rand()
Sorry if this is being ignorant but why has the rand() function not been changed? It's clearly not so random and I know that you can use srand() and seed the random function but why is that not just the norm?
rand() implement a linear congruential random number generator (LCRNG).
An LCRNG has the "formula":
R(i+1) = ( c1 * R(i) + k ) mod p
where
R(i) is the ith random number generated by the function;
c1 and k are carefully chosen constants
p is typically 2^32
By default, R(0) is 0, and thus the RNG will generate the same sequence of numbers every time.
srand() simply sets R(0) to something other than 0.
A "good" LCRNG such as the one provided by the C library has a maximal period. That is, the mod 2^32 operation means that the RNG generates numbers between 0 and 2^32 - 1, which is 2^32 numbers. Maximal period means that the RNG is guaranteed to generate every number exactly once before the sequence repeats. (Poorly chosen c and k can cause the period to become much less than 2^32 and thus not all numbers can be generated).
Since every number in the range 0...2^32-1 is generated by the C RNG, and since srand() takes a 32-bit value as parameter, all srand() does is change the "starting point" in the RNG "stream".
An LCRNG has the "formula":
R(i+1) = ( c1 * R(i) + k ) mod p
where
R(i) is the ith random number generated by the function;
c1 and k are carefully chosen constants
p is typically 2^32
By default, R(0) is 0, and thus the RNG will generate the same sequence of numbers every time.
srand() simply sets R(0) to something other than 0.
A "good" LCRNG such as the one provided by the C library has a maximal period. That is, the mod 2^32 operation means that the RNG generates numbers between 0 and 2^32 - 1, which is 2^32 numbers. Maximal period means that the RNG is guaranteed to generate every number exactly once before the sequence repeats. (Poorly chosen c and k can cause the period to become much less than 2^32 and thus not all numbers can be generated).
Since every number in the range 0...2^32-1 is generated by the C RNG, and since srand() takes a 32-bit value as parameter, all srand() does is change the "starting point" in the RNG "stream".
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When's this new version meant to be released?
Thanks for both of you're replies! It just seems that a lot of people seems to seed rand with (time(null)) so maybe that should just be what R should be set to by default. Just a though anywho.
Could I ask you to write a small c++ version using srand(time(null)) please? Simply outputing the value.
Thanks for both of you're replies! It just seems that a lot of people seems to seed rand with (time(null)) so maybe that should just be what R should be set to by default. Just a though anywho.
Could I ask you to write a small c++ version using srand(time(null)) please? Simply outputing the value.
time(0) (NULL is a macro for 0) returns the number of seconds passed since January 1st 1970. It is used to have a "random" number as seed. If you call srand(time(0)); only one in your program, you will get good random numbers as you can't know for sure which seed was set.C++0x had to be released at the end of the 2008 but they are still working on it
http://www.research.att.com/~bs/C++0xFAQ.html#when-standard
Let's just say it will be a while because even once its released it will take time for compiler vendors to implement it. The new standard only has about 500 more pages than the previous one.
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