Learn Lua in Neovim: Iterating Tables — pairs, ipairs, Gap Behavior & Word Counter | Episode 20

Learn Lua in Neovim: Iterating Tables — pairs, ipairs, Gap Behavior & Word Counter

In Lua, tables are versatile data structures that can be iterated using two primary functions: ipairs and pairs. This tutorial explains how ipairs iterates over array-like tables in order, stopping at the first nil gap, while pairs visits all keys in any order, making it ideal for dictionaries. We also demonstrate these concepts by building a word frequency counter that uses pairs to tally occurrences.

Code

-- Example table with array-like structure
local fruits = { "apple", "banana", "cherry", nil, "date" }

print("Using ipairs (stops at first nil gap):")
for i, fruit in ipairs(fruits) do
  print(i, fruit)
end
-- Output:
-- 1 apple
-- 2 banana
-- 3 cherry
-- Note: iteration stops before "date" because of the nil gap at index 4

print("\nUsing pairs (visits all keys in any order):")
for key, value in pairs(fruits) do
  print(key, value)
end
-- Output includes all keys, including index 5 with "date"

-- Word frequency counter example
local text = "hello world hello lua lua lua world"
local word_freq = {}

for word in text:gmatch("%w+") do
  -- Increment count for each word, or initialize to 1
  word_freq[word] = (word_freq[word] or 0) + 1
end

print("\nWord frequency count:")
for word, count in pairs(word_freq) do
  print(word, count)
end
-- Sample output:
-- hello 2
-- world 2
-- lua 3

Key Points

• ipairs iterates over array elements in order starting from index 1 and stops at the first nil value.
• pairs iterates over all keys in a table regardless of order or gaps, suitable for dictionaries.
• Tables with gaps (nil values between elements) cause ipairs to stop early, but pairs still visits all entries.
• Use ipairs when order matters and the table is a sequence; use pairs for general key-value iteration.
• Building a word frequency counter with pairs demonstrates practical use of table iteration for aggregating data.