read integers from a file into a vector in C++

Question

I am trying to read an unknown number of double values stored on separate lines from a text file into a vector called rainfall. My code won't compile; I am getting the error no match for 'operator>>' in 'inputFile >> rainfall' for the while loop line. I understand how to read in from a file into an array, but we are required to use vectors for this project and I'm not getting it. I appreciate any tips you can give on my partial code below.

vector<double> rainfall;    // a vector to hold rainfall data

// open file    
ifstream inputFile("/home/shared/data4.txt");

// test file open   
if (inputFile) {
    int count = 0;      // count number of items in the file

    // read the elements in the file into a vector  
    while ( inputFile >> rainfall ) {
        rainfall.push_back(count);
        ++count;
    }

    // close the file

Answer 1

I think you should store it in a variable of type double. Seems you are doing >> to a vector, which is not valid. Consider the following code:

// open file    
ifstream inputFile("/home/shared/data4.txt");

// test file open   
if (inputFile) {        
    double value;

    // read the elements in the file into a vector  
    while ( inputFile >> value ) {
        rainfall.push_back(value);
    }

// close the file

As @legends2k points out, you don't need to use the variable count. Use rainfall.size() to retrieve the number of items in the vector.

Answer 2

You cannot use >> operator to read in the whole vector. You need to read one item at a time, and push it into the vector:

double v;
while (inputFile >> v) {
    rainfall.push_back(v);
}

You do not need to count the entries, because rainfall.size() will give you the exact count.

Finally, the most C++ -ish way of reading a vector is with istream iterators:

// Prepare a pair of iterators to read the data from cin
std::istream_iterator<double> eos;
std::istream_iterator<double> iit(inputFile);
// No loop is necessary, because you can use copy()
std::copy(iit, eos, std::back_inserter(rainfall));

Answer 3

You could also do:

#include <algorithm>
#include <iterator>

...
std::istream_iterator<double> input(inputFile);
std::copy(input, std::istream_iterator<double>(),    
          std::back_inserter(rainfall));
...

assuming you like the STL.