44

I only know how I can generate a random boolean value (true/false). The default probability is 50:50

But how can I generate a true false value with my own probability? Let's say it returns true with a probability of 40:60 or 20:80 etc...

Hamid Pourjam
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Dark Side
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7 Answers7

68

One way is checking that the return value of Random.Next(100) is less than your desired probability. I can't speak to the true 'randomness' of this method though.

Proper example, using desired probability of 20%:

Random gen = new Random();
int prob = gen.Next(100);
return prob < 20;
E. Moffat
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    I would dare to assume that the predictability of these boolean values is just as random as `Random.Next()` would be. I don't know if that is the same as it's `randomness` though. – oɔɯǝɹ Jul 27 '16 at 20:00
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    For me, in ASP.Net the working syntax turned out a little more elaborate: `myRandomBool = new Random().Next(100) <= 50 ? true : false` – GerardV Jul 02 '19 at 19:32
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    @E. Moffat: `? true : false` is redundant code. – Alain Mar 04 '21 at 12:14
22

You generate a random number up to 100 exclusive and see if it's less than a given percent. Example:

if(random.Next(100) < 40) {
  // will be true 40% of the time
}

More generally, for a probability of X/Y, use an idiom like:

if(random.Next(Y) < X)
Hamid Pourjam
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Peter O.
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19

Here is an extension method that will provide a random bool with specified probability (in percentage) of being true;

public static bool NextBool(this Random r, int truePercentage = 50)
{
    return r.NextDouble() < truePercentage / 100.0;
}

you can use this like 

Random r = new Random();
r.NextBool(); // returns true or false with equal probability
r.NextBool(20); // 20% chance to be true;
r.NextBool(100); // always return true
r.NextBool(0); // always return false
Hamid Pourjam
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8

Assuming your probability is represented as double between 0.0 and 1.0, I would implement it more simply like this:

Random rand = new Random();
...
double trueProbability = 0.2;
bool result = rand.NextDouble() < trueProbability;

result will be true with the probability given by trueProbability

http://msdn.microsoft.com/en-us/library/system.random.nextdouble(v=vs.110).aspx

If this isn't "random enough", you can take a look at RNGCryptoServiceProvider:

http://msdn.microsoft.com/en-us/library/system.security.cryptography.rngcryptoserviceprovider(v=vs.110).aspx

Mike
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  • to be fully representative of the trueProbability shouldn't you do a <= instead of < test? – hubson bropa Jun 23 '15 at 22:51
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    @hubsonbropa No, since NextDouble() returns a value between 0 (inclusive) and 1 (exclusive). If you extrapolate to 100% probability, < 1 is sufficient. – Mike Jun 23 '15 at 22:57
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    @hubsonbropa Also, 0% should never be true. The only way to cover that case is to check for < 0. – Mike Jun 23 '15 at 23:31
6

I think it can help you 

Random gen = new Random();
bool result = gen.Next(100) < 50 ? true : false;
Ivan Suen
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2

For future knowledge:

40:60 would be:

var random = new Random();
return random.Next(10) < 4;

20:80 would be:

var random = new Random();
return random.Next(5) == 0

and 1:1 would be:

var random = new Random();
return random.Next(2) == 1;

Note: Just shorten the probability to the shortest variant - as for example: "random.Next(5) == 0" is quicker then "random.Next(100) <= 20 Though - if the probability changes from the user input - then it would look like:

[ModifierByChoice] bool GetProbability(int trueProbability, int falseProbability)
{
var random = new Random();
return random.Next(trueProbability, trueProbability + falseProbability) < trueProbability;
}
AssassinLV
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  • By reducing the fractions, the numbers written in the code suddenly become magical numbers. Unless you can and actually do prove that there is a significant slowdown by using larger numbers, the advice of reducing the fractions is counterproductive. Furthermore it may confuse beginning programmers that you sometimes use the `<` operator and sometimes the `==`, once even comparing to zero and the other time comparing to nonzero. – Roland Illig Feb 11 '19 at 18:22
-3
Random gen = new Random();
var boolVal = gen.Next(0, 1)==1? true : false;
alltej
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  • The question was: `But how can I generate a true false value with my own probability? Let's say it returns true with a probability of 40:60 or 20:80 etc...`. This answer is wrong. – Pang May 03 '16 at 01:28