C++17 if init for the while loop?

For some home work I'm doing the following:

int main() {

    ofstream file("log.txt");

    file << setw(5)  <<  "i"
         << setw(15) <<  "h"
         << setw(15) <<  "n"
         << setw(15) <<  "sum"
         << setw(15) <<  "diff"
         << endl;

    auto write2file = [&file](int i, double h, double n, double sum, double diff) {
        file << setw(5)  << i
             << setw(15) << h
             << setw(15) << n
             << setw(15) << sum
             << setw(15) << diff
             << endl;
    };

    double a = 0;
    double b = 2;
    int n = 1;
    double h = (b-a)/n;
    double sum = sum_analytic;
    double diff = 1;

    while (diff > pow(10, -4)) {
        h = (b-a)/++n;
        sum = ntgrt(a, b, n, h);
        diff = abs(sum - sum_analytic);

        static int i = 0;
        write2file(++i, h, n, sum, diff);
    }
}

Considering the C++17 if init feature (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r1.html) I'm kind of seing the same pattern here when I declare h, sum and diff outside the while loop but they are used only inside.

Is there a better way to write it, may be taking advantage of C++17 features?