Cookbooks
Lua
Chapter 9. Testing and Debugging
Printing debug output

Lua:
Printing debug output

How to:

Here’s the skinny on printing stuff in Lua:

print("Hello, Debug World!")  -- Pops a string onto the console

local number = 42
print("The number is:", number)  -- Combine strings and numbers

local table = {name = "Lua", year = 1993}
print(table)  -- Prints the table reference, not super helpful

Sample Output:

Hello, Debug World!
The number is: 42
table: 0x194a330

To get into the table and show its guts, do this:

for key, value in pairs(table) do
    print(key, "=", value)
end

Sample Output:

name = Lua
year = 1993

Deep Dive

Printing debug output isn’t new or fancy. It’s reliable like an old hammer. See, back in the day, fancy debuggers weren’t around. Coders printed to see where things went south. Lua’s print function is straightforward. It shoves stuff to stdout—that’s usually your terminal.

Alternatives? Lua has a bunch. There’s the heavier io.write() if you need more control, like skipping new lines. Modules like inspect spill your tables’ guts better than print can.

Implementation of print is basic in Lua’s C source code. It uses tostring on each arg and shoves it to stdout with a newline. LuaJIT, a just-in-time compiler version of Lua, uses the same print approach, but belt-and-braces.

See Also

Get the bigger picture:

Using an interactive shell (REPL)