Functions and Scope

Multiple Returns

Lua functions can return more than one value.

several results Assign each returned value to its own local name when both results matter.

Multiple Returns

total
multiple_returns.lua
Replay: real traced execution (multi-file project)
local total = 8

local function split_total(value)
  local left = value / 2
  local right = value - left
  return left, right
end

local first, second = split_total(total)

print("total=" .. total)
print("first=" .. first)
print("second=" .. second)

local total = 6

local function split_total(value)
  local left = value / 2
  local right = value - left
  return left, right
end

local first, second = split_total(total)

print("total=" .. total)
print("first=" .. first)
print("second=" .. second)

local total = 12

local function split_total(value)
  local left = value / 2
  local right = value - left
  return left, right
end

local first, second = split_total(total)

print("total=" .. total)
print("first=" .. first)
print("second=" .. second)

  1. total ← 8

    1local total→ 8 = 8 --@total=6, 1223local function split_total(value)4  local left = value / 25  local right = value - left6  return left, right7end89local first, second = split_total(total8)

  2. first ← 4.0, second ← 4.0

    9local first→ 4.0, second→ 4.0 = split_total(total8)1011print("total=" .. total8)12print("first=" .. first4.0)13print("second=" .. second4.0)
    outputtotal=8
first=4.0
second=4.0

  1. total ← 6

    1local total→ 6 = 623local function split_total(value)4  local left = value / 25  local right = value - left6  return left, right7end89local first, second = split_total(total6)

  2. first ← 3.0, second ← 3.0

    9local first→ 3.0, second→ 3.0 = split_total(total6)1011print("total=" .. total6)12print("first=" .. first3.0)13print("second=" .. second3.0)
    outputtotal=6
first=3.0
second=3.0

  1. total ← 12

    1local total→ 12 = 1223local function split_total(value)4  local left = value / 25  local right = value - left6  return left, right7end89local first, second = split_total(total12)

  2. first ← 6.0, second ← 6.0

    9local first→ 6.0, second→ 6.0 = split_total(total12)1011print("total=" .. total12)12print("first=" .. first6.0)13print("second=" .. second6.0)
    outputtotal=12
first=6.0
second=6.0

Follow the Two Results

  1. total starts as 8.
  2. split_total(total) receives 8.
  3. left becomes 8 / 2, which prints as 4.0.
  4. right becomes 8 - 4.0, which also prints as 4.0.
  5. Lua returns both values, so first=4.0 and second=4.0. | total | first | second | | --- | --- | --- | | 8 | 4.0 | 4.0 | | 6 | 3.0 | 3.0 | | 12 | 6.0 | 6.0 |

Exercise: multiple_returns.lua

Reproduce total=8, first=4.0, and second=4.0, then use the pinned total variants 6 and 12 to predict first=3.0 second=3.0 and first=6.0 second=6.0.