Input file not being read from input.txt file - c++

Question

#include <bits/stdc++.h>
#include <iostream>
#include <fstream>
using namespace std;
typedef long long int ll;
int main()
{
istream in("input.txt", "r", stdin);
ofstream out("output.txt", "w", stdout);
int t;
in >> t;
while (t--)
{
ll n;
in >> n;
ll arr[n];
for (int i = 0; i < n; i++)
{
in >> arr[i];
}
sort(arr, arr + n);
double sum = 0;
for (int i = 0; i < n - 1; i++)
{
sum += arr[i];
}
double ans = (sum / (n - 1)) + arr[n - 1];
out << setprecision(9) << ans << endl;
}
return 0;
}
Note that I also changed my tasks.json file.
I changed my code according to what #john and #PepjinKramer said, but my inputs are not being read from the input.txt file and so nothing is shown in output.txt.
How do I solve this problem?

Answer 1

Some feedback to show how to reduce the "cargo cult" content of much C++ code out there :
// #include <bits/stdc++.h> <== NO not standard C++
#include <algorithm> // for sort
#include <iostream>
#include <fstream>
#include <vector> // for resizable arrays
#include <numeric> // for accumulate
#include <iomanip> // for setprecission
// using namespace std; <== NO will become a problem in big projects (nameclashes0
// typedef long long int ll; <== NO just use standard types in code e.g. std::size_t or std::int64_t
int main()
{
//istream in("input.txt", "r", stdin); // reading from file doesn't need anything from stdin
//ofstream out("output.txt", "w", stdout); // writing to a file doesn't need anything from stdout
// std::ifstream input{ "input.txt" };
// std::ofstream output{ "output.txt" };
// for testing manual input
auto& input = std::cin;
auto& output = std::cout;
std::size_t number_of_averages; // give variables more meaningful names then just 't'
std::cout << "input number of averages to calculate = ";
input >> number_of_averages;
for(std::size_t n = 0; n < number_of_averages; ++n )
{
//ll n; NO sizes of containters should be std::size_t not long long
//ll n;
//in >> n;
std::size_t number_of_values;
std::cout << "input number of values = ";
input >> number_of_values;
// ll arr[n]; // NO C++ doesn NOT have variable length arrays, use std::vector
std::vector<double> values(number_of_values);
// for (int i = 0; i < n; i++) // Do NOT use index based loops if you don't have to (out of bound bugs)
//{
// use range based for loops and learn about (const) references
for(auto& value : values)
{
std::cout << "input number : ";
input >> value;
}
// sort(arr, arr + n); // No use iteraters
std::sort(values.begin(), values.end());
// NO for summing collections use <algorithm> accumulate
/*
double sum = 0;
for (int i = 0; i < n - 1; i++)
{
sum += arr[i];
}
*/
auto sum = std::accumulate(values.begin(), values.end(), 0.0);
auto average = sum / static_cast<double>(values.size());
std::cout << "average = ";
output << std::setprecision(9) << average << std::endl;
}
return 0;
}