konstantin - 10 months ago 33

Java Question

I am trying to understand a block of code i have how exactly it works. My code is an example of recursion and try to calculate the chance of a prime number as a number of a nth dice rolling outcome. My code is the following:

`public static double ev3(int sum, int rollsLeft) {`

if (rollsLeft == 0) {

if (sum == 3 ){

return 1;

}

else

return 0;

}

double rsum = 0;

for (int i = 1; i <= 6; i++) {

rsum += ev3(sum + i, rollsLeft - 1)/6.0;

}

return rsum;

}

I struggle to understand how the recursion exactly is working and if I am doing what I am suppose to. Why sum takes values beyond six.

Answer

Your function iterates over all possible permutations for some number of dice rolls and provides some metric for them. It's called recursively multiple times:

- once initially (not recursively)
- 6 times for all the possible results from the first dice roll
- 6
^{2}times for all the possible results from the second dice roll (6 times for each of the possible results from the first dice roll) - ...
- 6
^{n}times for all the possible results from the n-th dice roll

Your function evaluates some custom metric, but if you replace 100 with 1, -50 with 0, and generalize the `sum == 3 || sum == 5 || sum == 7 || sum == 11 || sum == 13 || sum == 17`

expression to an `isPrime(sum)`

function, you'll be able to calculate the probability of the sum of n dice rolls being a prime number.