How to set rand() result to values in a certain range including negatives?

问题

I am trying to dynamically allocate an array of doubles and set each element to a random value within a range of positive or negative numbers, but I am having difficulty.

Right now, I can only figure out how to set numbers 0 - max.

Here is what I have so far:

double *random_arr(int size, double min, double max) {
    double *array0 = calloc(size, sizeof(double));
    if (array0 == NULL) {
            exit(1);
    }
    for (int i = 0; i < size; i++)
            array0[i] = (max * rand() / RAND_MAX);
    return array0;
}

My best guess:

for (int i = 0; i < size; i++)
    array0[i]=((max + min) * rand() / RAND_MAX) - min;

回答1:

Range should be (max - min), and you can pre-calculate a multiplication factor dividing that by RAND_MAX.

double factor = (max - min) / RAND_MAX;
for (int i = 0; i < size; i++)
    array0[i] = (rand() * factor) + min;

回答2:

Assuming rand() returns a floating point number 0<=x<1, you simply need to scale by the range of your desired numbers and then add the minimum.

Example: We want the numbers from -100 to -10

Your range is your max - min which is -10 - (-100) = 90

Let's assume your random number is 0.5

0.5 * 90 + -100 (your minimum) = -55

Let's check edge cases:

random number = 0.0

0.0 * 90 + -100 = -100

random number = 0.9999999999...

0.99999999... * 90 + -100 = -10

Great!

来源：https://stackoverflow.com/questions/16410136/how-to-set-rand-result-to-values-in-a-certain-range-including-negatives