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POWhy does (int)55 == 54 in C++?
 

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  1. POWhy does (int)55 == 54 in C++?
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    <p>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.</p> <p>Whats the Sum of all even terms of the Fibonacci Sequence under 4 million? </p> <p>It wasn't working so I cut down to a test case of 100...</p> <p>Here's what I wrote...</p> <pre><code>// Problem2.cpp : Defines the entry point for the console application. // #include "stdafx.h" using namespace std; int _tmain(int argc, _TCHAR* argv[]) { cout &lt;&lt; "Project Euler Problem 2:\n\n"; cout &lt;&lt; "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:\n\n"; cout &lt;&lt; "1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...\n\n"; cout &lt;&lt; "Find the sum of all the even-valued terms in the sequence which do not exceed four million.\n\n"; cout &lt;&lt; "Answer: " &lt;&lt; Solve(); } double Solve() { int FibIndex = 0; double result = 0.0; double currentFib = GenerateNthFibonacciNumber(FibIndex); while (currentFib &lt; 100.0){ cout &lt;&lt; currentFib &lt;&lt; " " &lt;&lt; (int)currentFib &lt;&lt; " " &lt;&lt; (int)currentFib % 2 &lt;&lt; "\n"; if ((int)currentFib % 2 == 0){ result += currentFib; cout&lt;&lt;(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)); } </code></pre> <p>And here's the output...</p> <blockquote> <p>Project Euler Problem 2:</p> <p>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:</p> <p>1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...</p> <p>Find the sum of all the even-valued terms in the sequence which do not exceed four million.</p> <p>0 0 0 <br> 1 1 1 <br> 1 1 1 <br> 2 2 0<br> 3 3 1<br> 5 5 1<br> 8 8 0<br> 13 13 1<br> 21 21 1<br> 34 34 0<br> 55 54 0<br> 89 89 1<br> Answer: 99<br></p> </blockquote> <p>So I have three columns of debug code...the number returned from the generate function, (int)generatedNumber, and (int)generatedNumber % 2</p> <p>So on the 11th term we have</p> <p>55,54,0</p> <p>Why does (int)55 = 54?</p> <p>Thanks</p>
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    1. USJason Punyon
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    1. COFWIW, the Fibonacci sequence usually starts with 0. 0,1,1,2,3,5,...
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      1. USScottie T
    2. COThat's Project Euler's fault, I agree with you. The algorithm I wrote does generate the leading 0,1. But this doesn't cause any problem with the answer (being that its looking for the sum of even terms)
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      1. USJason Punyon
    3. COSince the double type has inherently limited precision you should be careful about using it for things like this because Binet's Formula uses infinite precision to get the result.
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