Random function in C

Random function in C

In the C programming language, the rand() is a function used for Pseudo Random Number Generator (PRNG). The random number generated by the rand() function is not random. It is a sequence that repeats itself in a given time, but the period is so immense that it is usually ignored. In other words, the C library function int rand(void) returns a pseudo random number in the sane of 0 to RAND_MAX. The RAND_MAX is a constant whose default value may vary between the implementations, but it will be granted to be atleast 32767. The rand() function works by remembering a seed value generally used to compute the upcoming random number and the next new seed.

Randomization is a fundamental technique in an algorithm design. It allows a program to run quickly whenever the average case behavior of an algorithm is comparatively better than the worst case behavior. Randomization is generally used in games, both in entertainment and gambling. The latter application always gives the known example of a programmer getting into a problem due to writing bad code.

Header file:

 #include <stdlib.h>

Stdlib.h is a header that defines variable types, macros, and various functions for performing general compiler functions. Some of the variable types include size_t, wchar_t, div_t, ldiv_t; macros include NULL, EXIT_FAILURE, EXIT_SUCCESS, RAND_MAX, MB_CUR_MAX;

Syntax:

 int rand (void)
 Return values: 

The rand function always returns the next pseudo random number in series. The range of the number series that is returned ranges between 0 and RAND_MAX. The rand() generates values in the range of [0, RAND_MAX). On some machines and operating systems that use the GNU C library, RAND_MAX is equal to INT_MAX or 2³¹-1 or might be as small as 32767. There are no strong guarantees about the quality of the random numbers that are generated by the rand() function, but the generated values should be good enough for casual use. They will have an advantage that as a part of the C language, it will be present in most places.

If the random numbers are generated using the rand() without calling the srand() function, the compiler will create the same sequence of numbers each time it runs.

E.g:

 #include <stdio.h>
 #include <conio.h>
 #include <stdlib.h>
 int main()
 {
 int i;
 printf (“ Print random numbers -> \n”);
 for ( i = 0; i < 10; i++)
 {
 printf (“%d”, rand()); //initializing random number funtion
 }
 printf (“\n”);
 return 0;
 }  

Output:

282930 939293 78382488 55325838348 838384

 #inlude <stdio.h>
 #include <conio.h>
 #include <stdlib.h>
 int main()
 {
 int i, n;
 time t;
 n = 5;
 srand ((unsigned) time(&t));
 for (i = 0; i < n; i++) //prints 5 random numbers from 0 to 49
 {
 printf (“%d \n”, rand() % 50);
 }
 return 0;
 } 

Output:

38 45 29 12 3 22

Many of the research on the pseudo random number generators over many years and better pseudo random number generators than rand are available. The Mersenne Twist runs about four times as fast as a rand in its C programming language, and it also passes a much wider battery of statistical tests.

There are many cryptographical secure pseudorandom number generators, out of which is the famous Blum Blum Shub. They cannot be predicted based on the output if seeded with a true random number value. Comparatively, cryptographic PRNG (Pseudo Random Number Generator) is slow for a day to day use.

srand():

You can set the seed for the rand function using srand() functions. This seed can only be set once, and before the first time, the rand() function will be called.

 Header file:
 stdlib.h 

Syntax:

int srand (unsigned int seed)

Arguments:

This only takes one argument:

Seed - an integer value used as a seed for a new series of pseudo random numbers.

Return values:

None

E.g.:

 #include <stdio.h>
 #include <stdlib.h>
 #include <time.h>
 #include <conio.h>
 int main()
 {
 srand (time(NULL));
 printf (“%d \n”, rand());
 srand (12);
 printf (“%d \n”, rand());
 return 0;
 } 

Output:

 1432462941
 1687063760 

Advantages of randomization

• Calculations can be performed with a few integer arithmetic instructions.
• Random numbers can be repeated,which and this is a great advantage when it comes to debugging that relies on random numbers.
• The lower bits of the random numbers may not be as random as the higher bits. In general, a programmer can never use parts of a pseudo random number as another random number.
• Some bit combination might not occur. If the constants are not chosen carefully, random numbers may not cover the range of possible numbers.