1. CMPT 201 Functions

2. Top Down Design (Divide and Conquer)
Break the algorithm into subtasks
Subtasks, or functions in C++, make programs
Easier to understand
Easier to change
Easier to write
Easier to test
Easier to debug
Easier for teams to develop

3. Predefined Functions C++ comes with libraries of predefined functions
Example: sqrt function
#include <cmath>
root = sqrt(9.0);
The number, 9, is called the argument
returns, or computes, the square root of a number

4. pow() #include <cmath>
cout<<“2.5 to the power 3.0 is “<< pow(2.5, 3.0);
The sequence of the two arguments matters!

6. Random Number Generation
Really pseudo-random numbers
Ri = (Ri-1 * 7) % 11
1. Seed the random number generator only once
#include <cstdlib>
#include <ctime>
srand(time(0));
2. The rand() function returns a random integer that is greater than or equal to 0 and less than RAND_MAX (32767);

7. Random Numbers Use % and + to scale to the number range you want
Generate a random number from 0-99
int num = rand() % 100;
Generate a random number from 1-6
int die = (rand() % 6) + 1;
Generating a random number x where 10<=x<=20?

8. Revisit: Guess a number
Generate a random lucky number between 1 and 10

9. User-Defined Functions
Function declaration
Type Function_Name(Type Parameter);
int product(int x, int y);

10. User-Defined Functions
Function definition
Type Function_Name(Type Parameter) { //code }
int product(int x, int y) // function header {
int result = x * y;
return result; }

11. The Function Call int main() { int number = product (5, 6); ... }
int product(int x, int y)
int result = x * y;
return result;

12. Note
When a return statement is executed, the function call ends.

13. Exercise (Multiple returns with if-else)
Build our own my_abs() function.

14. Functions that Return a Boolean Value
bool isPrime (int number);
//returns true if number is a prime number, false otherwise
Nested loops:
Consider using function calls (for inner loop) to make the program more readable

15. Note You can place the whole function definition before the main.
Procedural Abstraction
If a function is well designed, the programmer can use the function as if it were a black box.
char grade (int score)
// return a student’s grade based on his/her score

16. Local Variables vs. Global Variables
Are declared in a function
Cannot be used from outside the function
Have the function as their scope
Global variables
Available to all functions (including main)
Declared outside any function body
e.g., Global named constant

18. Call-by-value
int square (int n) { n = n * n; return n; } int main () int number = 10; int result = square(number); cout<<result<<“ “<<number;

19. Overloaded Functions
C++ allows more than one definition for the same function name
different numbers of arguments
different types of arguments
double avg(double n1, double n2) { return ((n1 + n2) / 2); }
double avg(double n1, double n2, double n3) { return (( n1 + n2 + n3) / 3); }

20. Overloaded Functions cout << avg( 10, 20, 30);
double avg(double n1, double n2) { return ((n1 + n2) / 2); }
double avg(double n1, double n2, double n3) { return (( n1 + n2 + n3) / 3); }
Which one are we calling? cout << avg( 10, 20);
cout << avg( 10, 20, 30);

21. To summarize.. Overloaded functions
Must have different numbers of formal parameters
AND / OR
Must have at least one different type of parameter