1. CS150 Introduction to Computer Science 1
Summary
We have still not finished talking about functions.
Today we will finish functions and start talking about reading and writing to files.
Don’t forget! Assignment is due on
Wednesday, October 29, 2003.
2/21/2019
CS150 Introduction to Computer Science 1

2. Testing a Program Using Stubs
When you have a large program with many functions, it is usually a good idea to create dummy functions or stubs.
These will have the proper prototypes, but they will not actually doing anything.
It is a good way of testing that the values are being passed to and from the functions correctly.
2/21/2019
CS150 Introduction to Computer Science 1

3. CS150 Introduction to Computer Science 1
Example
void computeSum(int, int, int&);
int main( void ) {
int x, y, sum;
x = 4;
y = 5;
sum = 0;
computeSum (x, y, sum);
cout << x << y << sum;
return 0; }
void computeSum(int num1, int num2, int& sum)
cout << “Function computeSum entered with the values ”;
cout << num1 << “, “ << num2 << “ and “ << sum << endl;
sum = 50;
2/21/2019
CS150 Introduction to Computer Science 1

4. CS150 Introduction to Computer Science 1
Last Word on Functions
Arguments can be input (receives) and/or output (sends)
When would an argument be both? Can be tricky…
How can you identify arguments that are used both for input and output?
Do we know any examples?
How do we identify them in our comments?
2/21/2019
CS150 Introduction to Computer Science 1

5. CS150 Introduction to Computer Science 1
Example
void computeSum(int, int, int&);
int main( void ) {
int x, y, sum;
x = 4;
y = 5;
sum = 0;
computeSum(x, y, sum);
cout << x << y << sum;
return 0; }
// Function: computeSum
// Purpose: computes the sum of
// two numbers
// In: num1, num2
// Out: sum
void computeSum(int num1, int num2, int& sum) {
sum = num1 + num2; }
2/21/2019
CS150 Introduction to Computer Science 1

6. CS150 Introduction to Computer Science 1
Example
void computeSumAndChange(int, int&, int&);
int main( void ) {
int x, y, sum;
x = 4;
y = 5;
sum = 0;
computeSumAndChange(x, y, sum);
cout << x << y << sum;
return 0; }
// Function: computeSumAndChange
// Purpose: computes the sum of two
// numbers and changes
// one of the values
// In: num1
// Out: sum
// In/Out: num2
void computeSumAndChange(int num1, int& num2, int& sum) {
sum = num1 + num2;
num2 = 7; }
2/21/2019
CS150 Introduction to Computer Science 1

7. CS150 Introduction to Computer Science 1
Constants Revisited
We know that we can declare constants in C++ as follows:
const type identifier = constant;
For example:
const int Pi = ;
There is another way of defining constants:
#define identifier replacement-text
#define Pi
2/21/2019
CS150 Introduction to Computer Science 1

8. CS150 Introduction to Computer Science 1
#define
#define is a compiler directive
It’s different from a variable or const
Form:
#define X Y
The compiler replaces all instances of X with Y
No memory space is created
Used in C primarily and in older C++ code
Not considered true C++ programming, but it’s useful
NOTE: NO SEMICOLON!!
2/21/2019
CS150 Introduction to Computer Science 1

9. 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.
2/21/2019
CS150 Introduction to Computer Science 1

10. 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
2/21/2019
CS150 Introduction to Computer Science 1

11. 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
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CS150 Introduction to Computer Science 1

12. 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”);
2/21/2019
CS150 Introduction to Computer Science 1

13. 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!
2/21/2019
CS150 Introduction to Computer Science 1

14. 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!
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CS150 Introduction to Computer Science 1

15. 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
2/21/2019
CS150 Introduction to Computer Science 1

16. 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;
2/21/2019
CS150 Introduction to Computer Science 1