Range Over Built in Lua

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range iterates over elements in a variety of built-in data structures. Let’s see how to use range with some of the data structures we’ve already learned.

Here we use range to sum the numbers in a table (array-like structure in Lua).

nums = {2, 3, 4}
sum = 0
for _, num in ipairs(nums) do
    sum = sum + num
end
print("sum:", sum)

range on tables provides both the index and value for each entry. Above we didn’t need the index, so we ignored it with the blank identifier _. Sometimes we actually want the indexes though.

for i, num in ipairs(nums) do
    if num == 3 then
        print("index:", i)
    end
end

range on tables can also iterate over key/value pairs. Here is how we can do it with Lua tables that act as maps.

kvs = {a = "apple", b = "banana"}
for k, v in pairs(kvs) do
    print(k, "->", v)
end

range can also iterate over just the keys of a table.

for k in pairs(kvs) do
    print("key:", k)
end

range on strings iterates over Unicode code points. The first value is the starting byte index of the rune and the second the rune itself.

for i = 1, #("go") do
    local c = ("go"):sub(i, i)
    print(i - 1, string.byte(c))
end

To run the Lua code, save it to a file (e.g., range.lua) and then execute it using the Lua interpreter.

$ lua range.lua
sum: 9
index: 2
a -> apple
b -> banana
key: a
key: b
0 103
1 111

Next example: Pointers.