File I/O (Ch. 7)

1. Programming File I/O (Ch. 7)

2. Using Input/Output Files • A computer file • is stored on a secondary storage device (e.g., disk) • is permanent until overwritten • can be used to provide input data or receive output data, or both • must reside in Project directory (not necessarily the same directory as the .cpp files) • must be opened before reading it

3. Using Input/Output Files • stream - a sequence of characters • interactive (iostream)  cin - input stream associated with keyboard  cout - output stream associated with display • file (fstream)  ifstream - defines new input stream (normally associated with a file)  ofstream - defines new output stream (normally associated with a file)

4. Declaring ifstream and ofstream Input File: (ifstream) ifstream fin; // declare input stream fin.open("data.txt"); // open the file fin >> varName; // reads a data from file // into variable “varName” // variable <type> matters! fin.close(); // closes the file Output File: (ofstream) ofstream fout; // declare output stream fout.open("data.txt");// open the file fout << varName; // writes the data in variable // “varName” to file. // variable <type> matters! fout.close(); // closes the file

5. Example 1 – input from file You can read and write integers, doubles, chars, etc. from files just like cin >>and cout <<: #include <iostream.h> #include <fstream.h> void main(void) { int data; ifstreamfin; fin.open("data.txt"); fin >> data; while (fin) // fin==false means no more data { cout << data << endl; fin >> data; } fin.close(); } Open the file before you use it Close the file after using it

6. data.txt 10 20 30 10 20 30 [EOF] File I/O: Example 1 • [EOF] is a special character: End Of File • When the file opens, the reading position is at the beginning • Every time you read a number, the reading position advances • When you read [EOF], fin becomes false If you want to use a DOS path name, you must put TWO backslashes fin.open(“d:\\MyData\\data.txt");

7. Example 2: output to file In the same way you can write data to a file #include <iostream.h> #include <fstream.h> void main(void) { int i; ofstreamfout; fout.open(“data2.txt"); fout << “Compute x^2” << endl; for (i=1; i<=3; i++) fout << i << “ “ << i*i << endl; fout.close(); } Compute x^2 1 1 2 4 3 9 Open with NotePad data2.txt Compute x^2 [CR] 1 1 [CR] 2 4 [CR] 3 9 [CR] [EOF] Inserted by the OS

8. File I/O: Example 3 - input #include <fstream.h> void main(void) { int i, size; int array[100] = {0}; ifstreamfin; fin.open("data.txt"); fin >> size; // read in number of data pts for (i=0; i<size; i++) fin >> array[i]; // read in elements fin.close(); // ........ use the array ........ // calculate average, etc . . . } Input an 1-D array of int The first element of the file is the size of the array, and then, the actual data. Example: data.txt 5 10 12 14 10 18

9. Checking for errors .fail() After opening a file • It is a good idea to make sure it was opened properly. • We can do this using the “.fail()” method Example: #include <iostream.h> #include <fstream.h> // input from file Void main() { ifstreamfin; // input file name is fin fin.open(“test.dat”); if (fin.fail()) // check if the open failed! { cout << “Error opening file \n”; } }

10. Common Mistakes to Avoid Remeber to open the file! Common mistake ifstream fin; fin >> size; // error!!!! didn’t open the file Correct ifstream fin; fin.open(“name-of-file”); fin >> size; // error!!!! didn’t open the file Close file only after done read/writing out data (not before) ofstream fout; fout.open(“name-of-file”); fout << size; // OK fout.close(); // Closed file . . . fout << more_data; // error!!!! file has been closed!