exc bad instruction - Simple Swift Fibonacci program crashing (Project Euler 2)

Question

I am trying to solve the second problem on Project Euler. The problem is as follows:

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Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be: 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ... By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

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I think I've written a solution, but when I try to run my code it crashes my Swift playground and gives me this error message:

Playground execution aborted: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

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var prev = 0
var next = 1
var num = 0
var sum = 0

for var i = 1; i < 400; i++ {
    num = prev + next
    if next % 2 == 0 {
        sum += next
    }
    prev = next
    next = num
}
print(sum)

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The weird thing is, if I set the counter on my loop to less than 93, it works fine. Explicitly setting the variable names to Double does not help. Anyone know what's going on here?

Answer 1

There is nothing weird about this at all. Do you know how large the 400 fibonacci number is?

176023680645013966468226945392411250770384383304492191886725992896575345044216019675

Swift Int64 or UInt64 simply cannot handle that large of a number. The later can go up to 18446744073709551615 at max - not even close.

If you change your variables to be doubles it works but will be inaccurate:

var prev : Double = 0
var next : Double = 1
var num : Double = 0
var sum : Double = 0

will yield

2.84812298108489e+83

which is kind of close to the actual value of

1.76e+83

Luckily you do not need to get values that big. I would recommend not writing a for loop but a while loop that calculates the next fibonacci number until the break condition is met whose values do not exceed four million.