32-bit). Try changing the types to long long and see if you get more accurate results to a larger range. Alternatively, you could change add unsigned to the type because the fib. sequence is strictly positive, so that’ll double the range of values.

This is architecture dependent, but using ranges from http://msdn.microsoft.com/en-us/library/s3f49ktz(v=vs.80).aspx, your ranges would be:

int: [–2,147,483,648, 2,147,483,647]
unsigned int: [0, 4,294,967,295]
long long: [–9,223,372,036,854,775,808, 9,223,372,036,854,775,807]
unsigned long long: [0, 18,446,744,073,709,551,615]

Edit: totally off-topic, but thanks for putting me over 1k you glorious voting bastards!