1. C++ Programming Lecture 10 Functions – Part IIBy
Ghada Al-Mashaqbeh
The Hashemite University
Computer Engineering Department

2. The Hashemite University
Outline
Introduction.
Random numbers generation in C++.
enum data type identification and usage.
Examples.
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3. Random Number Generation I
Random number generation is used mainly in simulation and game playing based application.
The rand() function is used in C++ to generate random integer numbers between 0 and a maximum specified value.
rand() function takes nothing (i.e. void) as its arguments and returns an unsigned integer.
In order to use this function you must Load <cstdlib> or <stdlib.h>
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4. Random Number Generation II
rand function syntax:
int i = rand();
Generates a pseudorandom number between 0 and RAND_MAX (usually 32767)
RAND_MAX is a symbolic constant defined in the stdlib header file.
0 <= rand() <= RAND_MAX.
A pseudorandom number is a preset sequence of "random" numbers.
The same sequence is generated upon every program execution, is this preferred?.
This repeated behavior is essentially in programming debug and verification in simulation and other random-based applications.
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5. Random Numbers Seeding I
srand function
Jumps to a seeded location in a "random" sequence.
Similar to rand() function, srand function is defined in the <stdlib.h> library.
Takes an unsigned integer as a seed (i.e. as an argument).
It does not return any value (returns void), it just change the random sequence (randomizing the rand() function).
Can be called more than once within the same program.
Still you need to use the rand() function to get the random numbers.
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6. Random Numbers Seeding II
srand syntax:
srand( seed );
seed can be any unsigned integer entered manually be the user or initialized through the program.
If the same seed is used every time the program is run we will get the same random sequence (i.e. the same without seed).
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7. Random Numbers Seeding III
To initialize seed value automatically use the following syntax:
srand( time( 0 ) );
time( 0 )
Returns the current calendar time in seconds.
time() function takes a pointer as an argument and returns unsigned integer.
Changes the seed every time the program is run, thereby allowing rand() to generate random numbers. So, it is much better than manual seeding.
Need to include the <ctime> or <time.h> library to use the time() function.
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8. Changing Random Numbers Range I
Scaling and shifting
Reduces random number to a certain range
Modulus ( % ) operator
Reduces number between 0 and RAND_MAX to a number between 0 and the scaling factor:
Number = offset (shift value) + rand() % scaling_factor
Example
i = rand() % 6 + 1;
Generates a number between 1 and 6
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9. Changing Random Numbers Range II
Scaling and shifting equations:
Lets assume the range is [min, max], then:
Offset = min.
Scaling factor = max – min + 1
Lets assume the range is [min, max)  [min, max-1], then:
Scaling factor = max – min
Lets assume the range is (min, max][min+1, max], then:
Offset = min + 1.
Continue for other possibilities .....
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10. Changing Random Numbers Range III
All the previous equations are used to generate integers but what about floating point random numbers?
First: convert the floating point range into an integer range by multiplying the range by 10^n where n is at least the number of digits after the decimal point.
Second: find the values of both the offset and scaling factor based on the equations in the previous slide.
Third: divide the resulting random number on 10^n to get a floating point random number in the specified range.
Note that n controls the difference value (lets call it the step width) between the generated random numbers.
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Examples
Generate random numbers in the following ranges:
100 <= n <= 200  int n = rand()%101
100 <= n < 500  int n = rand()%400
50 < n <= 200  int n = 51 + rand()%150
100 < n < 200  int n = rand()%99
0.01 <= n <= 0.08 
double n = (1 + rand()%8)/ with step width = 0.01
0.02 <= n <= 0.9 
double n = (20 + rand()%821)/ with step width = 0.001
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1 // Fig. 3.7: fig03_07.cpp
2 // Shifted, scaled integers produced by 1 + rand() % 6
3 #include <iostream> 4
5 using std::cout;
6 using std::endl; 7
8 #include <iomanip> 9
10 using std::setw; 11
12 #include <cstdlib> 13
14 int main()
15 {
16 for ( int i = 1; i <= 20; i++ ) {
cout << setw( 10 ) << ( 1 + rand() % 6 ); 18
if ( i % 5 == 0 )
cout << endl;
21 } 22
23 return 0;
24 }
Notice rand() % 6 . This returns a number between 0 and 5 (scaling). Add 1 to get a number between 1 and 6.
