Re^2: Randomizing Large Integers

Is 32K a limitation of the standard rand function?

The macro (uconfig.h, line 3368 in 5.8.8 or 4057 in 5.10.0) looks like only 15 bits are being used:

#define Drand01()  ((rand() & 0x7FFF) / (double) ((unsigned long)1 << 
+15))
                                ^^^^
[download]

Update: though, a somewhat closer look suggests this might be configuration dependent (at least, Configure checks for drand48()), so maybe I spoke too soon...

Update2: just ran a test build on a system which has drand48()... et voila, the macro ends up being defined as

#define Drand01()               drand48()
[download]

So, as often, the answer is: it depends :)

Comment on Re^2: Randomizing Large Integers Select or Download Code

Replies are listed 'Best First'.
Re^3: Randomizing Large Integers by mscharrer (Hermit) on May 19, 2008 at 20:29 UTC
15 bit of the input or of the output? I get the following behavior that lets me assume that 32 bit random integers work: `# perl -e 'printf "%010x\n", int rand 216' 00000052e2 # perl -e 'printf "%010x\n", int rand 232' 006f7ae0c5 # perl -e 'printf "%010x\n", int rand 264' 00ffffffff` [download] i.e. `rand 2N` seems to return a `N` bit random integer, up to `N = 32` but not above.	[reply] [d/l] [select]
Re^4: Randomizing Large Integers by BrowserUk (Patriarch) on May 19, 2008 at 21:25 UTC
The 15-bit rand function is a Win32 Perl (MS CRT-based) problem. See Re^3: Rand Wackiness! and thread at A better rand() for Win32 for some further discussion. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice. "Too many [] have been sedated by an oppressive environment of political correctness and risk aversion."	[reply]
Re^4: Randomizing Large Integers by ikegami (Patriarch) on May 20, 2008 at 16:59 UTC
15 bit of the input or of the output? 15 bits of entropy. For a given argument, rand will only spit out 2¹⁵ different results (spread over the requested range).	[reply]