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

Question

This is the first time I m trying random numbers with C  I miss C    Here is my code   int i  j   0  for i   0  i  lt   10  i          j   rand        printf  j    d n   j       with this code  I get the same sequence every time I run the code  But it generates different random sequences if I add srand   somevalue    before the for loop  Can anyone explain why

User · Answer

This is from http   www acm uiuc edu webmonkeys book c guide 2 13 html rand   Declaration   void srand unsigned int seed      This function seeds the random number generator used by the function rand  Seeding srand with the same seed will cause rand to return the same sequence of pseudo-random numbers  If srand is not called  rand acts as if srand 1  has been called

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You have to seed it  Seeding it with the time is a good idea   srand     include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt   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

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Random number generators are not actually random  they like most software is completely predictable  What rand does is create a different pseudo-random number each time it is called One which appears to be random  In order to use it properly you need to give it a different starting point     include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt   int main           initialize random seed       srand   time NULL        printf  random number  d n  rand       printf  random number  d n  rand       printf  random number  d n  rand       printf  random number  d n  rand        return 0

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call srand sameSeed  before calling rand    More details here

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None of you guys are answering his question        with this code i get the same sequance everytime the code but it generates random sequences if i add srand  somevalue   before the for loop   can someone explain why     From what my professor has told me  it is used if you want to make sure your code is running properly and to see if there is something wrong or if you can change something

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There s a lot of answers here  but no-one seems to have really explained why it is that rand   always generates the same sequence given the same seed - or even what the seed is really doing   So here goes   The rand   function maintains an internal state   Conceptually  you could think of this as a global variable of some type called rand state   Each time you call rand    it does two things   It uses the existing state to calculate a new state  and it uses the new state to calculate a number to return to you   state t rand state   INITIAL STATE   state t calculate next state state t s   int calculate return value state t s    int rand void        rand state   calculate next state rand state       return calculate return value rand state       Now you can see that each time you call rand    it s going to make rand state move one step along a pre-determined path   The random values you see are just based on where you are along that path  so they re going to follow a pre-determined sequence too   Now here s where srand   comes in   It lets you jump to a different point on the path   state t generate random state unsigned int seed    void srand unsigned int seed        rand state   generate random state seed       The exact details of state t  calculate next state    calculate return value   and generate random state   can vary from platform to platform  but they re usually quite simple   You can see from this that every time your program starts  rand state is going to start off at INITIAL STATE  which is equivalent to generate random state 1   - which is why you always get the same sequence if you don t use srand

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rand   returns the next  pseudo  random number in a series   What s happening is you have the same series each time its run  default  1     To seed a new series  you have to call srand   before you start calling rand     If you want something random every time  you might try   srand  time  0

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Seeding the rand    void srand  unsigned int seed       This function establishes seed as the seed for a new series of pseudo-random numbers  If you call rand before a seed has been established with srand  it uses the value 1 as a default seed        To produce a different pseudo-random series each time your program is run  do srand  time  0

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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

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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

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If I remember the quote from Knuth s seminal work  The Art of Computer Programming  at the beginning of the chapter on Random Number Generation  it goes like this    Anyone who attempts to generate random numbers by mathematical means is  technically speaking  in a state of sin    Simply put  the stock random number generators are algorithms  mathematical and 100  predictable  This is actually a good thing in a lot of situations  where a repeatable sequence of  random  numbers is desirable - for example for certain statistical exercises  where you don t want the  wobble  in results that truly random data introduces thanks to clustering effects   Although grabbing bits of  random  data from the computer s hardware is a popular second alternative  it s not truly random either - although the more complex the operating environment  the more possibilities for randomness - or at least unpredictability   Truly random data generators tend to look to outside sources  Radioactive decay is a favorite  as is the behavior of quasars  Anything whose roots are in quantum effects is effectively random - much to Einstein s annoyance

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Rand does not get you a random number  It gives you the next number in a sequence generated by a pseudorandom number generator  To get a different sequence every time you start your program  you have to seed the algorithm by calling srand   A  very bad  way to do it is by passing it the current time    include  lt stdio h gt   include  lt stdlib h gt   include  lt time h gt   int main         srand time NULL        int i  j   0      for i   0  i  lt   10  i              j   rand            printf  j    d n   j             return 0      Why this is a bad way  Because a pseudorandom number generator is as good as its seed  and the seed must be unpredictable  That is why you may need a better source of entropy  like reading from  dev urandom

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

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