How do I get a specific range of numbers from rand

Question

srand time null     printf   d   rand       Gives a high-range random number  0-32000ish   but I only need about 0-63 or 0-127  though I m not sure how to go about it  Any help

User · Answer

This answer does not focus on the randomness but on the arithmetic order. To get a number within a range, usually we can do it like this:

// the range is between [aMin, aMax]
double f = (double)rand() / RAND_MAX;
double result = aMin + f * (aMax - aMin);

However, there is a possibility that (aMax - aMin) overflows. E.g. aMax = 1, aMin = -DBL_MAX. A safer way is to write like this:

// the range is between [aMin, aMax]
double f = (double)rand() / RAND_MAX;
double result = aMin - f * aMin + f * aMax;

Based on this concept, something like this may cause a problem.

rand() % (max_number + 1 - minimum_number) + minimum_number
// 1. max_number + 1 might overflow
// 2. max_number + 1 - min_number might overflow

User · Answer

2 cents  ok 4 cents    n   rand   x   result l   limit  n RAND MAX   x l  Refactor    l 1   n RAND MAX     x l   l 1   Gives   x   l n RAND MAX  int randn int limit          return limit rand   RAND MAX      int i   for  i   0  i  lt  100  i            printf   d    randn 10         if    i   16   printf   n          gt  test 0 5 1 8 5 4 3 8 8 7 1 8 7 5 3 0 0 3 1 1 9 4 1 0 0 3 5 5 6 6 1 6 4 3 0 6 7 8 5 3 8 7 9 9 5 1 4 2 8 2 7 8 9 9 6 3 2 2 8 0 3 0 6 0 0 9 2 2 5 6 8 7 4 2 7 4 4 9 7 1 5 3 7 6 5 3 1 2 4 8 5 9 7 3 1 6 4 0 6 5

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Updated to not use a  define  double RAND double min  double max        return  double rand    double RAND MAX    max - min    min

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if you care about the quality of your random numbers don t use rand    use some other prng like http   en wikipedia org wiki Mersenne twister or one of the other high quality prng s out there  then just go with the modulus

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rand   will return numbers between 0 and RAND MAX  which is at least 32767   If you want to get a number within a range  you can just use modulo   int value   rand     66     0-65   For more accuracy  check out this article   It discusses why modulo is not necessarily good  bad distributions  particularly on the high end   and provides various options

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I think the following does it semi right  It s been awhile since I ve touched C  The idea is to use division since modulus doesn t always give random results  I added 1 to RAND MAX since there are that many possible values coming from rand including 0  And since the range is also 0 inclusive  I added 1 there too  I think the math is arranged correctly avoid integer math problems    define MK DIVISOR max    int   unsigned int RAND MAX 1  max 1     num   rand   MK DIVISOR 65

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You can use this   int random int min  int max      return min   rand      RAND MAX    max - min   1    1       From the      comp lang c FAQ list    Question 13 16      Q  How can I get random integers in a certain range       A  The obvious way   rand     N           POOR           which tries to return numbers from 0 to N-1  is poor  because the   low-order bits of many random number generators are distressingly   non-random   See question 13 18   A better method is something like   int   double rand       double RAND MAX   1    N        If you d rather not use floating point  another method is  rand      RAND MAX   N   1        If you just need to do something with probability 1 N  you could use  if rand    lt   RAND MAX 1u    N        All these methods obviously require knowing RAND MAX  which ANSI  defines in  lt stdlib h gt    and assume that N is much less than RAND MAX  When N is close to RAND MAX  and if the range of the random number   generator is not a multiple of N  i e  if  RAND MAX 1    N    0   all   of these methods break down  some outputs occur more often than   others   Using floating point does not help  the problem is that rand   returns RAND MAX 1 distinct values  which cannot always be evenly   divvied up into N buckets   If this is a problem  about the only thing   you can do is to call rand multiple times  discarding certain values   unsigned int x    RAND MAX   1u    N  unsigned int y   x   N  unsigned int r  do     r   rand      while r  gt   y   return r   x        For any of these techniques  it s straightforward to shift the range    if necessary  numbers in the range  M  N  could be generated with   something like  M   rand      RAND MAX    N - M   1    1         Note  by the way  that RAND MAX is a constant telling you what the   fixed range of the C library rand function is  You cannot set RAND MAX   to some other value  and there is no way of requesting that rand   return numbers in some other range        If you re starting with a random number generator which returns   floating-point values between 0 and 1  such as the last version of   PMrand alluded to in question 13 15  or drand48 in question   13 21   all you have to do to get integers from 0 to N-1 is   multiply the output of that generator by N    int  drand48     N        Additional links      References  K amp R2 Sec  7 8 7 p  168   PCS Sec  11 p  172   Quote from  http   c-faq com lib randrange html

