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.
(eof)
file2.dat:
(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 stream xxx to the external file fname
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
EOF: control-D in Unix and control-Z in DOS
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; }
while (ins.get(ch)) {
cout << ch; }

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

12. Solution to example 2 void main(){ ifstream ins; ofstream outs;
#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. cout << "Number of lines copied: " << count << endl;
ins.get(ch);
while (!ins.eof()) {
if (ch=='\n') {
count++;
cout << endl;
outs << endl;}
else {
cout << ch;
outs << ch;} }
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') {
cout << endl;
outs << endl;}
else {
cout << ch;
outs << ch;} }
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’) {
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:
Blanks: 2
Blanks: 4
Blanks: 0
1 3
Blanks: 3