Understanding std::accumulate

Question

I want to know why std::accumulate (aka reduce) 3rd parameter is needed. For those who do not know what accumulate is, it's used like so:

vector<int> V{1,2,3};  
int sum = accumulate(V.begin(), V.end(), 0);
// sum == 6

Call to accumulate is equivalent to:

sum = 0;  // 0 - value of 3rd param
for (auto x : V)  sum += x;

There is also optional 4th parameter, which allow to replace addition with any other operation.

Rationale that I've heard is that if you need let say not to add up, but multiply elements of a vector, we need other (non-zero) initial value:

vector<int> V{1,2,3};
int product = accumulate(V.begin(), V.end(), 1, multiplies<int>());

But why not do like Python - set initial value for V.begin(), and use range starting from V.begin()+1. Something like this:

int sum = accumulate(V.begin()+1, V.end(), V.begin());

This will work for any op. Why is 3rd parameter needed at all?

Answer 1

You're making a mistaken assumption: that type T is of the same type as the InputIterator.

But std::accumulate is generic, and allows all different kinds of creative accumulations and reductions.

Example #1: Accumulate salary across Employees

Here's a simple example: an Employee class, with many data fields.

class Employee {
/** All kinds of data: name, ID number, phone, email address... */
public:
 int monthlyPay() const;
};

You can't meaningfully "accumulate" a set of employees. That makes no sense; it's undefined. But, you can define an accumulation regarding the employees. Let's say we want to sum up all the monthly pay of all employees. std::accumulate can do that:

/** Simple class defining how to add a single Employee's
 *  monthly pay to our existing tally */
auto accumulate_func = [](int accumulator, const Employee& emp) {
   return accumulator + emp.monthlyPay();
 };

// And here's how you call the actual calculation:
int TotalMonthlyPayrollCost(const vector<Employee>& V)
{
 return std::accumulate(V.begin(), V.end(), 0, accumulate_func);
}

So in this example, we're accumulating an int value over a collection of Employee objects. Here, the accumulation sum isn't the same type of variable that we're actually summing over.

Example #2: Accumulating an average

You can use accumulate for more complex types of accumulations as well - maybe want to append values to a vector; maybe you have some arcane statistic you're tracking across the input; etc. What you accumulate doesn't have to be just a number; it can be something more complex.

For example, here's a simple example of using accumulate to calculate the average of a vector of ints:

// This time our accumulator isn't an int -- it's a structure that lets us
// accumulate an average.
struct average_accumulate_t
{
    int sum;
    size_t n;
    double GetAverage() const { return ((double)sum)/n; }
};

// Here's HOW we add a value to the average:
auto func_accumulate_average = 
    [](average_accumulate_t accAverage, int value) {
        return average_accumulate_t(
            {accAverage.sum+value, // value is added to the total sum
            accAverage.n+1});      // increment number of values seen
    };

double CalculateAverage(const vector<int>& V)
{
    average_accumulate_t res =
        std::accumulate(V.begin(), V.end(), average_accumulate_t({0,0}), func_accumulate_average)
    return res.GetAverage();
}

Example #3: Accumulate a running average

Another reason you need the initial value is because that value isn't always the default/neutral value for the calculation you're making.

Let's build on the average example we've already seen. But now, we want a class that can hold a running average -- that is, we can keep feeding in new values, and check the average so far, across multiple calls.

class RunningAverage
{
    average_accumulate_t _avg;
public:
    RunningAverage():_avg({0,0}){} // initialize to empty average

    double AverageSoFar() const { return _avg.GetAverage(); }

    void AddValues(const vector<int>& v)
    {
        _avg = std::accumulate(v.begin(), v.end(), 
            _avg, // NOT the default initial {0,0}!
            func_accumulate_average);
    }

};

int main()
{
    RunningAverage r;
    r.AddValues(vector<int>({1,1,1}));
    std::cout << "Running Average: " << r.AverageSoFar() << std::endl; // 1.0
    r.AddValues(vector<int>({-1,-1,-1}));
    std::cout << "Running Average: " << r.AverageSoFar() << std::endl; // 0.0
}

This is a case where we absolutely rely on being able to set that initial value for std::accumulate - we need to be able to initialize the accumulation from different starting points.

---

In summary, std::accumulate is good for any time you're iterating over an input range, and building up one single result across that range. But the result doesn't need to be the same type as the range, and you can't make any assumptions about what initial value to use -- which is why you must have an initial instance to use as the accumulating result.