Why is this vector not populating with values? [closed] - c++

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This function is intended to take a vector with x,y value pairs of the form {x1, x2, x3, y1, y2, y3} and return a vector with the values shuffled as such {x1, y1, x2, y2, x3, y3}. The size of the vector is 2*n, where n is the number of x values/y values respectively. By using print statements inside the function, I've already determined that the algorithm works.
vector<int> shuffle(vector<int>& nums, int n) {
vector<int> temp;
temp.reserve(2*n);
int xCounter = 0;
int yCounter = n;
for (int i=0; i<2*n; i+=2){
// populate arr x val
temp[i] = (nums[xCounter]);
// populate arr y val
temp[i+1] = (nums[yCounter]);
++xCounter;
++yCounter;
}
return temp;
}
int main()
{
vector<int> yoMomma = {1,2,3,1,2,3};
vector<int> ans;
ans = shuffle(yoMomma,yoMomma.size()/2);
return 0;
}

Answer 1

You called reserve, not resize, so your vector isn't actually increased in size, just capacity (pre-allocated memory that can hold elements in the future without having to reallocate). But that makes all your indexing assignments illegal; the vector still has a length of 0, you can't assign to any index. It happens to work because the reserve ensures there is memory there, but said memory would never be properly destructed (so it's a good thing you're using primitive types without destructors).
Change your loop to:
for (int i=0; i<n; ++i){
// populate arr x val
temp.push_back(nums[i]);
// populate arr y val
temp.push_back(nums[i+n]);
}
pushing new values onto the end, and saving a couple unnecessary variables, rather than indexing.