Weighted Loot

Some prizes should be rarer than others. This lesson gives each prize a weight, rolls one number, and walks a running total until the roll falls inside a prize’s share.

Contents

• Code
• Run
• Under the Hood
• Modify
• Make
• Good to Know

button Held -> chip pull
chip text -> decal Text

Code

const NAMES = ["Common", "Rare", "Epic"]
const WEIGHTS = [60, 30, 10]

@left in pull: exec
var prize: string = "Pull the lever"
@right out text = prize.Value

on pull {
  let roll = Random(1, 100)
  var total: int = 0
  var pick: int = 0
  for i, w in WEIGHTS {
    total += w
    if roll <= total {
      pick = i
      break
    }
  }
  prize = "${NAMES[pick]} (rolled ${roll})"
}

• NAMES and WEIGHTS are two const arrays, one entry per prize, in the same order. The weights add up to 100, so each is the chance in a hundred.
• roll is a number from 1 to 100.
• The loop adds each weight to total in turn: 60, then 90, then 100. The first time roll is at or under total, that prize wins and break stops the loop.
• So 1 to 60 is Common, 61 to 90 is Rare, and 91 to 100 is Epic.

The roll is 75. Which prize wins? What about 91?

Run

1. Compile the chip and paste it in-game. Place a Button (the lever) and a brick with a Text Decal.
2. Wire the Button’s Held to pull, and text to the Text Decal’s Text.
3. Pull a few times. Most pulls are Common, about one in three is Rare, and Epic turns up about once in ten.

Under the Hood

Open the chip. A const array that the circuit reads while it runs becomes an Array Var with its values typed in. WEIGHTS feeds the For Each Array loop. NAMES is read once, after the loop, by a Get Array Element with pick as its index.

Inside the loop, the roll comes in on a wire from the Random gate outside. It is rolled once, before the loop, so every iteration compares against the same number. A Random inside the loop would roll again on every iteration, and the weights would no longer mean what they say.

The arrays are const, so they cannot change while the game runs, and WEIGHTS.push(5) does not compile. Declare one with var to make it changeable.

Modify

1. Add a Legendary prize. Add "Legendary" to NAMES and a weight of 2 to WEIGHTS, and take 2 off Epic so the weights still add up to 100. Then check by hand which rolls give Legendary.

2. Color the prize by its rarity. A map literal, { key: value, ... }, can be a const too:

   const NAMES = ["Common", "Rare", "Epic"]
   const WEIGHTS = [60, 30, 10]
   const COLORS = { "Common": "aaaaaa", "Rare": "4080ff", "Epic": "c050ff" }
   
   @left in pull: exec
   var prize: string = "Pull the lever"
   @right out text = prize.Value
   
   on pull {
     let roll = Random(1, 100)
     var total: int = 0
     var pick: int = 0
     for i, w in WEIGHTS {
       total += w
       if roll <= total {
         pick = i
         break
       }
     }
     let won = NAMES[pick]
     prize = '<color="${COLORS[won]}">${won}'
   }
   

   COLORS[won] looks up the color for the prize’s name, the way NAMES[pick] looks up a name by its index.

3. Keep each prize and its weight together in one map, and loop over it:

   const LOOT = { "Common": 60, "Rare": 30, "Epic": 10 }
   
   @left in pull: exec
   var prize: string = "Pull the lever"
   @right out text = prize.Value
   
   on pull {
     let roll = Random(1, 100)
     var total: int = 0
     var won: string = ""
     for name, w in LOOT {
       total += w
       if roll <= total {
         won = name
         break
       }
     }
     prize = "${won} (rolled ${roll})"
   }
   

   The loop may visit the prizes in any order. That does not change the odds, because each prize still gets a stretch of the numbers as wide as its weight.

Make

Make a loot chest that never gives the same prize twice in a row. When the roll lands on the prize from last time, roll again, at most three times, and take whatever the third roll gives. Keep the last prize in a variable.

button Held -> chip pull
chip text -> decal Text

Good to Know

• The weights must add up to the top of the roll. If they add up to 90 and the roll goes to 100, a roll of 91 or more matches nothing, and the sign shows the first prize, since pick was never changed.
• Roll once, outside the loop. A roll inside the loop changes on every iteration.
• Weights set only the chances. Ten pulls can bring two Epics, or none.

See const containers at runtime in the reference.

Next: pick a lesson from the tree on the Learn page.