Reading from file into string while counting the lines C++ - c++

Question

I am trying to read a file into a string array. But I want to do it as if I do not know what the length of the document is. so I want to get a while loop to count the lines, and then another one to read the document.
When I do this, it works fine but it assumes I know what the length is going to be for the size of the arrays.
string count_lines;//dummy string to read the line temp
string votes[11];
string ID[11];
string whole_line[11];
int i = 0;
while (getline(file, count_lines))
{
whole_line[i] = count_lines;
ID[i].assign(count_lines, 0, 4);
votes[i].assign(count_lines, 6, 4);
cout << count_lines << endl;
i++;
}
But i tried to do this variation but it just prints blank lines with the same function as i printed the option above
string count_lines;//dummy string to read the line temp
string votes[11];
string ID[11];
string whole_line[11];
int i = 0;
while (getline(file, count_lines))
{
i++;
}
int k = 0;
while (getline(file, count_lines) && k < i)
{
whole_line[k] = count_lines;
ID[k].assign(count_lines, 0, 4);
votes[k].assign(count_lines, 6, 4);
cout << count_lines << endl;
i++;
}
I am not sure what i'm doing wrong.

Answer 1

Each call to std::geline (as well as << operator and read method) advances input position stored in the stream object. In the first while loop, you read the entire file, so after this loop, input position indicator points to the end of the file.
In order to start reading from the beginning in the second loop, you have to reset the position back to 0, using the ifstream::seekg method. This way you'll be able to "re-read" the entire file.
On the other hand, as pointed out in the comments, this isn't really the best way to read a file into memory, line by line. It would probably be better to use std::vector to store lines and append lines read with getline to it. Alternatively, you could read the entire file at once into a single buffer and split it into lines.

Answer 2

If you really are just looking to get the number of lines in your file, it's far more efficient to read the entire file into a buffer all at once, then just count the number of newline characters contained within. The following code is one of the more efficient ways this could be done.
#include <vector>
#include <algorithm>
#include <fstream>
int main()
{
std::ifstream textFile("your_file.txt", std::ios::binary);
size_t fileSize = textFile.tellg();
textFile.seekg(0, std::ios::beg);
std::vector<char> buffer(fileSize);
size_t numLines(0);
if (textFile.read(buffer.data(), fileSize))
{
numLines = std::count(buffer.begin(), buffer.end(), '\n');
}
return 0;
}