Pausing with coroutine.yield()

Part of the Logic & Flow section of Coddy's Lua journey — lesson 50 of 54.

The real power of coroutines comes from the coroutine.yield() function. This is what allows a coroutine to pause itself in the middle of execution and hand control back to the code that resumed it.

When a coroutine calls coroutine.yield(), it immediately stops running at that exact point. The next time you call coroutine.resume() on that coroutine, it will continue from right after the yield() call, as if nothing happened.

Here's a simple example:

function countToThree()
    print("One")
    coroutine.yield()
    print("Two")
    coroutine.yield()
    print("Three")
end

local co = coroutine.create(countToThree)
coroutine.resume(co)  -- Prints "One", then pauses
coroutine.resume(co)  -- Prints "Two", then pauses
coroutine.resume(co)  -- Prints "Three", then finishes

Notice how each resume() call runs the coroutine until it hits a yield(). The coroutine remembers exactly where it stopped, including all its local variables and execution state.

You can also pass values back when yielding. Any arguments you give to coroutine.yield() become the return values of coroutine.resume():

function giveNumbers()
    coroutine.yield(10)
    coroutine.yield(20)
end

local co = coroutine.create(giveNumbers)
local status, value = coroutine.resume(co)
print(value)  -- Prints: 10

status, value = coroutine.resume(co)
print(value)  -- Prints: 20

Challenge

Easy

Write a function createYieldingCoroutine that takes count and returns a string showing the output from a coroutine that yields values multiple times.

Create a coroutine that yields the string "Step X" for each step from 1 to count, then prints "Done" when finished. Resume the coroutine count + 1 times to get all yielded values and the final completion. Return all the yielded values concatenated with newlines.

Logic:

Create a function that uses a loop to yield "Step 1", "Step 2", etc., up to count
After all yields, the function should complete (no final yield for "Done")
Wrap this function in a coroutine using coroutine.create()
Resume the coroutine count times, collecting each yielded value
Concatenate all yielded values with newline characters between them
Return the concatenated string

Parameters:

count (number): The number of steps to yield

Returns: A string containing all yielded values separated by newlines (string). Format: Step 1\nStep 2\nStep 3

Cheat sheet

The coroutine.yield() function allows a coroutine to pause execution and return control to the caller. When coroutine.resume() is called again, the coroutine continues from right after the yield() call.

Basic usage of coroutine.yield():

function countToThree()
    print("One")
    coroutine.yield()
    print("Two")
    coroutine.yield()
    print("Three")
end

local co = coroutine.create(countToThree)
coroutine.resume(co)  -- Prints "One", then pauses
coroutine.resume(co)  -- Prints "Two", then pauses
coroutine.resume(co)  -- Prints "Three", then finishes

Passing values with coroutine.yield():

function giveNumbers()
    coroutine.yield(10)
    coroutine.yield(20)
end

local co = coroutine.create(giveNumbers)
local status, value = coroutine.resume(co)
print(value)  -- Prints: 10

status, value = coroutine.resume(co)
print(value)  -- Prints: 20

Arguments passed to coroutine.yield() become the return values of coroutine.resume(). The coroutine preserves all local variables and execution state between yields.

Try it yourself

function createYieldingCoroutine(count)
    -- Write code here
end

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in Logic & Flow

1Advanced Table Iteration

Iterating with pairs()Iterating with ipairs()pairs() vs. ipairs()Recap - Character Sheet

4Introduction to Metatables

What is a Metatable?setmetatable & getmetatable The __index Metamethod The __newindex Metamethod The __tostring Metamethod Arithmetic Metamethods Part 1 Arithmetic Metamethods Part 2 Recap - Read-Only Table

7Advanced String Manipulation

string.gsub() for Substitution Intro to String Patterns string.find()string.match()Iterating with string.gmatch()Recap - Log File Parser

2More Table Library Functions

table.concat()table construction & unpack()table.sort()Custom Sorting with Functions Recap - High Score Board

5Project: Text Adventure Engine

Project Setup: The Room Linking Rooms

Handling Invalid Moves

Displaying Exits

8Project: Contact List

Project Setup The Main Loop

Adding a Contact

Listing All Contacts

Searching for a Contact

Deleting a Contact

Handling Invalid Input

3Advanced Function Concepts

Returning Multiple Values Variadic Functions (...)Functions First-Class Values Anonymous Functions What is a Closure?Recap - Simple Event Handler

6Error Handling

The error() Function Protected Calls with pcall()Recap - Safe Division

9Coroutines for Beginners

What is a Coroutine?coroutine create & resumePausing with coroutine.yield()resume & yield Checking Coroutine Status Recap - Number Generator Recap - Vector Math