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crandomprobability

Probabilistic Running Based on Percentage in C

I am reviewing a code which I believe runs based on probability. I would like to verify that if it is true. Does the following code snippet runs 80% of the time? I dont quite get why use 1000 then if our job is to merely run a code 80% of the time.

if(rand()%1000<1000*0.8){
 ...
   }
Solution

  • It will run approximately 80% of the time.

    • rand() returns a number between 0 and RAND_MAX which is probably 2,147,483,647
    • rand() % 1000 reduces that range to 0-999, although some numbers in the first half or so of the range will be slightly more common because RAND_MAX is not evenly divisible by 1,000
    • 1000 * 0.8 is just 800

    The use of 1,000 here is arbitrary. A clearer way of representing 80% would be:

    if (rand() % 100 < 80)

    or just:

    if (rand() < RAND_MAX * 0.8)