C++ Multiplying elements in a vector - c++

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;

Answer 3

Another way is using inclusive_scan (C++17 and above)
The advantage is you can get multiplies of first "N" elements in a vector.
Below is the code. Explanation in comments.
#include <iostream>
#include <iterator>
#include <numeric>
#include <vector>
int main()
{
//INPUT VECTOR
std::vector<int> data{ 3, 1, 4, 1, 5, 9, 2, 6 };
//OUTPUT VECTOR WITH MULTIPLIES
//FIRST ELEMENT - 3
//SECOND ELEMENT - 3 * 1
//THIRD ELEMENT - 3 * 1 * 4
//FOURTH ELEMENT - 3 * 1 * 4 * 1
// ..
// ..
//LAST ELEMENT - 3 * 1 * 4 * 1 * 5 * 9 * 2 * 6
std::vector<int> multiplies(data.size());
//Multiply all numbers in given vector.
inclusive_scan(data.begin(), data.end(),
multiplies.begin(),
std::multiplies<>{});
//PRODUCT OF FIRST 5 ELEMENTS.
std::cout << "Product of first 5 elements :: " << multiplies[4] << std::endl;
//PRODUCT OF ALL ELEMENTS
std::cout << "Product of all elements :: " << multiplies[data.size() - 1] << std::endl;
}
Also there is an overload where the execution policy can be specified.
Sequential execution or Parallel execution. Need to include "execution" header.
//MULTIPLY ALL NUMBERS IN VECTOR WITH PARALLEL EXECUTION.
inclusive_scan(std::execution::par,data.begin(), data.end(),
multiplies.begin(),
std::multiplies<>{});