How to read a file into vector in C++?

Your loop is wrong:

for (int i=0; i=((Main.size())-1); i++) {

Try this:

for (int i=0; i < Main.size(); i++) {

Also, a more idiomatic way of reading numbers into a vector and writing them to stdout is something along these lines:

#include <iostream>
#include <iterator>
#include <fstream>
#include <vector>
#include <algorithm> // for std::copy

int main()
{
  std::ifstream is("numbers.txt");
  std::istream_iterator<double> start(is), end;
  std::vector<double> numbers(start, end);
  std::cout << "Read " << numbers.size() << " numbers" << std::endl;

  // print the numbers to stdout
  std::cout << "numbers read in:\n";
  std::copy(numbers.begin(), numbers.end(), 
            std::ostream_iterator<double>(std::cout, " "));
  std::cout << std::endl;

}

although you should check the status of the ifstream for read errors.

Just to expand on juanchopanza's answer a bit...

for (int i=0; i=((Main.size())-1); i++) {
    cout << Main[i] << '\n';
}

does this:

1. Create i and set it to 0.
2. Set i to Main.size() - 1. Since Main is empty, Main.size() is 0, and i gets set to -1.
3. Main[-1] is an out-of-bounds access. Kaboom.

Just a piece of advice. Instead of writing

for (int i=0; i=((Main.size())-1); i++) {
   cout << Main[i] << '\n';
}

as suggested above, write a:

for (vector<double>::iterator it=Main.begin(); it!=Main.end(); it++) {
   cout << *it << '\n';
}

to use iterators. If you have C++11 support, you can declare i as auto i=Main.begin() (just a handy shortcut though)

This avoids the nasty one-position-out-of-bound error caused by leaving out a -1 unintentionally.

1. In the loop you are assigning value rather than comparing value so

i=((Main.size())-1) -> i=(-1) since Main.size()

Main[i] will yield "Vector Subscript out of Range" coz i = -1.

2. You get Main.size() as 0 maybe becuase its not it can't find the file. Give the file path and check the output. Also it would be good to initialize the variables.