Displaying contents of vector - c++

Question

I am having trouble displaying the contents of a vector. I am unsure whether it is the way I read in the values from the text file, or whether my display function is just not working.
#include <iostream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
int main()
{
string line;
int n,length;
std::vector<int>arr1;
fstream file("t1.txt");
if(file.is_open())
{
while (getline(file,line))
{
cout << line << endl;
}
file << line;
length = line.length();
while(file >> n)
arr1.push_back(n);
for(int i =0; i < (int)arr1.size(); i++)
cout << arr1.at(i) << endl;
}
return 0;
}
Any help would be greatly appreciated.
the text file contains "5 2 5 5 -1 7 2 5 3 5 2 -2"

Answer 1

I suppose you have a problem with vector building (reading values from file to vector). You should delete (or comment) expression with getline, as well as writing line back to file (file << line), and use only file >> n.
Try the following shortened version of your program
#include <iostream>
#include <string>
#include <vector>
#include <fstream>
using namespace std;
int main()
{
string line;
int n;
std::vector<int> arr1;
fstream file("t1.txt");
if(file.is_open())
{
while(file >> n)
arr1.push_back(n);
for(int i =0; i < (int)arr1.size(); i++)
cout << arr1.at(i) << endl;
file.close();
}
return 0;
}
Problem of your initial code is that after you read line from your file (that comprises only one line) next reading (whether it is file >> ... or getline(...)) will provide no data because END OF FILE. To read data from file again you should re-open it
...
length = line.length();
file.close();
file.open("t1.txt");
while(file >> n)
arr1.push_back(n);
...
or rewind to the beginning
...
length = line.length();
file.clear();
file.seekg(0, ios_base::beg);
while(file >> n)
arr1.push_back(n);
...
But you should understand, that writing to file stream does not provide desired effect - the recorded data will be available only after transfer written data from the buffer to the file (in the general case, after the file closing).
EDIT:
Also I want to add, that C++ allows input to and output from vector without additional loops, just consider the following example
if(file.is_open())
{
// filling vector from the stream
istream_iterator<int> it(file); // iterator for int values in file
istream_iterator<int> eos; // end of straem
arr1.insert(arr1.begin(), it, eos);
// output vector to the stream (cout = standard output stream)
ostream_iterator<int> oit (cout,"\n"); // "\n" = new line is a separator for output
// also " " or any other separator can be used
copy(arr1.begin(), arr1.end(), oit);
}