[jjl]

This tutorial will demonstrate how to read a file line by line, then show how to manipulate the lines: convert to number, store in vectors, and even tokenize

How do i open a file?

#include <iostream>
#include <fstream> // needed for file functions

using namespace std;
int main()
{
	ifstream in("yourFile.txt"); // open file for reading

	if(!in.is_open()) // if file is not open - exit
		exit(1);

	in.close(); // close file

	//pause window
	cin.ignore();
	cin.get();

	return 0;
}

simple right?

But how do I read the file?

#include <iostream>
#include <fstream> 
#include <string> // used for getline and string
using namespace std;
int main()
{
	ifstream in("yourFile.txt"); 

	if(!in.is_open()) 
		exit(1);

	string line = ""; // The file will buffer every line into this string
	while(getline(in,line)) // loop through the file
	{
		cout<<line<<endl; // output every line to the screen
	}

	in.close(); 

	//pause window
	cin.ignore();
	cin.get();

	return 0;
}

Getline will loop through every line in the file and it will store that line temporarly in the string

So now you can read a file right? But what if you want to do data operations that involve actual numbers?

#include <iostream>
#include <fstream> 
#include <string> 
#include <vector>
using namespace std;

int main()
{
	ifstream in("yourFile.txt"); 

	if(!in.is_open()) 
		exit(1);

	string line = ""; 
	vector<double>numbers(0); // This vector will be used to store all of the data
	while(getline(in,line)) 
	{
		numbers.push_back(atoi(line.c_str())); //atoi expects a const char so you must convert
	}
	// you now have all of the lines stored in the vector as doubles
	// lets output the vector
	for(int i=0; i<numbers.size(); i++)
		cout<<numbers[i]<<endl; // output to screen

	in.close(); 

	//pause window
	cin.ignore();
	cin.get();

	return 0;
}

This method works fine if your file is in the following format

Quote

12345
12345
12345
12345

But sometimes our data is stored like this in our file

Quote

12345 12345 12345 12345
12345 12345 12345 12345

well how do we store the individual numbers? ... by tokenizing

example:

#include <iostream>
#include <fstream> 
#include <string> 
#include <vector>
#include <sstream>// used for istringstream
using namespace std;
 // this function takes a string pointer and pushes numbers into a
// vector by tokenizing the string at spaces ' '
void parse(string *ext, vector<int>&types)
{
	istringstream iss(*ext);
	string token;
	// you can replace ' ' with whatever you
	// would like to parse at
	while(getline(iss,token,' '))
		types.push_back(atoi(token.c_str())); // push numbers into vector
}
int main()
{
	ifstream in("yourFile.txt"); 

	if(!in.is_open()) 
		exit(1);

	string line = ""; 
	vector<int>numbers(0); 
	while(getline(in,line)) 
	{
		// call our function
		parse(&line,numbers);	
	}

	for(int i=0; i<numbers.size(); i++)
		cout<<numbers[i]<<endl; // output to screen

	in.close(); 

	//pause window
	cin.ignore();
	cin.get();

	return 0;
}

I hope this left you with a farther understand of file IO and using IO for numberic data