Re: random integer


"Richard Heathfield" <rjh@xxxxxxxxxxxxxxx> wrote in message
news:ea6dnVQWNKf-lLLbnZ2dnUVZ8trinZ2d@xxxxxxxxx
Phillip said:

bob@xxxxxxxxxxxxxx wrote:

For instance, if you want a number between 0 and 31999, you can't
just do rand()%32000 since 0-767 will be twice as likely.

Why ?

Pretend that you have four-bit ints, RAND_MAX is 15, and you want a
random number in the range 0 to 9. This is the same problem, just
scaled down a bit.

R R % 10
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9
10 0
11 1
12 2
13 3
14 4
15 5

As you can see, the numbers 0 to 5 are twice as likely to appear as the
numbers 6 to 9.

Now scale up.


if RAND_MAX is 100015 then numbers 0-5 will appear 10002 times, and 6-9 will
appera 10001 times.



.

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