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This is an automatically generated e-mail. To reply, visit:
<a href="http://reviewboard.kde.org/r/4992/">http://reviewboard.kde.org/r/4992/</a>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">Math can look evil sometimes but if we want correctness, than we must use it :)
The statement that pseudo-random number generator's problems are shadowing the errors using the proposed formulae, is possible, but I'm not convinced. Usually, pseudo-random generators are chosen so that they fulfill most of the mathematical requirements related to random numbers. And I think the uniformity is one of the first to be checked.
At least, if you go for the incorrect one, make it obvious in the docs - something like "this random function favors some of the values which disrupts the uniform distribution, but is good enough for the general use-case"</pre>
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<p style="margin-top: 0;">On August 13th, 2010, 11:03 a.m., <b>Ivan Cukic</b> wrote:</p>
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<a href="/r/4992/diff/3/?file=33701#file33701line55" style="color: black; font-weight: bold; text-decoration: underline;">trunk/KDE/kdelibs/kdecore/util/krandom.cpp</a>
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(Diff revision 3)
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<td colspan="2"><pre style="font-size: 8pt; line-height: 140%; margin: 0; ">int KRandom::random( int min, int max )</pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "> <span class="k">return</span><span class="p">(</span> <span class="n">random</span><span class="p">()</span> <span class="o">%</span> <span class="p">(</span><span class="n">max</span><span class="o">-</span><span class="n">min</span><span class="o">+</span><span class="mi">1</span><span class="p">)</span> <span class="o">+</span> <span class="n">min</span> <span class="p">);</span></pre></td>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">As far as the % goes, I'd say it is ok (apart from previous comments)</pre>
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<p>On August 13th, 2010, 11:06 a.m., <b>Ivan Cukic</b> wrote:</p>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">My mistake, it is not :)
Imagine the following:
random() gives from 0 to 10
min = 0
max = 7
this function would give priority to 0 1 and 2
0 1 2 3 4 5 6 7 8 9 10
______________________
0 1 2 3 4 5 6 7 0 1 2</pre>
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<p>On August 13th, 2010, 11:28 a.m., <b>Christoph Feck</b> wrote:</p>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">In other words, there cannot exist any mapping from a uniform random integer distribution in the range [a,b] to a different integer range [d,e] resulting in a uniform distribution.
Since even floats have limited precision in a computer, you cannot solve the problem mathematically correct anyway.</pre>
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<p>On August 13th, 2010, 6:41 p.m., <b>Ingo Klöcker</b> wrote:</p>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">You can solve this problem correctly. Here is a trivial (but terribly inefficient) solution which works for any 0 <= min <= max <= RAND_MAX:
int KRandom::random( int min, int max )
{
while ( true ) {
const int r = random();
if ( min <= r && r <= max )
return r;
}
}
Making this method work for any ints with max <= min + RAND_MAX (e.g. for min = -1 and max = 1) and more efficiently by throwing away at most max - min values in the range [0,RAND_MAX] is left as an exercise for the reader.</pre>
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<p>On August 13th, 2010, 7:27 p.m., <b>Thomas Lübking</b> wrote:</p>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">That's not necessary.
Sure: Ivans exmple looks horrible, but random() returns (ideally, the random generator ain't perfect as well) even distributed numbers across the (positive) long range (0-2**31-1 according to posix requirements)
so in worst case (max-min)/2 numbers would be biased by.00000000046566128752 == 1/(2**31-1)
(0,1&2 have a probability of 2/11 compared to 1/11 of the rest in the example, for 12 base numbers 0,1,2&3 would be biased by 1/12 since they appear by 2/12 instead of 1/12)
- Sorry, i must I must confess that i've no ida about proper english terms on this - bash me if they make no sense ;-)
You can however pretty safely assume that this imprecision is shadowed by the general random generator weakness.
If it's _really_ a problem you'd kick the overweightened values (preknown) by a chance of 1/(2**31-1) (ie. random() == n, you can pick your birthdate for n, it doesn't matter) and try again.</pre>
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<p>On August 13th, 2010, 8:53 p.m., <b>Thomas Lübking</b> wrote:</p>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">errr... "me" == "brain dead" == "bread"
you'd kick them in (max-min-1)/(2**31-1) of all cases, ie. if min <= random() <= max
(yes - it is f*** confusing, tststs ;-)</pre>
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<pre style="margin-left: 1em; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">Mapping no, but the following should be mathematically correct (c-like pseudo code - didn't try to compile):
int range = max - min + 1;
int timesInPool = RAND_MAX / range;
int value;
while ((value = random()) >= timesInPool) {
}
value %= range;
The standard example is to use
while (value > range) ...
but that is rather inefficient, especially for smaller ranges.
Theoretically, the above could end up in an endless loop, but with probability = 0. The worst case scenarion would be to have the range of RAND_MAX / 2 + 1.
Anyhow, a limit to number of whiles can be added just in case. The uniformity grows with exponent (I think) of the defined limit.
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<p>- Ivan</p>
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<p>On August 12th, 2010, 1:40 p.m., Sebastian Trueg wrote:</p>
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<div>Review request for kdelibs.</div>
<div>By Sebastian Trueg.</div>
<p style="color: grey;"><i>Updated 2010-08-12 13:40:52</i></p>
<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">The KRandom namespace is quite useful. However, applications need to be full of code snippets that calculate a random number in a range. IMHO it makes perfect sense to do this once in the library. Thus, I am proposing this patch to introduce two methods providing random numbers from a range.
Feel free to improve the implementation. :)</pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
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<li>trunk/KDE/kdelibs/kdecore/util/krandom.h <span style="color: grey">(1162164)</span></li>
<li>trunk/KDE/kdelibs/kdecore/util/krandom.cpp <span style="color: grey">(1162164)</span></li>
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<p><a href="http://reviewboard.kde.org/r/4992/diff/" style="margin-left: 3em;">View Diff</a></p>
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