fibonacci - Why does (int)55 == 54 in C++?

Question

So I'm learning C++. I've got my "C++ Programming Language" and "Effective C++" out and I'm running through Project Euler. Problem 1...dunzo. Problem 2...not so much. I'm working in VS2008 on a Win32 Console App.

Whats the Sum of all even terms of the Fibonacci Sequence under 4 million?

It wasn't working so I cut down to a test case of 100...

Here's what I wrote...

// Problem2.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
{
    cout << "Project Euler Problem 2:

";
    cout << "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:

";
    cout << "1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...

";
    cout << "Find the sum of all the even-valued terms in the sequence which do not exceed four million.

";
    cout << "Answer:  " << Solve();
}

double Solve() {
    int FibIndex = 0;
    double result = 0.0;
    double currentFib = GenerateNthFibonacciNumber(FibIndex);
    while (currentFib < 100.0){
        cout << currentFib << " " << (int)currentFib << " " << (int)currentFib % 2 << "
";
        if ((int)currentFib % 2 == 0){
            result += currentFib;
            cout<<(int)currentFib;
        }
        currentFib = GenerateNthFibonacciNumber(++FibIndex);
    }
    return result;
}

double GenerateNthFibonacciNumber(const int n){
    //This generates the nth Fibonacci Number using Binet's Formula
    const double PHI = (1.0 + sqrt(5.0)) / 2.0;
    return ((pow(PHI,n)-pow(-1.0/PHI,n)) / sqrt(5.0));
}

And here's the output...

Project Euler Problem 2:

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, ...

Find the sum of all the even-valued terms in the sequence which do not exceed four million.

0 0 0
1 1 1
1 1 1
2 2 0
3 3 1
5 5 1
8 8 0
13 13 1
21 21 1
34 34 0
55 54 0
89 89 1
Answer: 99

So I have three columns of debug code...the number returned from the generate function, (int)generatedNumber, and (int)generatedNumber % 2

So on the 11th term we have

55,54,0

Why does (int)55 = 54?

Answer 1

Casting to int truncates the number - same as if you'd called floor(currentFib). So even if currentFib is 54.999999... (a number so close to 55 that it will be rounded up when printed), (int)currentFib will produce 54.