Skip to main content

Speeding Ticket Calculator Notes

Essentials for the Speeding Ticket Calculator lab

Working with files​

The basics of working with file streams in C++.

Including headers​

// Use this header to include file streams
#include <fstream>
  • ifstream is used to read from a file
  • ofstream is used to write to a file

Opening the file​

ifstream fin;
fin.open("input-data.txt");

or the shorthand

ifstream fin("input-data.txt");

Error checking the file​

A file can fail to open if the file does not exist, or the user does not have permission to open that file.

Opening an output file can also fail. ofstream will create a new file, but it will not create new directories. So trying to write to directory/that/does/not/exist/output-file.txt will fail.

if(!fin.is_open()) {
    cerr << "Error opening file " << filename << endl;
    return 1;
}

Reading data out of a file​

ifstream like cin is an input stream, so we can use fin the same way as cin.

int value;
string text;
fin >> value >> text;

Closing the file​

You need to explicitly close the file once you are finished with it. It is not guaranteed the data will be written otherwise.

fin.close()

Usage example​

string filename = "input-data.txt";
ifstream fin(filename);

if(!fin.is_open()) {
    cerr << "Error opening file " << filename << endl;
    return 1;
}

int value;
string text;
if(fin >> value >> text) {
  cout << "Got: " << value << " and " << text << endl;
} else {
    cout << "Error reading int and string\n";
}


fin.close();

Reading all lines of data from a file​

In c++ we can use a while loop to read all the lines of data out of a file.

For instance, if we have this file:

Hello World 100 3.1415
Sample Text 200 2.7182
Another String 300 1.618
More Text 400 0.5772
Final Entry 500 1.4142

Notice all the values are separated by whitespace.

We can use the following loop to read out all the information

/* ...Open the file... */

string word1;
string word2;
int value;
double fraction;

// Exhaustively match these four values until fin fails
while(fin >> word1 >> word2 >> value >> fraction) {
    cout << word1 << " " << word2 << " " << value << " " << fraction << endl;
}

/* ...Close the file... */

This code will print the exact same output as the file.