C++ Multiplying elements in a vector

Question

I have been looking for a more optimal solution to the following and I cannot seem to find one.

Let's say I have a vector:

std::vector<double> vars = {1, 2, 3}

I want to perform 1 * 2 * 3 I know that I can do the following:

int multi = 1;

for(int i = 0; (i < vars.size()-1); i++)
{
    multi *= vars[i];
}

But, is there a more "C++11" way to do this? I really wanted to do this using lambda and so that I can calculate the multiply (product) of the vector without having another function inside the class, I'd rather have it calculated inside the function.

Answer 1

Yes, as usual, there is an algorithm (though this one's in <numeric>), std::accumulate (live example):

using std::begin;
using std::end;
auto multi = std::accumulate(begin(vars), end(vars), 1, std::multiplies<double>());

std::multiplies is in <functional>, too. By default, std::accumulate uses std::plus, which adds two values given to operator(). std::multiplies is a functor that multiplies them instead.

In C++14, you can replace std::multiplies<double> with std::multiplies<>, whose operator() is templated and will figure out the type. Based on what I've seen with Eric Niebler's Ranges proposal, it could possibly later look like vars | accumulate(1, std::multiplies<>()), but take that with a grain of salt.

Answer 2

You can use a ranged based for loop like:

std::vector<double> vars = {1, 2, 3}
int multi = 1;

for (const auto& e: vars)
    multi *= e;