How to sum some float

[mnm71]

In this code I want to read last n lines that contain float numbers and get an average of them, I get right float but the result of sum and average is not true.

#include <iostream>
using std::cout;
#include <fstream>
using std::ifstream;
#include <string>
using std::string;
using std::getline;
size_t count_lines(const char *filename)
{
   ifstream myfile("test.txt");
   string line;
   size_t count = 0;
   while ( getline(myfile, line) )
   {
      ++count;
   }
   return count;
}
int main()
{
   const char filename[] = "test.txt";
size_t i, count = count_lines(filename);
   ifstream myfile(filename);
   string line;
   for ( i = 0; i < count - 10; ++i )
   {
      getline(myfile, line); /* read and discard: skip line */
   }
   while ( getline(myfile, line) )
   {
      cout << line << "\n";
       cl =stof(line);   //The conversion works properly
       cl +=cl;     // The sum does not work properly
       cl = cl /10.0 //The avarage does not work properly
       cout<< "avarage:" << cl <<endl;
   }
   return 0;
}

[seeplus]

#include <iostream>
#include <fstream>
#include <vector>
#include <numeric>

int main()
{
	const char filename[] {"test.txt"};
	const int last {10};

	std::ifstream myFile(filename);

	if (!myFile.is_open())
		return (std::cout << "Cannot open file\n"), 1;

	std::vector<float> data;

	for (float f {}; myFile >> f; data.push_back(f));

	if (data.size() < last)
		return (std::cout << "Insufficient numbers\n"), 2;

	std::cout << "Average: " << std::accumulate(data.crbegin(), data.crbegin() + last, 0.0) / last;
}