File I/O

1. File I/O

2. Using Input/Output Files • A computer file • is stored on a secondary storage device (e.g., disk) • is permanent • 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. Two names The external file name: keyboard, screen, something.dat open The internal stream (or logical) name: cin, cout, stream-name

5. Basic operations #include <fstream> ifstream xxx; ofstream yyy; xxx.open(filename1); yyy.open(filename2); xxx >> … >> … yyy << … << … xxx.close(); yyy.close();

6. Overloaded << and >> in file I/O You can read and write integers, doubles, chars, etc. from files just like cin >> and cout << !

7. Example 1 Write a program which copies file1.dat to file2.dat. file1.dat: 1 2 3 4(eof) file2.dat: 4 3 2 1 (eof)

8. #include <iostream> #include <fstream> using namespace std; void main(){ ifstream fin; int A[4], r; fin.open("file1.dat"); // read data file of four integers for(r=0; r<4; r++) // into array fin >> A[r]; fin.close(); ofstream fout; fout.open("file2.dat"); // write data file for(r=3; r>=0; r--) // with numbers reversed fout << A[r] << ' '; fout.close(); }

9. Character I/O #include <fstream> ifstream xxx; char ch; • xxx.open(fname) • connects streamxxxto the external filefname • xxx.get(ch) • Gets the next character from the input stream xxx and places it in the character variable ch • xxx.put(ch) • Puts the character ch into the output stream xxx • xxx.eof() • tests for end-of-file condition • xxx.close() • disconnects the stream and closes the file Remark: two ‘invisible characters’, ‘\n’ (end of the line) and the end-of-file

10. A typical usage: ifstream ins; char ch; ins.get(ch); while (!ins.eof()) { cout << ch; ins.get(ch); } while (ins.get(ch)) { cout << ch; }

11. Example 2 indata.dat: a b c top10 methods to count spaces 1 3(eof) outdata.dat: a b c top10 methods to count spaces 1 3(eof) Output to screen: a b c top10 methods to count spaces 1 3 Number of lines copied: 4 Write a program which copies indata.dat to outdata.dat and counts the number of lines. Prints file to screen too.

12. Solution to example 2 #include <iostream> #include <fstream> using namespace std; void main(){ ifstream ins; ofstream outs; int count=0; char ch; ins.open("indata.dat"); // open the input file outs.open("outdata.dat"); // open the output file

13. count=0; ins.get(ch); while (!ins.eof()) { if (ch=='\n') { count++; cout << endl; outs << endl;} else { cout << ch; outs << ch;} ins.get(ch); } count++; ins.close(); outs.close(); cout << "Number of lines copied: " << count << endl; } The last line does not have a ‘\n’!

14. More compact C++ style count=0; while (ins.get(ch)) { if (ch=='\n') { count++; cout << endl; outs << endl;} else { cout << ch; outs << ch;} } count++; cout << endl; outs << endl;

15. Final refined solution! count=0; while (ins.get(next)) { if (next=='\n') { count++; cout << endl; outs << endl;} else { cout << next; outs << next;} } if (next != ‘\n’) { count++; cout << endl; outs << endl; } The previous solutions do not handle the case of empty file. File I/O is messy, use it moderately now!!!

16. Example 3 Write a program which counts the number of blanks on each line of indata.dat, outputs each line, and number of blanks on each line. indata.dat: a b c top10 methods to count spaces 1 3(eof) Output to screen: a b c Blanks: 2 top10 methods to count spaces Blanks: 4 Blanks: 0 1 3 Blanks: 3