How do you create pseudo random numbers sequentially in c/c++?

Question

srand(time(NULL));
for (it=hand.begin(); it < hand.end(); it++)
    (*it) = rand() % 13 + 1;

This code does not work to create many random numbers at a time. Is there a way to do it that isn't as complex as Mersennes and isn't operating system dependent?

I think this sentence *"This code does not work to create many random numbers at a time"* ....is not clear enough! — Nawaz, Apr 16 '11 at 17:19
Duplicated many times. You only seed (i.e. call `srand`) ***once***. Then you just call `rand` over and over again. Essentially all PRNG work that way. — dmckee --- ex-moderator kitten, Apr 16 '11 at 17:19
I just realized I posted the code wrong! I'm not seeding rand more than once. I'm seeding it once and then when generating many random numbers in a row they all come out the same because they're based on the current second. — Dash, Apr 16 '11 at 17:25
Duplicate: http://stackoverflow.com/questions/4859089/always-repeated-numbers-given-by-rand Near duplicate here: http://stackoverflow.com/questions/1068350/random-number-function-is-misfiring and http://stackoverflow.com/q/3159644/2509 — dmckee --- ex-moderator kitten, Apr 16 '11 at 17:32
@Dash: srand() is called ONCE. Put it in main. Call it on entry and never touch it again. As long as your application is not called more than once a second you will not repeat the numbers for the next 60 years. — Martin York, Apr 16 '11 at 18:20

score 4 · Accepted Answer · answered Apr 16 '11 at 17:19

PRNGs don't create many PRNs at once. Each output is dependent on the previous output, PRNGs are highly stateful.

Try:

srand(time(NULL)); // once at the start of the program

for( int i = 0; i < N; ++i )
    r[i] = rand();

Even APIs that return an entire block of output in a single function call, have just moved that loop inside the function.

score 2 · Answer 2 · answered Apr 16 '11 at 17:20

2

Call srand just once, at the start of your program. Then call rand() (not srand(rand())) to generate each random number.

answered Apr 16 '11 at 17:20

Anomie

92,546
13
126
145

score 1 · Answer 3 · answered Apr 16 '11 at 17:20

1

Boost.Random has lots of good random number generators which are easy to use.

answered Apr 16 '11 at 17:20

Benjamin Bannier

55,163
11
60
80

score 0 · Answer 4 · answered Apr 16 '11 at 17:30

0

George Marsaglia posted a Multiply With Carry PRNG some time ago in sci.math.

I cannot say how good it is or how well it behaves, but you might want to give it a try.

It should be OS and platform independent.

answered Apr 16 '11 at 17:30

pmg

106,608
13
126
198

score 0 · Answer 5 · edited Apr 17 '11 at 12:02

0

"please make sure you answer the question" OK

for (int i=n1; i < n2; ++i)
  { 
  int k; 
  do k = rand(); while (i !=k); 
  // k is a sequential pseudo random number
  }

There may be issues with efficiency...

edited Apr 17 '11 at 12:02

answered Apr 16 '11 at 21:15

phunctor

599
3
9

How do you create pseudo random numbers sequentially in c/c++?

5 Answers5