The for Loop

Most loops that count have the same three parts: a variable created before the loop, a condition, and a step at the end of the block:

local day = 1          -- start
while day <= 5 do      -- condition
    print("Day " .. day)
    day = day + 1      -- step
end

Counting like this is so common that Lua has a loop that does all three in one line, the for loop:

for day = 1, 5 do
    print("Day " .. day)
end

Both loops print the same:

Day 1
Day 2
Day 3
Day 4
Day 5

Read the first line as “for day from 1 to 5, do”. The loop gives the variable day the values 1, 2, 3, 4 and 5, one in each iteration. Both numbers are included: the loop starts at the first one and ends with the last one. You don't create the variable before the loop, you don't write a condition, and you don't increase the variable yourself: for does all that. So you can't forget the step and end up in an endless loop.

The start and the end are separated by a comma. for day = 1 to 5 do gives main.lua:1: ',' expected near 'to'. They can also be variables or calculations, for example for plot = 1, plots do.

The loop variable is often called i, but any name works. The for loop makes it local by itself, so don't write local in front of it:

for local i = 1, 3 do
    print(i)
end

main.lua:1: <name> expected near 'local'

The loop variable exists only inside the loop. After the loop's end, the name is unknown again, so it is nil:

for i = 1, 3 do
    print(i)
end
print(i)

1
2
3
nil

If you need the value after the loop, use a while loop with a variable created before it, as in the previous exercises. On the other hand, each loop can have its own i, because every i disappears when its loop ends.

for or while? Use for when you know how many times the loop will run: from 1 to 10, or once for every day of a week. Use while when you don't know it in advance. Emma fills the trough for Bella the cow with water from the well, 8 liters per bucket. How many buckets does she need for at least 50 liters?

local water = 0
local buckets = 0
while water < 50 do
    water = water + 8
    buckets = buckets + 1
end
print("Buckets: " .. buckets)
print("Water: " .. water .. " liters")

Buckets: 7
Water: 56 liters