Question

I’m new to c++ and programming in general. Feel free to leave any hints, tips, or suggestions about my code!!

I am trying to copy my doubles from a vector to an array. I used copy() and it is copying all the elements except for the last. So the last element in the array stays exactly the same as it was before I used copy. So when I try to add the sums of the elements in the array, I don’t get the correct sum.

Here is my code:

vector<double> myVector; 
double myArray[4];
double myDouble = 0.0;

    ...//(add elements to vector)

    copy(&myVector[0], &myVector[4], myArray);

for(int i = 0; i < 4; i++)
{
    myDouble += myArray[i];
    if(i == 4)
        cout << "The sum of your values is " << fixed << setprecision(2) << myDouble << endl;
}

Thank you! Let me know if I need to be more specific.

Answer 1

The call to std::copy() copies four elements. This seems to fill your array. Note that your array contains exactly 4 elements and the last valid index is 3. Your vector myVector seems to include at least 5 elements: If it contains less then 5 elements, the expression myVector[4] is illegal. That said, the end iterator in a sequence always refers to the element behind the last value of the sequence, i.e., sequences are half-open: the begin is included, the end is the first element not included.

That said, you probably want to copy like this:

std::copy(myVector.begin(), std::min(4, myVector.size()) + myVector.begin(), myArray);

Of course, to get the sum of the elements in the vector you would actually use

double sum = std::accumulate(myVector.begin(), myVector.end(), 0.0);