Executing the program again gives the same "random" dice rolls.
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1 // Fig. 3.9: fig03_09.cpp
2 // Randomizing die-rolling program
3 #include <iostream> 4
5 using std::cout;
6 using std::cin;
7 using std::endl; 8
9 #include <iomanip> 10
11 using std::setw; 12
13 #include <cstdlib> 14
15 int main()
16 {
17 unsigned seed; 18
19 cout << "Enter seed: ";
20 cin >> seed;
21 srand( seed ); 22
23 for ( int i = 1; i <= 10; i++ ) {
cout << setw( 10 ) << 1 + rand() % 6; 25
if ( i % 5 == 0 )
cout << endl;
28 } 29
30 return 0;
31 }
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Enter seed: 67
Enter seed: 432
Notice how the die rolls change with the seed.
Program Output
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15. The Hashemite University
Enumeration I
It is the counterpart of symbolic constants found in C (to assign integer values to symbolic constants).
The main difference is that enum can define a new data type within the program.
Enumeration - set of integers with identifiers
enum typeName {constant1, constant2…};
Constants start at 0 (default), incremented by 1
Unique constant names
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Enumeration II
If you don't specify values for enum constants, the values start at zero and increase by one with each move down the list.
E.g:
enum MyEnumType { ALPHA, BETA, GAMMA }; ALPHA has a value of 0, BETA has a value of 1, and GAMMA has a value of 2.
If you want, you may provide explicit values for enum constants, as in
enum Size { SMALL = 10, MEDIUM = 100, LARGE = 1000 }; Or
enum Size { SMALL = 10, MEDIUM, LARGE }; //here MEDIUM will have the value of 11 and LARGE will have the value of 12
You can assign positive and negative integer values to enum constants (but not floating point values which will be a syntax error).
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Enumeration III
There are two kinds of enum type declarations.
One kind creates a named type, as in
enum MyEnumType { ALPHA, BETA, GAMMA };
If you give an enum type a name, you can use that type for variables, function arguments and return values, and so on:
E.g:
enum MyEnumType x; /* legal in both C and C++ */
MyEnumType y; // legal only in C++
The other kind creates an unnamed type. This is used when you want names for constants but don't plan to use the type to declare variables, function arguments, etc.
For example, you can write enum { HOMER, MARGE, BART, LISA, MAGGIE };
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18. C++ enum type conversion rules
There is an implicit conversion from any enum type to int.
E.g:
enum MyEnumType { ALPHA, BETA, GAMMA };
Then the following lines are legal:
int i = BETA; // give i a value of 1
int j = 3 + GAMMA; // give j a value of 5
On the other hand, there is not an implicit conversion from int to an enum type:
MyEnumType x = 2; // syntax error
MyEnumType y = 123; // syntax error
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Enumeration Example I
#include<iostream.h>
int main() {
enum cars{BMW, MAZDA, KIA, BENZ};
cars mycar;
int money, i,
BMW_price = 1000,
MAZDA_price = 100,
KIA_price = 500,
BENZ_price = 2000;
cout << "Enter the car type (BMW = 0, MAZDA = 1, KIA = 2, BENZ = 3):\n";
cin >> i;
mycar = (cars)i;
cout << "Enter the money balance:\n";
cin >> money;
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20. Enumeration Example II
switch(mycar) {
case BMW:
if(money >= BMW_price)
cout << "You cannot bye car " << mycar << endl;
else
break;
case MAZDA:
if(money >= MAZDA_price)
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21. Enumeration Example II
case KIA:
if(money >= KIA_price)
cout << "You cannot bye car " << mycar << endl;
else
break;
case BENZ:
if(money >= BENZ_price) }
return 0;
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Example Output
Try it and find the output by your self.
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23. The Hashemite University
Additional Notes
This lecture covers the following material from the textbook:
Fourth Edition:
Chapter 3: Sections 3.8 and 3.9
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