random number [0, 99]
in [C++]
|
From: quickcur on 8 Feb 2006 06:02 Hi, I am using the int version of rand() with MS Visual Studio. I would like to generate random number in the range of [0, 99]. Right now I am using rand() * 100 / (RAN_MAX + 1) I found that I got too much 0. 0 appears more than any other numbers. How can I fix it? Thanks, [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Ulrich Eckhardt on 8 Feb 2006 09:28 quickcur(a)yahoo.com wrote: > Hi, I am using the int version of rand() with MS Visual Studio. I would > like to generate random number in the range of [0, 99]. Right now I am > using > > rand() * 100 / (RAN_MAX + 1) > > I found that I got too much 0. 0 appears more than any other numbers. > How can I fix it? Several things on aove code: - RAND_MAX not RAN_MAX - RAND_MAX+1 might overflow - rand()*100 might overflow, too The easiest way is to call rand() until it gives a number in your range, obviously, but this could mean many calls. A more sophisticated way is to first determine the largest multiple of your range that is smaller or equal to RAND_MAX, call rand() until the value is in that range and then simply apply the modulo operator to the result. The reason is easy: Assume RAND_MAX=9, i.e. rand() gives you numbers from 0 to 9. If you need numbers in the range 0 to 3, the following mapping applies: 0 -> 0 1 -> 1 2 -> 2 3 -> 3 4 -> 0 5 -> 1 6 -> 2 7 -> 3 8 -> reroll 9 -> reroll Uli [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Carlos Moreno on 8 Feb 2006 09:30 quickcur(a)yahoo.com wrote: > Hi, I am using the int version of rand() with MS Visual Studio. I would > like to generate random number in the range of [0, 99]. Right now I am > using > > rand() * 100 / (RAN_MAX + 1) > > I found that I got too much 0. 0 appears more than any other numbers. > How can I fix it? Integer division. Plus, try a Google search on this newsgroup for plenty of discussions on *the* right way to do what you're trying to do (even if based on rand(), which is a poor RNG to begin with) -- I'm talking about discarding random numbers above the highest multiple of your number (100 in this case). HTH, Carlos -- [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Thomas Maeder on 8 Feb 2006 09:34 quickcur(a)yahoo.com writes: > Hi, I am using the int version of rand() with MS Visual Studio. I would > like to generate random number in the range of [0, 99]. Right now I am > using > > rand() * 100 / (RAN_MAX + 1) > > I found that I got too much 0. 0 appears more than any other numbers. > How can I fix it? Learn how integer arithmetics works in C++. The expression rand()*100 may cause an overflow, resulting in a number below RAND_MAX. Dividing that number by RAND_MAX+1 will always give 0. A naive, but better approach is int randValue_0_100(std::rand()%100); This will produce numbers in the range 0..99, but the lower numbers will occur slightly more often because RAND_MAX typically isn't a multiple of 100. To fix for that, something like int const upper_rand_limit(RAND_MAX/100*100); int randValue; do { randValue = std::rand(); } while (randValue>=upper_rand_limit); int randValue_0_100(randValue%100); should work. To be really sure about the quality of your pseudo-random numbers, you have to carefully look at how it's documented and/or implemented, of course. [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ]
From: Maxim Yegorushkin on 8 Feb 2006 12:23
quickcur(a)yahoo.com wrote: > Hi, I am using the int version of rand() with MS Visual Studio. I would > like to generate random number in the range of [0, 99]. Right now I am > using > > rand() * 100 / (RAN_MAX + 1) > > I found that I got too much 0. 0 appears more than any other numbers. > How can I fix it? (unsigned)rand() % 100; [ See http://www.gotw.ca/resources/clcm.htm for info about ] [ comp.lang.c++.moderated. First time posters: Do this! ] |