Замена максимального double элемента на СГ полож. элем.

#include <iostream>
#include <windows.h>
#include <ctime>
#include <algorithm>
#include <numeric>
#include <string>

using namespace std;

void set_arr(double *);

void f(double *);

void show(double *);

int main()
{
    SetConsoleCP(1251);
    SetConsoleOutputCP(1251);
    srand(time(NULL));
    system("color 0A");

    auto arr = new double[10u + rand() % 11u];
    set_arr(arr);
    cout << "Исходный массив" << endl;
    show(arr);
    f(arr);
    cout << "Обработанный массив" << endl;
    show(arr);

    system("pause");
    return 0;
}

void set_arr(double *a)
{
    auto len = _msize(a) / (2u * sizeof(a));
    auto filler = []()
    {
        return -10 + 20 * (rand() / static_cast<double>(RAND_MAX));
    };
    generate(a, a + len, filler);
    a[rand() % len] = rand() % 11;
}

void f(double *a)
{
    auto len = _msize(a) / (2u * sizeof(a));
    auto count_positive = 0u;
    auto max = a[0u];
    auto collection = [&count_positive, &max](double p, double t) -> double
    {
        if (t > 0)
        {
            if (t > max)
                max = t;
            ++count_positive;
            return p *= t;
        }
        else
        {
            return p;
        }
    };
    auto p = accumulate(a, a + len, 1., collection);
    auto avg_geom = pow(p, 1. / count_positive);
    auto swp = [max, avg_geom](double &t)
    {
        if (t == max)
        {
            t = avg_geom;
        }
    };
    for_each(a, a + len, swp);
}

void show(double *a)
{
    auto len = _msize(a) / (2u * sizeof(a));
    for (auto it = a; it != a + len; ++it)
    {
        cout << *it << endl;
    }
    cout << endl;
}