How can you cheaply choose from a dynamic array randomly, with an oracle generated seed number, in Solidity?

Question

In JS you could use this and call it a day.

let randomArrayEntry = myArray[Math.floor(Math.random() * myArray.length)];

However, in Solidity everything is deterministic unless you call an outside function for a random number. Easy in theory, but the problem is that it is not currently practical with available services to choose a number matching your array length.

If my array length is 23, and the oracle spits out 1823792836199027320224 as the seed we can reduce it down to the last two numbers % 10**2. Now we have the number 24 and we now have a range from 0-99 of possible numbers. How can we then choose fairly from the array using this number without spending a bunch of gas iterating over it?

The first solution I came up with is to check for the first two numbers in a row less than 24, and then that becomes the answer. The problem is that the randomly generated number might not have any two numbers in a row less than our array length. This becomes much more likely if our number is 2 or 3. The transaction would have to revert or fetch another random number as a fallback. This seems like a poor plan.

Edit 1:

The current solution (found after) is to just take the remainder.

uint256 randomArrayEntry = (randomNumber + myArray.length + 10) % myArray.length;

You multiply the random number by 10 so you don't divide zero or less than the array length (no remainder).

Answer 1

Good Vibes is right. Modulo and you're done.

The discussion gives me the impression that you don't fully understand how the remainder function solves it. There is no divide by zero, no need to multiply x 10. Just do:

random % array.length and you will get a number from 0 to array.length-1 which is precisely what you want.

0 % <anything> = 0
3 % 9 = 3
23 % 23 = 0
24 % 23 = 1
1212121212456453535356453121212121 % 23 = something from 0-22

It's not free, of course, but it's a simple math op. Further efforts to complicate the process will probably add expensive steps instead of improving performance.

This is as random as the random number that feeds it.

Hope it helps.