Changing Items

An item of a list works like a variable: you can give it a new value with =. On the left, write the list's name and the index in square brackets, just as when you read the item:

local plots = {"carrot", "potato", "pumpkin"}
plots[2] = "tomato"
print(plots[1], plots[2], plots[3])

This prints:

carrot	tomato	pumpkin

The second item is now "tomato", and the old "potato" is gone. The other items stay as they were. Numbers in a list change the same way. Lua has no +=, so you write the item on both sides:

local eggs = {4, 6, 5}
eggs[3] = eggs[3] + 2
print(eggs[3])

This prints:

7

Don't write == instead of =. plots[2] == "tomato" is a comparison, not a command, and on a line of its own, it ends with syntax error near '=='.

Adding an item to the end. The place right after the last item has the index #plots + 1. When you assign a value there, the list grows by one item:

local plots = {"carrot", "potato"}
plots[#plots + 1] = "wheat"
plots[#plots + 1] = "corn"
print(#plots, plots[3], plots[4])

This prints:

4	wheat	corn

After the first assignment, the list has three items, so the second one puts "corn" at index 4. This way, you can also fill a list that starts empty: local harvest = {}, and then harvest[#harvest + 1] = … as many times as you need.

Don't leave holes. A list should have its items at the indexes 1, 2, 3… with no gaps. If you assign an item further away than #plots + 1, or set an item in the middle to nil, the list gets a hole, and # stops being reliable:

local crops = {"carrot", "potato", "pumpkin", "wheat"}
crops[2] = nil
print(#crops)
local more = {"carrot", "potato", "pumpkin"}
more[5] = "corn"
print(#more)

This prints:

4
3

The first list has only three crops left, but # says 4. The second one has four crops, but # says 3: it doesn't see "corn" at all. With a hole, # may count up to the hole or past it, and Lua doesn't promise which. So add items only at #t + 1, and remove them the way the next exercise shows.