Manually calculating norm

Question

I have this code shown below which is supposed to manually calculate the norm of a randomlly generated vector. However, I keep getting the output being printed to the terminal as 0. Why is the case?

#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <ctime>
using namespace std;

double ComputeNorm(const vector<double>& x) {
    double result = 0.0;
    for (auto a : x) {
        double result = result + a*a;
    }
    return sqrt(result);
}


int main() {
    // Declare variables
#if 0
    int n;
    cin >> n
#else
    int n = 1000;
#endif

    if (n == 0) {
        cout << "N must be > 0" << endl;
    }

    vector<double> x(n, 0.0);

    // Seed the random number generate with the current epoch time
    srand(time(0));

    // Generate random numbers and print them to the screen
    generate(x.begin(), x.end(), [] { return (double)rand()/RAND_MAX; });

    // Print out the values computed by BLAS and manually
//    cout << "BLAS Norm:   " << cblas_dnrm2(n, &x[0], 1) << endl;
    cout << "Manual norm: " << ComputeNorm(x) << endl;
//    cout << "x(n): " << x[n] << endl;
    return 0;
}

Answer 1

        double result = result + a*a;

This declares a new variable inside the loop, so the other result variable doesn't change, which is why 0 is returned.
To fix it just do result = result + a*a or result += a*a:

double ComputeNorm(const vector<double>& x) {
    double result = 0.0;
    for (auto a : x) {
        result = result + a*a;
    }
    return sqrt(result);
}

Answer 2

In the for loop in your ComputeNorm function, you have been redeclaring result in every iteration, while the result outside of you loop was never changed.

Instead, you might want result = result + a*a there.

Answer 3

I can't comment because I don't have enough reputation, but what I want to comment is that you are declaring the local variable resulttwice inside the function `ComputeNorm.

That may be the root of your problem.