C++ function for picking from a list where each element has a distinct probability

Question

I have an array of structs and one of the fields in the struct is a float. I want to pick one of the structs where the probability of picking it is relative to the value of the float. ie

struct s{
  float probability;
  ...
}

s sArray[50];

What is the fastest way to decide which s to pick? Is there a function for this? If I knew the sum of all the probability fields (Note it will not be 1), then could I iterate through each s and compare probability/total_probability with a random number, changing the random number for each s? ie

if( (float) (rand() / RAND_MAX) < probability)...

Answer 1

Find out RAND_MAX as you say. Generate a random number up to RAND_MAX. Iterate through the array counting up the probabilities until you equal or exceed your generated random number. (With only 50 element performance shouldn't be an issue, otherwise store the sums of the probabilities once in another array and then do a bisection search into that for the random value.)

Answer 2

float p = (rand() / static_cast<float>(RAND_MAX)) * total_probability;
s* current = &sArray[0];
while ( (p -= current->probability) > 0)
    ++current;
// `current` now points to your chosen target