The official playground cannot run Random

Developers
1

When I use the playground on the official website to run
import Random "mo:base/Random";
It will prompt this error, is it because of a version problem?

截屏2021-07-01 下午10.28.05
截屏2021-07-01 下午10.28.052128×794 98.6 KB

btw, which Homie knows how to use Random, can anyone provide some code examples?

2

Random depend on the raw_rand system API, which is not implemented in the interpreter, so you are getting an assertion failure.

You can check this example for random usage: examples/motoko/random_maze at master · dfinity/examples · GitHub

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3

Hi @chenyan
Thanks for your reply, I have a question, please help.

I know that the range of the return value in rangeFrom is determined by the parameter p, the return value of rangeFrom: [0 … 2^p-1], if p is 3, then the range of the random number is [0…8].
If the random number range I want to obtain is [0…99] or [5…10], how should I determine this parameter p first.For example, I expect to get a random number from 0 to 99, but I need to know what p is first.
Thanks

4

I think this code should almost do what you want:

(or set p to be the binary logarithm of the range, draw a number, if in range, return it + mix, otherwise reject and try again)

5

Hey @claudio , I am trying to get a random number between [0…max) according to the code example you sent me.
But when I call the getMax function, the returned result is (opt 0). Is there any solution?
This is my code, thanks :pray:

func bit(b : Bool) : Nat {
    if (b) 1 else 0;
};

func chooseMax(f : Random.Finite, max : Nat) : ? Nat {
    assert max > 0;
    do ? {
      if (max == 1) return ? 0;
      var k = bit(f.coin()!);
    
      var n = max / 2;
      while (n > 1) {
        k := k * 2 + bit(f.coin()!);
        n := n / 2;
      };

      if (k < max)
        return ? k
      else chooseMax(f, max) !; // retry
    };
  }; 

public func getMax(max:Nat):async ?Nat{
  let entropy = await Random.blob(); 
  var f = Random.Finite(entropy);
  var result= chooseMax(f,max);
  return result;
};
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6

@claudio @chenyan can one of you please DM me. I have an issue related to this problem. Thank you.

1 Like
7

Oh dear, I’m sorry I never replied to your message - I must have overlooked a notification.

Turns out there is a bug in Random.mo that probably explains why you were seeing this behaviour.

Thanks to @LightningLad91 for investigating and prompting me again.

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