Transfer practice · drain(initial) must use repeat to subtract 1 at least once, stopping at zero or below. Repair the amount subtracted. Compare drain(0) with a while-based drain: repeat runs its body before checking.

Charge a lamp without getting stuck: practice

v3 · Activity 1 of 1 · while · repeat · termination · 11 min Runs here

Chapter outline

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Mini-mission

Use a familiar idea in a different task. Read the changed requirement before running.

Your task

drain(initial) must use repeat to subtract 1 at least once, stopping at zero or below. Repair the amount subtracted. Compare drain(0) with a while-based drain: repeat runs its body before checking.

Interface: chargeTo(initial:number,target:number,increment:number) -> number

Input	Expected
Positive charge`3`	`0`
Zero begins with an iteration`0`	`-1`

See another example

local charge = 0
while charge < 5 do
    charge = charge + 1
end
return charge

Each iteration increases charge. After five updates, charge < 5 is false and the loop stops.

Syntax in this activity

New syntax: while ... do, repeat ... until

Earlier ideas used here

numeric-for
comparison
state-updates

Explain more

while checks its condition before each iteration. The body must change state so the condition eventually becomes false.

repeat checks its until condition after the body, so it runs at least once. That is useful only when one attempt is appropriate even at the initial boundary.

This exercise accepts positive whole-number increments. Invalid increments must return the initial charge immediately. The browser and server enforce external limits too, but your program should still have a valid termination argument.

Explain this code

Select a word or symbol in this authored example. Your editor draft stays unchanged.

  chargeToinitial, target, increment
     increment  0   initial 
     charge  initial
     charge  increment  target 
        charge  charge  increment
    
     charge

 chargeTo

Choose a highlighted part to see what it means.

Free experiment

This is a private experiment copy. It does not replace your answer or complete the activity.

These inputs apply when your scratchpad returns the function described by the activity interface.

Test different inputs

Font size Plain text

local charge = 0
while charge < 5 do
    charge = charge + 1
end
return charge

Luau keyword highlighting · Ctrl/⌘ + Enter: Run · Esc: leave editorTab inserts indentation

Experiment Output

Your output will appear here.

Reset starter

Replace this code with the starter? Download your draft to keep a copy.

Code

Luau

Font size Plain text

local function drain(initial)
    local charge = initial
    repeat
        charge = charge - 2
    until charge <= 0
    return charge
end
return drain

Luau keyword highlighting · Ctrl/⌘ + Enter: Run · Esc: leave editorTab inserts indentation

Your output will appear here.

Ready

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Your task

drain(initial) must use repeat to subtract 1 at least once, stopping at zero or below. Repair the amount subtracted. Compare drain(0) with a while-based drain: repeat runs its body before checking.

Interface: chargeTo(initial:number,target:number,increment:number) -> number

Input	Expected
Positive charge`3`	`0`
Zero begins with an iteration`0`	`-1`

See another example

local charge = 0
while charge < 5 do
    charge = charge + 1
end
return charge

Each iteration increases charge. After five updates, charge < 5 is false and the loop stops.

Syntax in this activity

New syntax: while ... do, repeat ... until

Earlier ideas used here

numeric-for
comparison
state-updates

Explain more

while checks its condition before each iteration. The body must change state so the condition eventually becomes false.

repeat checks its until condition after the body, so it runs at least once. That is useful only when one attempt is appropriate even at the initial boundary.

Explain this code

Select a word or symbol in this authored example. Your editor draft stays unchanged.

  chargeToinitial, target, increment
     increment  0   initial 
     charge  initial
     charge  increment  target 
        charge  charge  increment
    
     charge

 chargeTo

Choose a highlighted part to see what it means.

Free experiment

This is a private experiment copy. It does not replace your answer or complete the activity.

These inputs apply when your scratchpad returns the function described by the activity interface.

Test different inputs

Font size Plain text

local charge = 0
while charge < 5 do
    charge = charge + 1
end
return charge

Luau keyword highlighting · Ctrl/⌘ + Enter: Run · Esc: leave editorTab inserts indentation

Experiment Output

Your output will appear here.