Cosine Similarity of Two Vectors

#include <iostream>
#include <vector>
#include <cmath>
#include <numeric>

template <class T>
double VectorCosine(const std::vector<T> &In1, const std::vector<T> &In2) {
    if(In1.size() != In2.size()) {
        return -2;
    }
    double la=sqrt(inner_product(In1.begin(), In1.end(), In1.begin(), 0));
    double lb=sqrt(inner_product(In2.begin(), In2.end(), In2.begin(), 0));
    double val=inner_product(In1.begin(), In1.end(), In2.begin(), 0);

    if((std::abs(la-0.0)<0.0001) || std::abs(lb-0.0)<0.0001)
        return -3;

    return val/(la*lb);
}

int main() {
    int a[]={3, 2, 0, 5, 0, 0, 0, 2, 0, 0}, b[]={1, 0, 0, 0, 0, 0, 0, 1, 0, 2};
    std::vector<int> v_a(a, a+sizeof(a)/sizeof(a[0])), v_b(b, b+sizeof(b)/sizeof(b[0]));

    double s=VectorCosine(v_a, v_b);

    std::cout<<s<<std::endl;

    return 0;
}

Cosine similarity really is a measure of the(cosine of the) angle between x and y. Thus, if the cosine similarity is 1,

the angle between x and y is 0, and x and y are the same except for magnitude(length). If the cosine similarity is 0,

then the angle between x and y is 90, and they do not share any terms.