Lua Academy · Lesson

Transforming Tables: map, filter, reduce

Build functional higher-order helpers for table transformation.

Lesson 4 of 413 steps

Transforming Tables: map, filter, reduce is a free Lua Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Lua Academy learning path, and your progress syncs across the web and the CoddyKit app. The Lua Academy course includes 4 lessons in total.

Functional Patterns in Lua

Lua has no built-in map/filter/reduce, but they are trivial to implement using closures and generic for loops.

map: Transform Every Element

map applies a function to each element and returns a new table of results.

local function map(t, fn)
  local result = {}
  for i, v in ipairs(t) do result[i] = fn(v) end
  return result
end
local doubled = map({1,2,3}, function(x) return x * 2 end)
print(table.concat(doubled, ", "))  -- 2, 4, 6

filter: Keep Matching Elements

filter keeps only elements for which the predicate returns true.

local function filter(t, pred)
  local result = {}
  for _, v in ipairs(t) do
    if pred(v) then result[#result+1] = v end
  end
  return result
end
local evens = filter({1,2,3,4,5}, function(x) return x % 2 == 0 end)
print(table.concat(evens, ", "))  -- 2, 4

reduce: Fold to a Single Value

reduce combines elements left-to-right using an accumulator.

local function reduce(t, fn, init)
  local acc = init
  for _, v in ipairs(t) do acc = fn(acc, v) end
  return acc
end
local sum = reduce({1,2,3,4,5}, function(a,b) return a+b end, 0)
print(sum)  -- 15

Chaining Operations

Chain map, filter, and reduce for expressive data pipelines.

local nums = {1,2,3,4,5,6,7,8,9,10}
local result = reduce(
  map(
    filter(nums, function(x) return x % 2 == 0 end),
    function(x) return x * x end
  ),
  function(a,b) return a + b end, 0
)
print(result)  -- 220

forEach: Side Effects

A forEach iterates without building a new table — useful for side effects like printing.

local function forEach(t, fn)
  for i, v in ipairs(t) do fn(v, i) end
end
forEach({"a","b","c"}, function(v, i) print(i, v) end)

flatMap: Map then Flatten

flatMap maps each element to a list, then concatenates all lists.

local function flatMap(t, fn)
  local result = {}
  for _, v in ipairs(t) do
    for _, item in ipairs(fn(v)) do
      result[#result+1] = item
    end
  end
  return result
end
local expanded = flatMap({1,2,3}, function(x) return {x, x*10} end)

zip: Combine Two Tables

zip pairs elements from two tables into a table of pairs.

local function zip(a, b)
  local result = {}
  for i = 1, math.min(#a, #b) do
    result[i] = {a[i], b[i]}
  end
  return result
end
local zipped = zip({1,2,3}, {"a","b","c"})

Working with Dictionary Tables

For key-value tables, iterate with pairs instead of ipairs and collect key-value pairs.

local function mapValues(t, fn)
  local result = {}
  for k, v in pairs(t) do result[k] = fn(v) end
  return result
end
local doubled = mapValues({x=1,y=2}, function(v) return v*2 end)

Lazy Evaluation Concept

For large datasets, consider lazy iterators (closures that yield one element at a time) to avoid building large intermediate tables.

Performance Note

Each map/filter creates a new table allocation. For tight loops, in-place mutation may be more efficient, but sacrifices immutability and clarity.

reduce Question

What does reduce({1,2,3,4}, function(a,b) return a+b end, 0) return?

Answer it in the interactive lesson

Recap: map/filter/reduce

map transforms, filter selects, reduce folds. These three higher-order functions form the foundation of functional data processing in Lua.

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Frequently Asked Questions

Is the “Transforming Tables: map, filter, reduce” lesson free?

Yes — any 3 lessons of the Lua Academy learning path, including “Transforming Tables: map, filter, reduce”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Lua Academy course includes 4 lessons in total.

What will I learn in “Transforming Tables: map, filter, reduce”?

Build functional higher-order helpers for table transformation. You practise Lua Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Lua Academy?

No prior experience is required. Lua Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Transforming Tables: map, filter, reduce” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

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All lessons in this course

table.sort with Comparators
table.concat and table.move
Deep Copy and Shallow Copy
Transforming Tables: map, filter, reduce

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