Looping over a List

So far, you've written indexes as numbers: crops[1], crops[2]. But an index can also be a variable. If i is 2, then crops[i] is the same as crops[2]. And a for loop can take i through all the indexes, from 1 to the length of the list:

local crops = {"carrot", "potato", "pumpkin"}
for i = 1, #crops do
    print(i .. ". " .. crops[i])
end

This prints:

1. carrot
2. potato
3. pumpkin

In the first iteration, i is 1 and the loop prints crops[1]. In the second iteration, it prints crops[2], and so on up to #crops, the last index. The loop works for a list of any length: for a hundred crops, it runs a hundred times, and for an empty list, it doesn't run at all, because for i = 1, 0 runs zero times.

Since Lua counts from 1, the loop goes from 1 to #crops. Two mistakes are common. Starting at 0 gives an extra iteration at the start, where the item is nil:

local crops = {"carrot", "potato", "pumpkin"}
for i = 0, #crops do
    print(i, crops[i])
end

This prints:

0	nil
1	carrot
2	potato
3	pumpkin

Going up to #crops + 1 gives an extra iteration at the end. With .., the program prints all the crops and then stops:

local crops = {"carrot", "potato", "pumpkin"}
for i = 1, #crops + 1 do
    print("Crop: " .. crops[i])
end

Crop: carrot
Crop: potato
Crop: pumpkin
main.lua:3: attempt to concatenate a nil value (field '?')

With the index in a variable, a loop can do more than print the items in order. It can go backwards, with the step -1, from the last index to the first:

local crops = {"carrot", "potato", "pumpkin"}
for i = #crops, 1, -1 do
    print(crops[i])
end

This prints:

pumpkin
potato
carrot

It can work with two lists at once. When the values that belong together have the same index in both lists, one i reads both of them. Here, days[i] is how many days crops[i] takes to grow:

local crops = {"carrot", "potato", "pumpkin"}
local days = {3, 4, 7}
for i = 1, #crops do
    print(crops[i] .. " grows " .. days[i] .. " days")
end

This prints:

carrot grows 3 days
potato grows 4 days
pumpkin grows 7 days

And it can change the items, because eggs[i] = … works with a variable index too. This loop doubles every number in the list:

local eggs = {4, 6, 5}
for i = 1, #eggs do
    eggs[i] = eggs[i] * 2
end
print(eggs[1], eggs[2], eggs[3])

This prints:

8	12	10