I'm trying to figure out the best way to implement a loot drop system for my roguelike. I need individual enemies to have lists of items they can drop, each with different odds of dropping. Here's how I'm doing it now:

 public static Item checkDrop(List<LootChance> loot_list)
        {
            if (checkLootList(loot_list))
            {
                float roll = (float)rand.NextDouble() * 100;
                float chance_count = 0;

                foreach (LootChance lc in loot_list)
                {
                    if (roll <= lc.PercentChance)
                    {
                        return lc.Loot;
                    }
                    chance_count += lc.PercentChance;
                }
            }
            else
            {
                Log.add("Loot odds exceed 100%!");
            }

            return null;
        }

This works for now, but it will stop me from ever having the total odds of all possible loot items to exceed 100. I could see there being other, possibly better ways of handling this. How is this kind of thing usually calculated?

Normalise the values in the list.

Say we have a chance to drop eggs or widgets, with eggs being 10 times more likely than widgets. We have 10 + 1 possibilities, so eggs is 10/11 and widgets 1/11 chance.

Either use a roll from 1 to 11 or multiply up by 100 to get a percentage. Make the last item drop always if the others fail to avoid rounding error, if you use a percentage.

Thanks for the reply!

I avoided doing that because I use the remaining unused percent to decide when to drop nothing at all. Should I be making that a separate check? Like first roll to see if the enemy will drop an item, and after that roll to see what the item is?

Yeah, that's what I would do (use 2 checks). That's also easily expandable to have a distribution of number of items to drop (10% drop nothing, 50% drop 1 item, 30% drop 2, 10% drop 3, etc.).

So would the best way to do this be to hold all the possible loot outcomes in a collection that gets generated once when the enemy is instantiated, then just roll for which element of the collection to access?

I'd only generate the collection when you are about to use it (i.e. when the enemy is killed), and then throw it away.

Yeah I guess that would make more sense. Thanks for the help!