Iterators and Generic For

Nested Iteration

Nested loops let one iteration pattern run inside another.

nested loops The inner loop completes for each outer loop value before the outer loop advances.

Nested Iteration

innerLimit
nested_iteration.lua
Replay: real traced execution (multi-file project)
local innerLimit = 2
local visits = 0
local lastLabel = ""

for outer = 1, 2 do
  for inner = 1, innerLimit do
    visits = visits + 1
    lastLabel = outer .. ":" .. inner
  end
end

print("innerLimit=" .. innerLimit)
print("visits=" .. visits)
print("lastLabel=" .. lastLabel)

local innerLimit = 1
local visits = 0
local lastLabel = ""

for outer = 1, 2 do
  for inner = 1, innerLimit do
    visits = visits + 1
    lastLabel = outer .. ":" .. inner
  end
end

print("innerLimit=" .. innerLimit)
print("visits=" .. visits)
print("lastLabel=" .. lastLabel)

local innerLimit = 3
local visits = 0
local lastLabel = ""

for outer = 1, 2 do
  for inner = 1, innerLimit do
    visits = visits + 1
    lastLabel = outer .. ":" .. inner
  end
end

print("innerLimit=" .. innerLimit)
print("visits=" .. visits)
print("lastLabel=" .. lastLabel)

  1. innerLimit ← 2, visits ← 0, lastLabel ← (empty)

    1local innerLimit→ 2 = 2 --@innerLimit=1, 32local visits→ 0 = 03local lastLabel→ (empty) = ""

  2. outer = 1, 2 do

    pass 1 of 2
    5for outer1 = 1, 2 do6  for inner = 1, innerLimit do7    visits = visits + 1

  3. visits ← 1, lastLabel ← 1:1

    pass 1 of 4
    5for outer = 1, 2 do6  for inner1 = 1, innerLimit2 do7    visits→ 1 = visits + 18    lastLabel→ 1:1 = outer1 .. ":" .. inner19  end
    All 4 passes — pass 1 is the card above
    passinneroutervisitslastLabel
    1110 → 11:1
    2211 → 21:2
    3122 → 32:1
    4223 → 42:2

  4. outer = 1, 2 do

    pass 2 of 2
    5for outer2 = 1, 2 do6  for inner = 1, innerLimit do7    visits = visits + 1

  5. print("innerLimit=" .. innerLimit)

    12print("innerLimit=" .. innerLimit2)13print("visits=" .. visits4)14print("lastLabel=" .. lastLabel2:2)
    outputinnerLimit=2
visits=4
lastLabel=2:2

  1. innerLimit ← 1, visits ← 0, lastLabel ← (empty)

    1local innerLimit→ 1 = 12local visits→ 0 = 03local lastLabel→ (empty) = ""

  2. outer = 1, 2 do

    pass 1 of 2
    5for outer1 = 1, 2 do6  for inner = 1, innerLimit do7    visits = visits + 1

  3. visits ← 1, lastLabel ← 1:1

    pass 1 of 2
    5for outer = 1, 2 do6  for inner1 = 1, innerLimit1 do7    visits→ 1 = visits + 18    lastLabel→ 1:1 = outer1 .. ":" .. inner19  end

  4. outer = 1, 2 do

    pass 2 of 2
    5for outer2 = 1, 2 do6  for inner = 1, innerLimit do7    visits = visits + 1

  5. visits ← 2, lastLabel ← 2:1

    pass 2 of 2
    5for outer = 1, 2 do6  for inner1 = 1, innerLimit1 do7    visits→ 2 = visits + 18    lastLabel→ 2:1 = outer2 .. ":" .. inner19  end

  6. print("innerLimit=" .. innerLimit)

    12print("innerLimit=" .. innerLimit1)13print("visits=" .. visits2)14print("lastLabel=" .. lastLabel2:1)
    outputinnerLimit=1
visits=2
lastLabel=2:1

  1. innerLimit ← 3, visits ← 0, lastLabel ← (empty)

    1local innerLimit→ 3 = 32local visits→ 0 = 03local lastLabel→ (empty) = ""

  2. outer = 1, 2 do

    pass 1 of 2
    5for outer1 = 1, 2 do6  for inner = 1, innerLimit do7    visits = visits + 1

  3. visits ← 1, lastLabel ← 1:1

    pass 1 of 6
    5for outer = 1, 2 do6  for inner1 = 1, innerLimit3 do7    visits→ 1 = visits + 18    lastLabel→ 1:1 = outer1 .. ":" .. inner19  end
    All 6 passes — pass 1 is the card above
    passinneroutervisitslastLabel
    1110 → 11:1
    2211 → 21:2
    3312 → 31:3
    4123 → 42:1
    5224 → 52:2
    6325 → 62:3

  4. outer = 1, 2 do

    pass 2 of 2
    5for outer2 = 1, 2 do6  for inner = 1, innerLimit do7    visits = visits + 1

  5. print("innerLimit=" .. innerLimit)

    12print("innerLimit=" .. innerLimit3)13print("visits=" .. visits6)14print("lastLabel=" .. lastLabel2:3)
    outputinnerLimit=3
visits=6
lastLabel=2:3