Question

I’m starting to write C++ and I’m trying to read a text file that is formatted like this:

32.0      12.43
503.2     3.212

etc.

In Java, I could use a Scanner with .nextFloat(), and put the contains into an Array. I am trying to achieve the same thing with C++. How is this done?

Answer 1

To read from standard input

#include <iostream>

int main()
{
   float f1, f2, f3, f4;
   std::cin >> f1 >> f2 >> f3 >> f4;       
}

To read from a file

#include <fstream>

int main()
{
   std::ifstream fin("file.txt"); //fin is an arbitrary identifier
   float f1, f2, f3, f4;
   fin >> f1;
   fin >> f2;
   fin >> f3;
   fin >> f4;  // or fin >> f1 >> f2 >> f3 >> f4; no difference     
}

To read all floats into an array (vector) of floats, you can use this:

#include <fstream> 
#include <vector>
#include <iterator> //for istream_iterator and back_inserter
#include <algorithm> //for copy

int main()
{
    std::ifstream fin("input.txt");
    std::vector<float> v; //could also initialize with the two-iterator constructor
    std::copy(std::istream_iterator<float>(fin), 
              std::istream_iterator<float>(), 
              std::back_inserter(v)); //push_back into vector
}

Read more about stream input/output here and elsewhere.
I would also strongly advise that you read a good C++ book