C++17 if init for the while loop?

Question

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?

Answer 1

Then what you want is a for loop:

for(double diff = 1; diff > pow(10, -4);)
{
    double h = (b-a)/++n;
    double sum = ntgrt(a, b, n, h);
    diff = abs(sum - sum_analytic);
    ....
}

There was no point in adding this capability to while since you can just use for and leave off the increment statement.

Answer 2

I'd start with some C++98 features to reduce repetition.

template <class I, class F>
void write2file(std::ostream &os, I i, F h, I n, F sum, F diff) {
    using std::setw;

    os  << setw(5) << i
        << setw(15) << h
        << setw(15) << n
        << setw(15) << sum
        << setw(15) << diff
        << "\n";
}

// ...
write2file(file, "i", "h", "n", "sum", "diff");


// ...
write2file(file, ++i, h, n, sum, diff);

It would appear that when they're used, i and n always have the same value. I'd just use one of the two rather than both.

Your pow(10, -4) would probably be better off as 1e-4.

I think some good use of C++98 can probably clean up the loop as well. I'd move it into a function template, something on this general order:

template <typename F>
void show_convergence(double a, double b, F f, double sum_a, std::ostream &os) {
    double diff = 1.0;

    for (int n=1; diff > 1e-4; n++) {
        double h = (b-a)/n;
        double sum = f(a, b, n, h);
        diff = abs(sum-sum_a);
        write2file(os, n, h, sum, diff);
    }
}

Technically, this wouldn't have to be a template--but it's fairly harmless, and avoids the ugliness of a pointer to a function (which can also lead to a fair degree of inefficiency). Then you'd call this something like this:

show_convergence(0, 2, ntgrt, sum_analytic, file);