Multiple function calls return different results for same input arguments

Question

#include <iostream>
#include <cmath>
#include <numeric>
#include <vector>
#include <algorithm>

bool isPointWithinSphere(std::vector<int> point, const double &radius) {

    std::transform(point.begin(), point.end(), point.begin(), [](auto &x)    {return std::pow(x,2);});

    return std::sqrt(std::accumulate(point.begin(), point.end() + 1, 0,     std::plus<int>())) <= radius;   
}


int countLatticePoints(std::vector<int> &point, const double &radius, const     int &dimension, int count = 0) {


     for(int i = -(static_cast<int>(std::floor(radius))); i <= static_cast<int>(std::floor(radius)); i++) {
        point.push_back(i);

        if(point.size() == dimension){
            if(isPointWithinSphere(point, radius)) count++;
        }else count = countLatticePoints(point, radius, dimension, count);

        point.pop_back();
    }

    return count;
}

MAIN

int main() {
std::vector<int> vec {};
std::cout << countLatticePoints(vec, 2.05, 2) << std::endl;
std::cout << countLatticePoints(vec, 1.5, 3) << std::endl;
std::cout << countLatticePoints(vec, 25.5, 1) << std::endl;
std::cout << countLatticePoints(vec, 2.05, 2) << std::endl;
}

The above program run returns the following results:

I'm trying to understand why my first function call using the same input parameters returns 13 (correct answer) as the result, yet when I call the function again at a later time with the same exact input arguments, I get 9 as the answer ?

Can't think of any reason this should happen.

The Dark · Accepted Answer · 2016-05-20 01:07:55Z

2

std::accumulate works from [first, last). This means it does not include last, so that it is easy to read an entire collection. You dont want to use point.end() + 1 as that means it will try to process point.end().

Doing so means you are reading outside of the vector boundaries and causes undefined behavior.

Change the line to

return std::sqrt(std::accumulate(point.begin(), point.end(), 0,     std::plus<int>())) <= radius;

answered May 20, 2016 at 1:07

The Dark

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