Why do I always get the same sequence of random numbers with rand()?

You have to seed it. Seeding it with the time is a good idea:

srand()

#include <stdio.h>#include <stdlib.h>#include <time.h>int main (){  srand ( time(NULL) );  printf ("Random Number: %d\n", rand() %100);  return 0;}

You get the same sequence because rand() is automatically seeded with the a value of 1 if you do not call srand().

Edit

Due to comments

rand() will return a number between 0 and RAND_MAX (defined in the standard library). Using the modulo operator (%) gives the remainder of the division rand() / 100. This will force the random number to be within the range 0-99. For example, to get a random number in the range of 0-999 we would apply rand() % 1000.

rand() returns pseudo-random numbers. It generates numbers based on a given algorithm.The starting point of that algorithm is always the same, so you'll see the same sequence generated for each invocation. This is handy when you need to verify the behavior and consistency of your program.

You can set the "seed" of the random generator with the srand function(only call srand once in a program) One common way to get different sequences from the rand() generator is to set the seed to the current time or the id of the process:

srand(time(NULL)); or srand(getpid()); at the start of the program.

Generating real randomness is very very hard for a computer, but for practical non-crypto related work, an algorithm that tries to evenly distribute the generated sequences works fine.

To quote from man rand :

The srand() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by rand(). These sequences are repeatable by calling srand() with the same seed value.

If no seed value is provided, the rand() function is automatically seeded with a value of 1.

So, with no seed value, rand() assumes the seed as 1 (every time in your case) and with the same seed value, rand() will produce the same sequence of numbers.