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colmmacc's profile
Colm MacCárthaigh
Colm MacCárthaigh
Colm MacCárthaigh
@colmmacc

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Colm MacCárthaigh

@colmmacc

AWS, Apache, Crypto, Irish Music, Haiku, Photography

Seattle
notesfromthesound.com
Joined April 2008

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    1. Colm MacCárthaigh‏ @colmmacc 29 Jun 2018
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      Have you ever needed to generate a random number in code? whether it's for rolling a dice, or shuffling a set, this tweet thread is here for you! There's no reason that it should be easy or obvious, very experienced programmers repeat common mistakes. I did, before I learned ...

      38 replies 947 retweets 2,806 likes
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    2. Colm MacCárthaigh‏ @colmmacc 29 Jun 2018
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      O.k. let's start with the most common problem, and the most common mistakes: how do we pick a random number between 0 and N inclusive, let's say N = 5, so like a dice that starts at zero because we're nerds.

      1 reply 1 retweet 53 likes
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    3. Colm MacCárthaigh‏ @colmmacc 29 Jun 2018
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      A common solution is to r = rand() % (N + 1). Easy, right? Wrong! This solution is biased. To see how, imagine that RAND_MAX is "15". 0 % 6 == 0, 6 % 6 == 0, and 12 % 6 == 0 , so there are three rand() values each that return 0. Same works for 1, 2, 3 ...

      6 replies 6 retweets 63 likes
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    4. Colm MacCárthaigh‏ @colmmacc 29 Jun 2018
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      But there are only two rand() values that return 4: 4 % 6 and 10 % 6. Same for 5. So 0, 1, 2, 3 are all 50% more likely than 4 or 5. That's bias! Of course with a big RAND_MAX, this bias diminishes, but it's still there. So don't do it this way.

      1 reply 2 retweets 42 likes
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    5. Colm MacCárthaigh‏ @colmmacc 29 Jun 2018
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      O.k. so what if we use a float to avoid this? It's tempting! We can build a float between 0.0 and 1.0 by doing f = (float) rand() / (float) RAND_MAX. So we build a little macro or function for that, and now we can just do r = randFloat() * 5, right?

      2 replies 1 retweet 20 likes
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    6. Colm MacCárthaigh‏ @colmmacc 29 Jun 2018
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      No! This doesn't work other, because FLOATING POINT IS LIES. The distribution between 0.0 and 1.0 is not evenly spread and there's all sorts of rounding fuzziness that will make r non-uniform. No good.

      5 replies 8 retweets 89 likes
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    8. Beast‏ @beast_pixels 1 Jul 2018
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      Replying to @e2021da17 @ReednJ @colmmacc

      Yes the logic is faulty, it doesn't matter whether the value range between 0.0 and 1.0 has a uniform distribution or not. All it means is that you get a loss of accuracy, that's why you generally should use doubles rather than floats for it.

      2 replies 0 retweets 0 likes
      Colm MacCárthaigh‏ @colmmacc 1 Jul 2018
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      Replying to @beast_pixels @ReednJ

      Wow! WHY?? Don't use doubles either! or ever use a floating-point scalar (even a bignum) if you want a uniform distribution of ints at the end. It won't be.

      6:32 AM - 1 Jul 2018
      1 reply 0 retweets 0 likes
        1. Beast‏ @beast_pixels 1 Jul 2018
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          Replying to @colmmacc @ReednJ

          Dividing a number by the maximum is mathematical operation that by definition is uniform. That 32 bit floats don't have enough accuracy to store all possible results is an different issue. (Using fixed points would fix that, but could also be affected by your modulo problem.)

          0 replies 0 retweets 0 likes
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