How do I read from a specific line in a file? - c++

Question

#include <iostream>
#include <fstream>
using namespace std;
int main() {
int k = 0;
int n;
int y[0];
cout << "write n\n";
cin >> n;
ifstream infile;
infile.open("xxx.in.txt");
infile >> y[0];
infile.close();
for(int x = 0; x < n; x++)
for(int j = 1; j < n; j++)
if(y[x] > y[j])
k = k++;
ofstream outfile;
outfile.open("xxx.out.txt");
outfile << k;
outfile.close();
}
filexxx.in contains two lines (it is a text file).
The first line has n amount of numbers, the second line has random numbers spaced apart.
I want to read from the second line.
How do I do this?

Answer 1

There are a few ways to get input from the second line of your text file.
If you know how long the first line is you could use istream::seekg.
ifstream infile;
infile.open("xxx.in.txt");
infile.seekg(length_of_first_line); // This would move the cursor to the second line.
// Code to read line 2.
If you do not know the value or it could change. Then you would need to use istream::getline and discard the first line.
char buffer[256];
ifstream infile;
infile.open("xxx.in.txt");
infile.getline(buffer, 256); // Read in the first line.
// Code to read line 2.

Answer 2

Input from text file:
With the use of <vector>, <fstream> and <sstream> you can use something along the lines of the following function:
using namespace std;
vector<vector<int> > getNumbers(string FileName) {
vector<vector<int> > Numbers; //Stores all the file's numbers
ifstream infile(FileName.c_str());
string String;
int a;
while (getline(infile, String)) {
vector<int> Line; //Stores each line's numbers
stringstream Stream(String);
while (Stream >> a) //Extracts numbers from 'Stream'
Line.push_back(a); //Each number is added to Line
Numbers.push_back(Line); //Each Line is added to Numbers
}
return Numbers;
}
It returns a two-dimensional vector of ints, where in getFileNumber[x][y] the x value is the line number (starting at value 0 for line 1), and the y value is the position of the number on the line (0 for the first number).
For example, in a file containing:
464 654334 35665 3456332 4454
2456 788654 3456 5775
The number 3456 would be getFileNumbers[1][2]
Output to another text file:
To copy the entire contents of the getFileNumbers vector to a text file, you'd do this:
std::vector<std::vector<int> > Numbers = getFileNumbers("xxx.in.txt");
std::ofstream Output("xxx.out.txt");
for (unsigned int y = 0; y < Numbers.size(); ++y) {
for (unsigned int x = 0; x < Numbers[y].size(); ++x)
Output << Numbers[y][x] << ' ';
Output << '\n';
}
Line specific output:
If you wanted to output line x of a file with std::cout, you'd do this:
std::vector<std::vector<int> > Numbers = getFileNumbers("xxx.in.txt");
for (unsigned int i = 0; i < Numbers[x].size(); ++i)
std::cout << Numbers[x][i] << ' ';
If you wanted to output line x to a file, you'd simply replace std::cout with a std::ofstream or std::fstream object.