1. CS149D Elements of Computer Science
Ayman Abdel-Hamid
Department of Computer Science
Old Dominion University
Lecture 21: 11/12/2002
Lecture 21: 11/12/2002
CS149D Fall 2002

2. Outline Reading and writing data files (section 3.6)
Lecture 21: 11/12/2002
CS149D Fall 2002

3. Data Files1/2 Reading or writing large amounts of data
Results from scientific or engineering experiments usually are stored in data files. These data files are later processed to extract desired information
#include <fstream.h> #information related to manipulating a file
fstream inputfile,outputfile; #declare two file streams
//associate inputfile with file on disk “input.txt” in read mode
inputfile.open (“input.txt”,ios::in);
//associate outputfile with file on disk “output.txt” in write mode (file is overwritten if already exists)
outputfile.open(“output.txt”,ios::out);
//shorthand form
fstream inputfile (“input.txt”,ios::in);
Lecture 21: 11/12/2002
CS149D Fall 2002

4. Data Files2/2 Alternatively can use
ifstream inputfile; //input file stream
ofstream outputfile; //output file stream
inputfile.open(“input.txt);
outputfile.open(“output.txt”);
Lecture 21: 11/12/2002
CS149D Fall 2002

5. File I/O
Once file object have been successfully associated with file, can read or write information the same as reading from the keyboard, or writing to the monitor
fstream inputfile;
inputfile.open (“input.txt”,ios::in);
//read from input file
inputfile >> time >> height;
fstream outputfile;
outputfile.open (“output.txt”,ios::out);
// write to output files
outputfile << time << height;
//Close file after we are done
inputfile.close();
Outputfile.close();
Lecture 21: 11/12/2002
CS149D Fall 2002

6. Reading Data Files1/4 File consists of a number of lines (records)
Each line has one or more fields to be read by the program
File Structure Alternatives
The number of records (lines) in the file is known, usually stored as the first line in the file  use a for loop to read the information
The last line in the file contains a special data value (usually not occurring in the allowable data values)  use a while loop with a condition that is true as long as the data value read is not the special data value
A special end-of-file indicator is inserted at the end of every file. We use the function eof to detect the end-of-file indicator
Lecture 21: 11/12/2002
CS149D Fall 2002

7. Reading Data Files2/4 Sample input file 3 1 200 2 300 3 400
# The number of records is stored as the first line in the file
fstream valuesfile;
//open file for input
valuesfile.open("sensor1.dat",ios::in);
//read number of data lines in input file
valuesfile>>num_data_pts;
//read the first data line
valuesfile>>id>>value;
//for loop to read remaining data values from file
for (int k=2; k <=num_data_pts;k++) {
valuesfile >> id >> value;
//process values read from file }
Sample input file 3
1 200
2 300
3 400
Lecture 21: 11/12/2002
CS149D Fall 2002

8. Reading Data Files3/4 Sample input file 1 200 2 300 3 400 -1 -1
// input file terminated with a negative id value in last line
int num_data_pts = 0, id;
fstream valuesfile;
//open file for input
valuesfile.open("sensor_trailer.dat",ios::in);
//read the first data line
valuesfile>>id>>value;
//while loop to read remaining data lines
while (id >= 0) {
//process the current input value ….
num_data_pts++;
//read next line
valuesfile >> id >> value; }
Sample input file
1 200
2 300
3 400
-1 -1
Lecture 21: 11/12/2002
CS149D Fall 2002

9. Reading Data Files4/4 Sample input file 1 200 2 300 3 400
//end-of-file checked by using eof function
int num_data_pts = 0, id;
fstream valuesfile;
//open file for input
valuesfile.open("sensor.dat",ios::in);
//read the first data line
valuesfile>>id>>value;
//while loop to read remaining data lines
while (!valuesfile.eof()) {
//process the current input value ….
num_data_pts++;
//read next line
valuesfile >> id >> value; }
Sample input file
1 200
2 300
3 400
Lecture 21: 11/12/2002
CS149D Fall 2002