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Homework Help: Project Euler Problem 002: Fibonacci sequence (in C++)

  1. May 5, 2012 #1
    1. The problem statement, all variables and given/known data
    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.

    2. Relevant equations

    3. The attempt at a solution
    #include <cstdlib>
    #include <iostream>
    using namespace std;

    int main()

    int n;
    int fibnum;
    int sum;
    int num2;
    int num1;
    int limit;

    num2 = 1;
    num1 = 0;
    fibnum = 0;
    sum = 0;

    cout << "n?" << endl;
    cin >> limit;

    for(n = 0; n < limit; n += 1)
    fibnum = num2 + num1;
    if(fibnum % 2 == 0)
    sum += fibnum;

    num1 = num2;
    num2 = fibnum;


    cout << sum << endl;

    return EXIT_SUCCESS;


    This chunk of code allows me to get the correct sum when "limit" is a relatively small number (e.g. 10 will return 44 and 5 will return 10), but doesn't work for really big numbers...like 4,000,000. I'm not looking for a solution; I know that if my algorithm were designed correctly I would have already gotten the correct answer, but clearly there is some glaring flaw that I'm not noticing. Am I on the right track with this method at least, or am I wasting my time beating my head over my desk with this approach?
  2. jcsd
  3. May 5, 2012 #2


    User Avatar

    Staff: Mentor

    What do you mean by "doesn't work"? What's happening?

    Have you checked how large Fibonacci numbers get?
    Last edited: May 5, 2012
  4. May 5, 2012 #3
    When I punch in sufficiently large number for the program, I get clearly bogus results like -98123786458 for example; this problem does not arise with numbers that are not particularly big. And yes I have checked how large Fibonacci numbers get O_O.
  5. May 5, 2012 #4


    User Avatar

    Staff: Mentor

    You are using either a 32 or 64 bit integers, are they sufficiently large to store Fibonacci numbers? How many bits needed to store 4000000th Fibonacci number?
  6. May 5, 2012 #5
    It asks for the sum of every part of the sequence that does not exceed 4 million. It doesn't ask for the sum of the first 4 million in the sequence.

    For example, if I had a sequence
    s_n = 2^n, n from 0 to infinity

    then the sum of the first 5 would be
    1 + 2 + 4 + 8 + 16
    whereas the sum of all values that do not exceed five in the sequence would be
    1 + 2 + 4

    Do you see the difference?

    In this circumstance, you should probably look into doing a while loop since you do not know exactly how many iterations you will need to do.

    On a side note, you can initialize a value to a variable when you declare it, e.g.
    int num2 = 1;

    And to answer the question of why your code fails with such an insane limit, take a look at this:
    So you plug in n+1 to see what the nth fib is. Let's plug in 100...
    The answer is 573147844013817084101

    Your finite-sized integers are overflowing. They simply cannot hold a number that large.

    Let's see what Wolfram says 4,000,000 is:
    2.63331342366565668198735005895985564737121509 × 10^835950
    Last edited: May 5, 2012
  7. May 6, 2012 #6
  8. May 8, 2012 #7
    Code (Text):
    for n=0 ... 4000000
        if is.even( fibonacci(n) )
            s += fibonacci(n)
    print s
    [tex]\approx 5.404785360620536653971914908\cdot 10 ^{1204119}[/tex]
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