1. CS150 Introduction to Computer Science 1
Summary
Don’t forget! Assignment is due on
Wednesday, October 29, 2003.
2/24/2019
CS150 Introduction to Computer Science 1

2. Ifstreams and Ofstreams
So far, we have been reading user input from the keyboard.
We have also been writing the output of our programs directly to the screen.
What if we want to read in a large amount of data that is stored in a file.
What if we want to write a large amount of data and store it in a file.
2/24/2019
CS150 Introduction to Computer Science 1

3. Steps for External files
include<fstream>
Declare file variables (pointers) that will correspond to the files you are using
Input files type: ifstream
Output files type: ofstream
Open file
Use file for input/output
Close files
2/24/2019
CS150 Introduction to Computer Science 1

4. Declaring File pointers
ifstream ifil;
ofstream ofil;
File variables or pointers are the ways that you refer to the files you are using.
Can specify which input/output file to use.
May input from more than one file
May output to more than one file
2/24/2019
CS150 Introduction to Computer Science 1

5. CS150 Introduction to Computer Science 1
Opening files
<fileptr>.open(<filename>)
Same syntax for both input and output files
Filename may be string literal or char array variable
Example:
ifstream ifil;
ifil.open(“input.dat”);
2/24/2019
CS150 Introduction to Computer Science 1

6. CS150 Introduction to Computer Science 1
Checking for errors
Should check for errors opening file
if(ifil.fail()) {
cout << “Error opening input file “;
exit(1); }
Note: you must open files first before using them!
2/24/2019
CS150 Introduction to Computer Science 1

7. CS150 Introduction to Computer Science 1
Using file pointers
Use input file pointer wherever you use cin
Examples:
ifil >> ch;
Output output file ptr wherever you use cout
ofil << ch;
Make sure to use correct file pointer!
2/24/2019
CS150 Introduction to Computer Science 1

8. CS150 Introduction to Computer Science 1
Closing files
Must do at end when file is finished
ifil.close();
ofil.close();
Same syntax for input and output files
2/24/2019
CS150 Introduction to Computer Science 1

9. CS150 Introduction to Computer Science 1
Example
#include<fstream>
#include<iostream>
#define inFile "in.txt"
int main( void ) {
ifstream ifil; //input stream
string name;
ifil.open(inFile);
if (ifil.fail())
cout << "*** Error opening file" << endl;
exit (1); }
while (!ifil.eof())
ifil >> name;
cout << name << " ";
ifil.close();
return 0;
2/24/2019
CS150 Introduction to Computer Science 1