Custom iterators in Lua

The generic for drives any function that returns next values.

-- The generic for drives any iterator function you write.
local function torch_burn(hours)
  local left = hours
  return function()
    if left > 0 then
      left = left - 1
      return hours - left, left
    end
  end
end

for hour, remaining in torch_burn(4) do
  print(string.format("Hour %d: %d hours of light left",
    hour, remaining))
end

How it works

1. The factory returns a closure holding the remaining hours.
2. Returning nil ends the loop — that is the whole protocol.
3. Two returns per step feed two loop variables.

Keywords and builtins used here

• do
• end
• for
• function
• if
• in
• local
• print
• return
• then
• torch_burn

The run, in numbers

Lines

15

Characters to type

317

Tokens

57

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 53 seconds.

Type this snippet

Step 3 of 3 in Closures, step 16 of 27 in Language basics.

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