using std::accumulate, getting a "too many arguments" error - c++

Question

std::accumulate is supposed to be able to take either three or four arguments. In the former case it's just when you want to add the numbers in a container; in the latter case it's when you want to first apply a function and then add them. I've written code that generates a vector of random doubles and then does some stuff to them: first it performs an x->x^2 transform using std::transform, then adds them up with std::accumulate, and lastly combines the two actions into one using the four-argument version of std::accumulate.
Everything works except for step 3. Looking at the example code to be found at http://www.cplusplus.com/reference/numeric/accumulate/, I can't see any reason why this shouldn't work, but I'm getting a "Too many arguments error" when compiling (I'm using XCode. For some reason it doesn't tell me the line number, but I've narrowed it down to the second usage of std::accumulate). Any insights?
#include <numeric>
#include <time.h>
#include <math.h>
using std::vector;
using std::cout;
using std::endl;
double square(double a) {
return a*a;
}
void problem_2_1() {
vector<double> original;
//GENERATE RANDOM VALUES
srand((int)time(NULL));//seed the rand function to time
for (int i=0; i<10; ++i) {
double rand_val = (rand() % 100)/10.0;
original.push_back(rand_val);
cout << rand_val << endl;
}
//USING TRANSFORM
vector<double> squared;
squared.resize(original.size());
std::transform(original.begin(), original.end(), squared.begin(), square);
for (int i=0; i<original.size(); ++i) {
std::cout << original[i] << '\t' << squared[i] << std::endl;
}
//USING ACCUMULATE
double squaredLength = std::accumulate(squared.begin(), squared.end(), 0.0);
double length = sqrt(squaredLength);
cout << "Magnitude of the vector is: " << length << endl;
//USING 4-VARIABLE ACCUMULATE
double alt_squaredLength = std::accumulate(original.begin(), original.end(), 0.0, square);
double alt_length = sqrt(alt_squaredLength);
cout << "Magnitude of the vector is: " << alt_length << endl;
}

Answer 1

The fourth argument to that std::accumulate overload needs to be a binary operator. Currently you are using a unary one.
std::accumulate performs a binary operation between successive elements in a container, hence the need for a binary operator. The fourth argument replaces the default binary operation, addition. It does not apply a unary operation and then perform addition. If you want to square the elements and then add them, you would need something like
double addSquare(double a, double b)
{
return a + b*b;
}
Then
double x = std::accumulate(original.begin(), original.end(), 0.0, addSquare);