Control Flow Details

Elseif Chain

elseif checks ordered conditions until one branch matches.

ordered choices Put the most specific or earliest rule first, then let later branches handle the rest.

Elseif Chain

size
elseif_chain.lua
Replay: real traced execution (multi-file project)
local size = 12
local label = ""

if size < 10 then
  label = "small"
elseif size <= 15 then
  label = "medium"
else
  label = "large"
end

print("size=" .. size)
print("label=" .. label)

local size = 4
local label = ""

if size < 10 then
  label = "small"
elseif size <= 15 then
  label = "medium"
else
  label = "large"
end

print("size=" .. size)
print("label=" .. label)

local size = 20
local label = ""

if size < 10 then
  label = "small"
elseif size <= 15 then
  label = "medium"
else
  label = "large"
end

print("size=" .. size)
print("label=" .. label)

  1. size ← 12, label ← (empty)

    1local size→ 12 = 12 --@size=4, 202local label→ (empty) = ""

  2. label ← medium

    5  label = "small"6elseif size12 <= 15 then7  label→ medium = "medium"8else

  3. print("size=" .. size)

    12print("size=" .. size12)13print("label=" .. labelmedium)
    outputsize=12
label=medium

  1. size ← 4, label ← (empty)

    1local size→ 4 = 42local label→ (empty) = ""

  2. label ← small

    4if size4 < 10 then5  label→ small = "small"6elseif size <= 15 then

  3. print("size=" .. size)

    12print("size=" .. size4)13print("label=" .. labelsmall)
    outputsize=4
label=small

  1. size ← 20, label ← (empty)

    1local size→ 20 = 202local label→ (empty) = ""

  2. label ← large

    7  label = "medium"8else9  label→ large = "large"10end

  3. print("size=" .. size)

    12print("size=" .. size20)13print("label=" .. labellarge)
    outputsize=20
label=large

Pick a Size Label

  1. size starts at 12.
  2. The first check, size < 10, is false.
  3. The elseif check, size <= 15, is true.
  4. label becomes medium. | Size | First matching branch | Label | | --- | --- | --- | | 4 | if | small | | 12 | elseif | medium | | 20 | else | large |

Exercise: elseif_chain.lua

Use if, elseif, and else to label a size as small, medium, or large