how to sum up a vector of vector int in C++ without loops

Question

I try to implement that summing up all elements of a vector<vector<int>> in a non-loop ways.
I have checked some relevant questions before, How to sum up elements of a C++ vector?.
So I try to use std::accumulate to implement it but I find it is hard for me to overload a Binary Operator in std::accumulate and implement it.
So I am confused about how to implement it with std::accumulate or is there a better way?
If not mind could anyone help me?
Thanks in advance.

Answer 1

You need to use std::accumulate twice, once for the outer vector with a binary operator that knows how to sum the inner vector using an additional call to std::accumulate:

int sum = std::accumulate(
    vec.begin(), vec.end(),                       // iterators for the outer vector
    0,                                            // initial value for summation - 0
    [](int init, const std::vector<int>& intvec){ // binaryOp that sums a single vector<int>
        return std::accumulate(
            intvec.begin(), intvec.end(), // iterators for the inner vector
            init);                        // current sum
                                          // use the default binaryOp here
    }
);

Answer 2

In this case, I do not suggest using std::accumulate as it would greatly impair readability. Moreover, this function use loops internally, so you would not save anything. Just compare the following loop-based solution with the other answers that use std::accumulate:

int result = 0 ;
for (auto const & subvector : your_vector)
    for (int element : subvector)
        result += element;

Does using a combination of iterators, STL functions, and lambda functions makes your code easier to understand and faster? For me, the answer is clear. Loops are not evil, especially for such simple application.