Difference between revisions of "LlFrand"
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// Sometimes it is useful to get a random integer over a given range. This is a suprisingly tricky and emotive subject | // Sometimes it is useful to get a random integer over a given range. This is a suprisingly tricky and emotive subject | ||
// The random number given by llFrand(); does NOT include the stated float. | |||
// and has caused endless discussion on the scripting groups. | // and has caused endless discussion on the scripting groups. | ||
// Because an LSL floating point number is only a subset of real numbers and does not have infinite granularity, | // Because an LSL floating point number is only a subset of real numbers and does not have infinite granularity, | ||
Revision as of 09:05, 19 January 2009
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Summary
Function: float llFrand( float mag );| 0.0 | Forced Delay |
| 10.0 | Energy |
Returns a float that is pseudo random number in the range [0.0,mag) or (mag, 0.0].
The sign of mag matches the return.
| • float | mag | – | Any valid float value |
Specification
returns a pseudo random number in range [0.0, mag) or (mag, 0.0], depending upon the sign of mag.
Caveats
The random number generator is not a source of entropy.
Examples
<lsl>
// Tosses a coin, giving a *near* 50:50 chance of a result.
integer coin_toss() {
if( llFrand(1.) < .5 ) return TRUE; return FALSE;
}
// Sometimes it is useful to get a random integer over a given range. This is a suprisingly tricky and emotive subject
// The random number given by llFrand(); does NOT include the stated float. // and has caused endless discussion on the scripting groups. // Because an LSL floating point number is only a subset of real numbers and does not have infinite granularity, // in most solutions one integer output will be over-represented by a tiny amount or else an integer will be produced // outside of the range. The probability of over-representation or over extension for a given call, is inversely // proportional to the range and is typically a maximum of 1 / 8388608 when the range is 1.
// Here are some solutions.
// Random integer generator // Contributed by Mephistopheles Thalheimer // Returns a psuedo-random integer in the range of min to max inclusive.
// Rationale: Expands the range by 1.0 to ensure equal bin spacing on ends relative to the middle of // the range and then uses an integer cast to round towards zero. The while loop discards the // improbable outlier that occurs when llFrand( n ) == n.
// Caveats: This function is not range checked and will fail if max < min // It is serious overkill as per Hg discussion below.
integer random_integer( integer min, integer max ) {
integer r;
do { r = min + (integer)( llFrand( max - min + 1 ) ); } while( r == max + 1 ) ; // Non recursive catch of the improbable
return r;
}
// Random integer generator and discussion
// Contributed by Hg Beeks
// Returns a psuedo-random integer in the range of min to max inclusive but is slightly more efficient ( but with risk ) to Mephs above.
// While it IS technically possible to get the maximum value in llFrand and cause this to return a value above the range, // the likelyhood of this occuring is 1:1,000,000 for every whole number in the range - You will have a 1:5,000,000 chance of // getting 5 if doing llFrand(5), at the very least likelyhood. Testing also seems to imply that llFrand goes past the six-digit // lead that float returns, which means that the likelyhood could, in fact, be lower than this. However, decreasing // the +1 to .999999 in fact affects the probability and would make the highest value you want just a smidgen less likely.
integer fast_random_integer( integer min, integer max ) {
return min + (integer)llFrand( max - min + 1 );
}
default
{
touch_start(integer total_number)
{
// When touched, say "Heads" with probability 0.5,
// otherwise, say "Tails."
if ( coin_toss() )
llSay(0, "Heads");
else
llSay(0, "Tails");
integer n1 = random_integer( 2, 8 ); // Return a random number between 2 and 8
llSay( PUBLIC_CHANNEL, "I chose a " + (string)n1 );
integer n2 = fast_random_integer( 3, 10 ); // Returns a (slightly faster) random integer between 3 and 10.
llSay( PUBLIC_CHANNEL, "I chose a " + (string)n2 );
}
}
</lsl>
<lsl>
// Simple integer random number tester // Contributed by Mephistopheles Thalheimer
// This is a random number tester designed to give a quick visual explanation and proof of why some // random integer functions just do not work. // In general, with any random number generator, if you can see a pattern emerging, then chances are, // the function is not random.
// The test case given "silly_random_integer( .. )" shows the type of pitfalls that can happen. Superficially, // it would seem like a good candidate. I thought so, and in fact mooted it in a discussion, however, a bit of thought reveals // that the first and last bin are only collecting rounded results from half the float space as the rest of the integers. // They are therefore under-represented in output, and the generator is flawed.
integer fast_random_integer( integer min, integer max )
{
return min + (integer)llFrand( max - min + 1 );
}
integer silly_random_integer( integer min, integer max ) {
return min + (integer)( llRound( llFrand( max - min ) ) ); // Looks good, but does not work
}
// Simple integer random number tester
// Contributed by Mephistopheles Thalheimer
list bins;
integer MIN = 2; // The minimum integer you want integer MAX = 5; // The maximum integer you want
integer NUMBER_OF_TRYS = 10000; // The bigger the better.. but slower
default {
state_entry()
{
llSay( PUBLIC_CHANNEL, "Bin tester ready.");
bins = [];
}
touch_start(integer total_number)
{
llSay( PUBLIC_CHANNEL, "Started, be patient" );
integer i;
integer r;
integer range = MAX - MIN;
for( i = 0; i <= range; ++i )
{
bins += [ 0 ];
}
integer v;
integer out_of_range;
for( i = 0; i < NUMBER_OF_TRYS; ++i )
{
r = silly_random_integer( MIN, MAX ); // Replace this with the function you are testing
// Note the output on this one has about 0.5 expected hits on the first and last bin
//r = random_integer( MIN, MAX );
//r = fast_random_integer( MIN, MAX );
if( r > MAX
Useful Snippets
Pseudo-random_Number_Generator - Suitable for apps which require repeatable results that feel random.
Notes
The random number generator is not a source of entropy.
The sequence of random numbers are shared across the entire process, and not independently seeded. Therefore, the pseudo random number generation is not suitable for any application which requires completely predictable or completely unpredictable results.
See Also
Functions
| • | llListRandomize |
Deep Notes
Signature |
|---|
function float llFrand( float mag ); |