1 Jul 2015

Reading Data From Files with C++

I want to read some data from a text file of unknown length using C++. If the file is space delimited this is quite easy. I can use infile to break each line up into tokens divided by the spaces and then load them into temp variables.

If our data file is structured as follows:

s0675405@achray loadtest $ head spaces.txt
1 4
2 5 
55 56
2131 5
5544 1
11 465
5 0
4 49875
6 1

I can use the following code to load the data and print each value to the terminal:

#include <string>
#include <fstream>

int main()
{

  string path = "/home/test/";

  ifstream infile;  
                     
  stringstream ss;
  ss << path << "spaces.txt";                
  infile.open(ss.str().c_str());
  
  int a, b;
  while (infile >> a >> b)
  {
    cout << a << " " << b << endl;
  }

}

Which results in:

s0675405@achray driver_functions_swdg $ ./Load.out 
1 4
2 5
55 56
2131 5
5544 1
11 465
5 0
4 49875
6 1

If more data is included on each row it is easy to expand this to add in more variables, which can be operated on however you wish now that they are in memory. But what if our data is not space delimited?

Using the file commas.txt:

s0675405@achray loadtest $ head commas.txt 
1,1,1,4
2,2,2,5 
5,5,55,56
2,2,2131,5
5,5,5544,1
1,1,11,465
5,5,5,0
4,4,4,49875
6,6,6,1

Will result in us getting no data printed to the terminal, as 1,4 is identified as a single token, but is an invalid integer and so is ignored.

So we need to use a slightly different approach, which explicitly defines the comma as our delimiter:

#include <string>
#include <fstream>

int main()
{
  string path = "/home/test/";

  ifstream infile;  
                     
  stringstream ss;
  ss << path << "commas.txt";                
  infile.open(ss.str().c_str());
   
  int a, b, c, d;
  char delimiter;
  while ((infile >> a >> delimiter >> b >> delimiter >> c >> delimiter >> d) && (delimiter == ','))
  {
    cout << a << " " << b << " " << c << " " << d << endl;
  }

}

And this method again parses the file correctly:

s0675405@achray driver_functions_swdg $ ./Load.out 
1 1 1 4
2 2 2 5
5 5 55 56
2 2 2131 5
5 5 5544 1
1 1 11 465
5 5 5 0
4 4 4 49875
6 6 6 1