how to compute the length of a vector code example

Example 1: vector length c++

#include 

int main () {
    std::vector v;
    auto size = v.size();
}

Example 2: vector length formula

||v|| = sqrt(a^2+b^2)

Example 3: finding the size of vector in c++

vector a;
//to directly find the size of the vector;
//use  a.size(;

cout <<" " << a.size();