Thread: Random Number

  1. #1
    Registered User New001's Avatar
    Join Date
    May 2004
    Posts
    6

    Random Number

    How do I generate a random intager in C++?
    Also could I set the number to be in a certain range, such as between 0 and 25.

  2. #2
    erstwhile
    Join Date
    Jan 2002
    Posts
    2,227
    faq.
    CProgramming FAQ
    Caution: this person may be a carrier of the misinformation virus.

  3. #3
    Registered User big146's Avatar
    Join Date
    Apr 2003
    Posts
    74

    Random Numbers

    Hi there,

    #include<cstdlilb>

    int random;
    random = 1 + rand() % 25;
    will generate random numbers in the range of 1 to 25.
    But these will be fairly predictable numbers.

    #include <ctime>
    #include <cstdlib>

    srand(time(0) ); //uses clock to seed the random number

    dice1 = 1 + rand() % 6;
    more random- uses clock seconds to seed.

    Hope this helps.

  4. #4
    Registered User New001's Avatar
    Join Date
    May 2004
    Posts
    6
    Ok thanks

  5. #5
    Registered User Russell's Avatar
    Join Date
    May 2004
    Posts
    71
    A better seed:
    Code:
          1 #include <iostream>
          2 
          3 extern "C" {
          4   #include <time.h>
          5 }
          6 
          7 int main(int argc,char* argv[])
          8 {
          9   struct timespec seed;
         10   for (int i=0;i<=10;++i) {
         11     clock_gettime(CLOCK_REALTIME,&seed);
         12     srand(seed.tv_sec+seed.tv_nsec);
         13     std::cout<<rand()%10000<<std::endl;
         14   }
         15 
         16   return 0;
         17 }

  6. #6
    Code Goddess Prelude's Avatar
    Join Date
    Sep 2001
    Posts
    9,897
    >struct timespec seed;
    >clock_gettime(CLOCK_REALTIME,&seed);
    Neither of these are standard. A good, portable method still uses the current time, but instead uses a pseudo-hash of the bytes in a time_t value:
    Code:
    /*
      Based off of Ben Pfaff's implementation that was based off
      of Lawrence Kirby's implementation. :-)
    */
    #include <cstddef>
    #include <ctime>
    #include <limits>
    
    unsigned
    time_seed()
    {
      time_t         timeval; /* Current time. */
      unsigned char *ptr;     /* Type punned pointed into timeval. */
      unsigned       seed;    /* Generated seed. */
      size_t         i;
    
      timeval = std::time(0);
      ptr = reinterpret_cast<unsigned char *>(&timeval);
      seed = 0;
      for (i = 0; i < sizeof timeval; i++) {
        seed = seed * (std::numeric_limits<unsigned char>::max() + 2U) + ptr[i];
      }
    
      return seed;
    }
    You would use it like this:
    Code:
    std::srand(time_seed());
    >std::cout<<rand()%10000<<std::endl;
    Using the low order bits of a random number may not result in a good distribution. I've seen long strings of repeating numbers when using modulus with rand. A better way would be to divide by RAND_MAX. Here's a nifty function to do that:
    Code:
    #include <cstdlib>
    #include <iostream>
    
    double
    random()
    {
      return static_cast<double>(std::rand()) / RAND_MAX;
    }
    
    int
    main()
    {
      std::srand(time_seed());
      for (int i = 0; i < 10; i++) {
        std::cout<< static_cast<int>(random() * 10) <<std::endl;
      }
    }
    My best code is written with the delete key.

  7. #7
    Registered User
    Join Date
    Mar 2002
    Posts
    1,595

    Smile

    Wait a minute. Assume, for sake of simplicity, that RAND_MAX is 10,000 (I know that that isn't the real value of RAND_MAX) and that rand() returns a value between 0 and RAND_MAX. Division of the return value by RAND_MAX will result in values ranging from 1 to 0.0001. The number of times 0 appears in the first placeholder to the right of the decimal has to be greater than the number of times any other digit appears in that spot given that 0 is in that spot for all numbers less than 0.1, which is well over half of all the possible values obtainable by division of RAND_MAX. Multiplication of that value by 10 will promote the first digit to the right of the decimal to integer status, meaning the probability of having 0 as the final result of (rand()/RAND_MAX) * 10 has to be over 50%, too. Therefore the chance of having a string of zeros as a result of that process should be significantly higher than a string of any given digit using a modulus with a constant (other than 1). Or am I missing something here? (Wouldn't be the first time.)

