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

Answer 4

The input operator >> is not defined for inputting doubles into a std::vector.
Instead construct the std::vector with two tokenizing input iterators for the input file.
Here's an example of how you can do it by using only 2 lines of code:
std::ifstream inputFile{"/home/shared/data4.txt"};
std::vector<double> rainfall{std::istream_iterator<double>{inputFile}, {}};
Another solution is to define the input operator function as:
std::istream& operator>> (std::istream& in, std::vector<double>& v) {
double d;
while (in >> d) {
v.push_back(d);
}
return in;
}
Then you can use it as in your example:
std::vector<double> rainfall;
inputFile >> rainfall;

Answer 5

Consider using this input reader:
#include <iterator>
#include <algorithm>
#include <vector>
#include <iostream>
#include <fstream>
template<typename T>
std::vector<T> parse_stream(std::istream &stream) {
std::vector<T> v;
std::istream_iterator<T> input(stream);
std::copy(input, std::istream_iterator<T>(), std::back_inserter(v));
return v;
}
int main(int argc, char* argv[])
{
std::ifstream input("/home/shared/data4.txt");
std::vector<int> v = parse_stream<int>(input);
for(auto &item: v) {
std::cout << item << std::endl;
}
return 0;
}
It allows you to use other stream types as well and is generic over type read.