Lua's Repeat Loop

Like the While Loop, the Repeat Loop is a "condition-controlled" loop. Unlike the While Loop, the Repeat Loop guarantees that the loop body is executed at least one time, then stops when the controlling condition is true.

Repeat loops follow the pattern:

repeat
    -- loop body
    -- repeat as long as boolean expression is true
until [boolean expression]

As a simple example, let's sum up all numbers from 0 to 4 using a repeat loop:

local x = 0

local result = 0

repeat
    result = result + x

    x = x + 1
until x >= 5

print(result) -- 10

The break statement can be used to terminate a repeat loop early, just like a while loop. The following is equivalent to the previous example.

local x = 0

local result = 0

repeat
    result = result + x

    x = x + 1

    if x >= 5 then
        break
    end
until false

print(result) -- 10

Note that in this case we set the boolean expression to false, so that it continues to execute until the break is encountered. As while the while loop, care must be taken to prevent infinite loops.

Finally, let's implement the final example from the while loop section using a repeat loop, showing that the repeat loop also terminates when a return statement is encountered:

local function sum()
    local x = 0
    local result = 0

    repeat
        result = result + x

        x = x + 1

        if x >= 5 then
            return result
        end
    until false
end

print(sum()) -- 10