C++ FUNCTIONS TO GENERATE RANDOM NUMBERS

In C++ we can use some of miscellaneous and useful functions of stdlib.h to generate random numbers which are rand( ), srand( ),random( )and randomize( ).

            Simply to generate a random number, you can use rand( ) functions that produces a random number in the range 0 to RAND_MAX. The RAND_MAX is also defined in stdlib.h which gives you the maximum value returned by rand( ). For example if you execute this code given below it will generate 5 random numbers.

#include <iostream.h>

#include <stdlib.h>

int main()

{

int num,i;

     for(i=0;i<5;++i)

     {

          num=rand();

          cout<<num<<endl;

     }

}

Output:

Now if you run this same program you will notice same random numbers this program is generating.

2^nd Run – 

3^rd Run – 

4^th Run – 

So, in order to generate a random number when ever you run the program you need to use srand( ) function in the above program. The srand( ) function starts generating a number from a point that depends on the value we pass to srand( ) as argument. Therefore, to generate a different number every time this function is executed we need seed (start) the function with a truly variable value such as system time. Now we can change the above code as follows:

#include <iostream.h>

#include <stdlib.h>

#include <conio.h>

# include <time.h> //to get system time through time_t

int main()

{

int num,i;

unsigned int sval;

time_t t;//t is a time type variable

sval=(unsigned)time(&t);

srand(sval);

     for(i=0;i<5;++i)

     {

          num=rand();

          cout<<num<<' ';

     }

getch();

}

Output:

1^st Run -

2^nd Run –

3^rd Run –

Note** Remember one thing: srand( ) function should be used just once before the loop starts.

Generating a random number with a specified range:

The rand( ) function by default generates a number in range 0 to RAND_MAX. Now if you want to generate a number with our own specified range say – 5 to 14, then you can change the rand( ) as below:

(rand( ) % U) + L

(rand( ) % 10) + 5

Here your L = 5 and U = 10

random( ) and rnadomize( ):

in order to generate in users’ specified range we can use two more macros, also defined in stdlib.h they are random( ) and randomize( ). These two also having similar function as that of rand( ) and srand( ).

To generate a random number between 0 – 9 you can write:

random(10);

Output can be any one of these:

0          1          2          3          4          5          6          7          8          9

Because random( ) function generates a random number maximum up to n – 1.

So, to generate a random number between L – U (where L is your lower limit and U is your upper limit)

You should write:

 random(U – L + 1) + L

Example: random(100 – 50 + 1) + 50 (It will generate a number within the range 50 – 100)

Note** randomize ( ) function internally implements a macro that calls time function of time.h. So, you need to use time.h header file in your program to implement random ( ) function.

Example 1:

//Program to generate 5 random numbers in the range 5 - 14

//program is using srand( ) and rand( ) functions.

#include <iostream.h>

#include <stdlib.h>

#include <conio.h>

# include <time.h>

int main()

{

int num,i;

unsigned int sval;

time_t t;

sval=(unsigned)time(&t);

srand(sval);

     for(i=0;i<5;++i)

     {

          num=(rand()%10) + 5;

          cout<<num<<' ';

     }

getch();

}Output:

1^st Run

2^nd Run

3^rd Run

4^th Run

Example 2:

//Program to generate 5 random numbers in the range 5 - 14

//program is using randomize( ) and random( ) functions.

#include <iostream.h>

#include <stdlib.h>

#include <conio.h>

# include <time.h>

int main()

{

int num,i;

randomize();

     for(i=0;i<5;++i)

     {

          num=random(14-5+1)+5;

          cout<<num<<' ';

     }

getch();

}

Output:

1^st Run

2^nd Run

3^rd Run

4^th Run

Note** For official explanation of any inbuilt library function or keyword you can always refer to the HELP menu of TurboC++ 3.0 IDE. The main reason behind writing all these contents only is to explain the topics in best possible way using examples and other ways of illustration. That's why wherever it is possible I have added snapshots of screen outputs or snaps of program code with graphical illustrations. Please refer to all those resources.