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As others have noted  simply using a modulus will skew the probabilities for individual numbers so that smaller numbers are preferred   A very ingenious and good solution to that problem is used in Java s java util Random class   public int nextInt int n        if  n  lt   0          throw new IllegalArgumentException  n must be positive         if   n  amp  -n     n      i e   n is a power of 2         return  int   n    long next 31    gt  gt  31        int bits  val      do           bits   next 31           val   bits   n        while  bits - val    n-1   lt  0       return val      It took me a while to understand why it works and I leave that as an exercise for the reader but it s a pretty concise solution which will ensure that numbers have equal probabilities   The important part in that piece of code is the condition for the while loop  which rejects numbers that fall in the range of numbers which otherwise would result in an uneven distribution

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Taking the modulo of the result  as the other posters have asserted will give you something that s nearly random  but not perfectly so   Consider this extreme example  suppose you wanted to simulate a coin toss  returning either 0 or 1  You might do this   isHeads     rand     2      1    Looks harmless enough  right  Suppose that RAND MAX is only 3  It s much higher of course  but the point here is that there s a bias when you use a modulus that doesn t evenly divide RAND MAX  If you want high quality random numbers  you re going to have a problem   Consider my example  The possible outcomes are   rand    freq  rand     2 0       1 3   0 1       1 3   1 2       1 3   0   Hence   tails  will happen twice as often as  heads    Mr  Atwood discusses this matter in this Coding Horror Article

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Just using rand   will give you same random numbers when running program multiple times  i e  when you run your program first time it would produce random number x y and z  If you run the program again then it will produce same x y and z numbers as observed by me   The solution I found to keep it unique every time is using srand    Here is the additional code    include lt stdlib h gt   include lt time h gt   time t t  srand  unsigned  time  amp t    int rand number   rand      65   1 - 0    0   i e Random numbers in range 0-65    To set range you can use formula   rand      max number   1 - minimum number    minimum number  Hope it helps

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check here  http   c-faq com lib randrange html  For any of these techniques  it s straightforward to shift the range  if necessary  numbers in the range  M  N  could be generated with something like  M   rand      RAND MAX    N - M   1    1

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double scale   1 0     double  RAND MAX   1 0   int min  max      rval    int  rand     scale    max - min   1    min

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rand      max number   1 - minimum number    minimum number   So  for 0-65   rand      65   1 - 0    0    obviously you can leave the 0 off  but it s there for completeness    Note that this will bias the randomness slightly  but probably not anything to be concerned about if you re not doing something particularly sensitive

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Just to add some extra detail to the existing answers   The mod   operation will always perform a complete division and therefore yield a remainder less than the divisor      x   y   x -  y   floor  x y      An example of a random range finding function with comments   uint32 t rand range uint32 t n  uint32 t m           size of range  inclusive     const uint32 t length of range   m - n   1          add n so that we don t return a number below our range     return  uint32 t  rand     length of range   n       Another interesting property as per the above      x   y   x  if x  lt  y   const uint32 t value   rand range 1  RAND MAX      results in rand     RAND MAX   1    TRUE for all x   RAND MAX  where x is the result of rand   assert value    RAND MAX   result of rand

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The naive way to do it is   int myRand   rand     66     for 0-65   This will likely be a very slightly non-uniform distribution  depending on your maximum value   but it s pretty close   To explain why it s not quite uniform  consider this very simplified example  Suppose RAND MAX is 4 and you want a number from 0-2  The possible values you can get are shown in this table   rand        rand     3 --------- ------------ 0           0 1           1 2           2 3           0   See the problem  If your maximum value is not an even divisor of RAND MAX  you ll be more likely to choose small values  However  since RAND MAX is generally 32767  the bias is likely to be small enough to get away with for most purposes   There are various ways to get around this problem  see here for an explanation of how Java s Random handles it

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If you don t overly care about the  randomness  of the low-order bits  just rand     HI VAL   Also    double rand      double RAND MAX      lazy way to get  0 0  1 0

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You can change it by adding a   in front of the rand function in order to change to code  For example    rand     50   will give you a random number in a range of 50  For you  replace 50 with 63 or 127

[c] How do I get a specific range of numbers from rand()?

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