I'm trying to implement a linear congruential pseudorandom number generator (<http://en.wikipedia.org/wiki/Linear_congruential_generator>), since $RANDOM and /dev/random aren't standardized. I'm referring to the Shell & Utilities volume of POSIX.1-2008, but I'm running into some odd issues. This works fine:
But I want to use the generated integer in an arithmetic expansion, for modular arithmetic (I know it's a sloppy way of limiting the range of values, but it's good enough for my purpose).
I've tried the following, but it simply repeats "1103527590 0" over and over:
Similarly, this repeats "1103527590":
It looks like, if I try to store or use the output of rand(), either $rand_x is being reset to 1 each time or the arithmetic substitution in rand() isn't being used.
But if I access $rand_x directly, as in the following script, I get the correct sequence:
However I'd like to avoid exposing $rand_x like that (rand() will eventually become a library function). So does anyone have any idea why using echo and capturing/using the output of rand() isn't working?
I'm testing this in GNU Bash and dash on GNU/Linux. When I get this working, I'll test it in more shells.
---------- Post updated at 23:58 ---------- Previous update was at 23:26 ----------
Quote:
Originally Posted by PehJota
It looks like, if I try to store or use the output of rand(), either $rand_x is being reset to 1 each time or the arithmetic substitution in rand() isn't being used.
The former seems to be the case. If I echo $rand_x from rand() and just store the output of rand(), $rand_x is reset to 1 for some reason.
The following script illustrates this:
Note that in rand(), $rand_x is now also printed to standard error (file descriptor 2) so it appears on the terminal even though standard output is stored in $r.
This is in response to a private message I received (since I can't send PMs until I have 10 posts...).
I've implemented this linear congruential generator in C to generate the correct sequence of integers (that is, what I'm expecting from this shell function):
And here is the printed sequence:
So it's generating the correct number. I guess the problem is that this number is 32 bits long and would exceed a signed long integer on my i686 system. Oops.
I changed my modulus, multiplier, and increment so the returned value doesn't exceed 2^31. So now this works:
For some reason "echo" still just resets $rand_x to its original value... I wish I knew why it did that. But I'll just use "return".
I'll test this script in a few different shells now to see if the signed long issue affects numbers before they're wrapped with modulo. If not, I think this problem is solved.
EDIT:
Harish: I know it's an infinite loop. I just interrupt it after a few iterations.
Ygor: Oh, I didn't realize command substitution used a subshell (I see it now in POSIX.1-2008) or that variables are set differently in a subshell. Yeah, that explains the weird behavior with "echo" then. Thanks!
EDIT 2:
I used these new values in three different shells so far (GNU Bash, dash, and BusyBox ash) and in C (modifying the above code). I generated 1000 pseudorandom numbers using all four test cases, and all four results were equal. I think the results are undefined if there is overflow in the intermediate arithmetic though. Oh well; I'll test more shells to be sure.
EDIT 3:
Well I've tested 8 shells (GNU Bash, dash, BusyBox ash, zsh, csh, tcsh, ksh, and mksh) across two PC architectures, haha. All give the same 1000 numbers (seeded at 1), so the arithmetic seems portable. Anyway, thanks again Ygor and Harish!
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