  8. #8
    & the hat of GPL slaying Thantos's Avatar
    Join Date
    Sep 2001
    Posts
    5,681
    1) The range of numbers is 0 to 1 when using the divison method.
    2) Every RAND_MAX I've seen is odd. I'm fairly confident this is by design to avoid problems such as above.

  9. #9
    Registered User
    Join Date
    Mar 2002
    Posts
    1,595
    But that begs the argument (argument here being my side of the debate, not speaking in anger) as posed.
    Last edited by elad; 05-25-2004 at 11:11 AM.

  10. #10
    & the hat of GPL slaying Thantos's Avatar
    Join Date
    Sep 2001
    Posts
    5,681
    Well really it could be a lot less than .0001 since you are storing it into a double. In set notation it would look like [0,1].

    If RAND_MAX was 10000 then yes you could have problems. However that is completely outside the bounds of what is. Values I've seen used for RAND_MAX:
    16383
    32767
    65535
    2147483647

    Notice a pattern here?

  11. #11
    Registered User
    Join Date
    Oct 2001
    Posts
    2,934
    >for all numbers less than 0.1, which is well over half of all the possible values obtainable by division of RAND_MAX.

    Why do you say well over half? I would say 1/10, or 10%.

  12. #12
    Registered User Russell's Avatar
    Join Date
    May 2004
    Posts
    71
    Quote Originally Posted by Prelude
    >struct timespec seed;
    >clock_gettime(CLOCK_REALTIME,&seed);
    Neither of these are standard.
    No, I didn't think so.

    Thanks for that extra information. =)

  13. #13
    Registered User
    Join Date
    Mar 2002
    Posts
    1,595
    swoopy---Thanks for getting me to think through the problem from a different angle and therefore helping me see the error of my initial logic. You are correct. I am wrong.

    If rand() returns a value from 0 to RAND_MAX and RAND_MAX is defined as 10000 then you can generate a table as follows:

    0/10000 = 0
    1/10000 = 0.0001
    10/10000 = 0.001
    100/10000 = 0.01
    1000/10000 = 0.1
    2000/10000 = 0.2
    3000/10000 = 0.3

    Therefore there are 1000 possible values of rand(), 0 to 999, that will give results less than 0.1, or 10% of all possible values of rand(). Likewise there are 1000 possible values of rand(), 1000 to 1999, that will give end results 0.1 to < 0.2 which is 10%, and similar for 0.2 to 0.3, etc. The same logic should hold for any value assigned to RAND_MAX. Now Preludes code makes sense.

    Thank you.

  14. #14
    Registered User
    Join Date
    Oct 2001
    Posts
    2,934
    Elad, both you and Prelude are much more logical thinkers than myself. For the part of your quote I posted, I thought maybe I had interpreted it wrong, so wanted some clarification from you. It's fun to learn new things from threads of this nature.

    It's understandable why dividing by RAND_MAX gives an even distribution, but it's unclear why modulus doesn't. However since Prelude has explained this before, I was aware modulus was to be avoided.

  15. #15
    Registered User
    Join Date
    Mar 2002
    Posts
    1,595
    swoopy, I'm not sure why Preludes indicates that rand()/RAND_MAX is less likely to give a string of equal values on repeated values than rand()/modulus, either; but it clearly isn't the reason I initially came up with, as you pointed out. Maybe the problem is I'm misinterpretting what Prelude wrote in the first place.

Popular pages Recent additions subscribe to a feed

Similar Threads

  1. rapid random number generation problem
    By Newton in forum C Programming
    Replies: 17
    Last Post: 09-19-2008, 02:08 PM
  2. Random number in range generation.
    By hebali in forum C Programming
    Replies: 19
    Last Post: 03-04-2008, 10:46 AM
  3. adding a number to a number
    By bigmac(rexdale) in forum C Programming
    Replies: 11
    Last Post: 10-24-2007, 12:56 PM
  4. random number tutorial
    By 7stud in forum C++ Programming
    Replies: 3
    Last Post: 07-26-2005, 02:41 PM
  5. Random Number Generator
    By Ikurik in forum C++ Programming
    Replies: 16
    Last Post: 08-17-2003, 07:34 PM