LlFrand

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函数名
Function: float llFrand( float mag );
参数：float mag – 任何有效的浮点值
返回值：返回一个在[0.0，mag]或（mag，0.0].[1]范围内的伪随机浮点数

注意事项
随机数生成器不是熵的来源。 随机数序列在整个模拟器过程中是共享的，而不是独立的种子。因此，伪随机数的产生不适合任何需要完全可预测或完全不可预测结果的应用。 应该记住，当把一个整数作为mag传递给llFrand时，它将被隐式地类型转换为一个float。 范围[-224，+224]之外的许多整数不能用浮点表示（这是浮点类型的固有限制）；例如，在该范围之外不会出现奇数整数。因此，当将得到的浮点数转换为整数时，不可能生成大于224+1的均匀分布值，也不可能生成大于9*223+1或总计约7500万个不同的整数。可能需要两个llFrand调用来获得所需的整数范围；请参见下面的示例

示例

// *near* 50:50 chance of "Heads" vs. "Tails" integer coin_toss() { if (llFrand(1.0) < 0.5) return TRUE; return FALSE; } // Sometimes it is useful to get a random integer over a given range. This is // a surprisingly tricky and emotive subject and has caused endless discussion // on the scripting groups. The primary cause of probability errors when // employing llFrand is to have a varying bin size on the edges of the range. // // As the bracket notation indicates, [0.0, mag), the function is inclusive of // the 0.0 and exclusive of the entered value. Because an LSL floating point // number is only a subset of real numbers and does not have infinite // granularity, this schema will work for any float greater than float // t = 1.175494351e-38; at which value the function will return only zero. At a // float beyond this, a math error occurs. // Random integer generator: // Contributed by Mephistopheles Thalheimer, // original function posted by Hg Beeks // Returns a pseudo-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. // Caveats: // This function is not range checked and will fail if max < min integer random_integer(integer min, integer max) { return min + (integer)(llFrand(max - min + 1)); } say(string message) { llSay(0, message); } default { touch_start(integer total_number) { // *near* 50:50 chance of "Heads" vs. "Tails" if (coin_toss()) say("Heads"); else say("Tails"); integer n1 = random_integer(2, 8); // Return a random number between 2 and 8 say("I chose a " + (string)n1); } } // Example for generating an uniformly distributed integer with more than // 16 million possible values; in particular, this code will generate // 500,000,000 possible values, ranging from 0 to 499,999,999 inclusive. // // The method consists of not using llFrand() on a number larger than 16,777,216 // (2^24) but to divide the range into two numbers that are both less than that, // using a scheme of the form (integer)llFrand(a)*b + (integer)llFrand(b), where // a*b gives the desired range. // // For prime ranges, or ranges with a prime factor greater than 2^24, a rejection // scheme should be used (use this method to generate a number slightly above the // target range, and reject it and generate a new one if it exceeds the maximum). default { state_entry() { integer rand = (integer)llFrand(500)*1000000 + (integer)llFrand(1000000); llOwnerSay("Here's a random number between 0 and 499,999,999 inclusive: " + (string)rand); } } The following code produces the most possibilities for random negative integers (suitable for use as channel numbers for example) integer rand = 0x80000000 | (integer)llFrand(65536) | ((integer)llFrand(65536) << 16); // 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 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 } say(string message) { llSay(0, message); } // 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_TRIES = 10000; // The bigger the better.. but slower default { state_entry() { say("Bin tester ready."); bins = []; } touch_start(integer total_number) { say("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_TRIES; ++i) { // Replace the next line with the function you are testing r = silly_random_integer(MIN, MAX); // Note the output on the next line has about 0.5 expected hits on the first and last bin // r = random_integer(MIN, MAX); if (r > MAX || r < MIN) { out_of_range++; } else { v = llList2Integer(bins, r - MIN); bins = llListReplaceList(bins, [++v], r - MIN, r - MIN); } } for (i = 0; i <= range; ++i) { say("Bin #" + (string)(i + MIN) + " = " + (string)llList2Integer(bins, i)); } say("Number out of range = " + (string)out_of_range); } } //Exponential distribution // // Contributed by Pat Perth on October 5th 2013 // No rights reserved // // Return an exponentially distributed random number with expected value 'mean_value' // // Reference: Wikipedia article "Exponential distribution", in particular the section // entitled "Generating exponential variates" at // // http://en.wikipedia.org/wiki/Exponential_distribution (visited on October 5th 2013) // // The exponential distribution is often an appropriate way to simulate waiting times // of the sort "It takes about x seconds for <something> to happen." For // example, if you want to trigger a rain shower "about every seven days", use // // time_between_rain = random_exponential(7.0 * 24.0 * 60.0 * 60.0) ; // // in an llSleep(...) or llSetTimerEvent(...) call. // // Please notice the negative sign in the return value. float random_exponential(float mean_value) { return -mean_value * llLog(llFrand(1.0)); }

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