C中的偏置随机数生成器函数

Question

Trying to create the following function in C:

bool randBool(double bias)

which returns either 0 or 1 randomly.

The part that is tripping me up is that I would like to allow for the user to input a "bias" in the range [-1.0, 1.0], which represents the likelihood that the output will be 0 or 1 .

Here are a few examples of how the inputted bias should influence the function:

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randBool(-1.0) should return 0 100% of the time.

randBool(1.0) should return 1 100% of the time.

randBool(-0.5) is 50% more likely to return 0 than 1 .

randBool(0.05) is 5% more likely to return 1 than 0 .

randBool(0.0) is no more likely to return 0 than 1 .

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I am almost certain that this is a probability problem, but I am not so familiar with the topic, so I am stumped on how to implement this function.

Answer 1

Something like this :

bool randBool(double bias) {
    return rand() < ((RAND_MAX + 1.0) * ((bias + 1) / 2));
}

The ((bias + 1) / 2) part is to get the bias in the [0,1] range instead of [-1,1]. That could be avoided if the bias parameter is changed to already be in the [0,1] range.

The bias is then defined as the probability of a 1 being returned. With :

• 0.0 (corresponds to your bias -1.0 ) : all zeroes
• 0.25 (corresponds to your bias -0.5 ) : 25% ones, 75% zeroes
• 0.5 (corresponds to your bias 0.0 ) : even mix of ones and zeroes
• 0.525 (corresponds to your bias 0.05 ) : 52.5% ones, 47.5% zeroes
• 1.0 (corresponds to your bias 1.0 ) : all ones

NOTES on RAND_MAX

• If RAND_MAX < INT_MAX , then RAND_MAX + 1 can be used instead of RAND_MAX + 1.0 .

• If RAND_MAX + 1.0 cannot be represented by a double without rounding (ref. Are all integer values perfectly represented as doubles? ), then the presented solution is not reliable.
  An approach on such platforms, could be to re-scale the result of rand() to a range that can be represented by either an int or a double (depending on whether you use RAND_MAX + 1 or RAND_MAX + 1.0 ).
  Or alternatively, use a different random number generator (that doesn't have this issue), which is probably a good idea anyway, given the many low quality implementations of rand() out there.