07. Hello, World

The traditional first program in any language prints Hello, world! and exits. It is tiny, but writing it proves the language is installed, the editor is set up, and the terminal can run files. This chapter writes it, breaks it to see an error, then fixes it.

The whole program

Open exercises/07/01-hello.lua in VS Code. The file contains one line:

print("Hello, world!")

In the VS Code terminal, run it:

lua exercises/07/01-hello.lua

The terminal prints:

Hello, world!

That is the whole program. The rest of this chapter explains each part of that line.

What print does

print is a function — a named piece of code that does a job when you ask for it. The job of print is to write whatever you give it to the terminal, followed by a new line.

The parentheses ( and ) after the name are how you ask the function to run. Anything between them is what you give it.

print("Hello, world!")
--    ^^^^^^^^^^^^^^^
--    this is what we are giving to print

In Lua, the things you give a function are called arguments. The program above gives print one: the string "Hello, world!".

Strings

The text "Hello, world!" is a string — a sequence of characters. In Lua, strings are written between double quotes " or single quotes '. Both work:

print("Hello, world!")
print('Hello, world!')

The quotes are not part of the string. They mark where the string starts and where it ends. When the string prints, only what is between the quotes appears.

Printing more than one thing

print can take more than one argument. Separate them with commas:

print("Hello,", "world!")

The terminal prints Hello,, a tab, then world!, all on one line. print puts a tab between each argument and a new line at the end.

For a different separator, build one string yourself with .. (two dots), which sticks strings together:

print("Hello, " .. "world!")

Now the output is Hello, world! with one space between the words, because the space is inside the first string.

Comments

A comment is text Lua ignores when it runs the program. Comments are for the humans reading the code. A single-line comment starts with --:

-- This line is a comment. Lua does not run it.
print("Hello, world!")  -- A comment can also sit at the end of a line.

A comment that spans many lines starts with --[[ and ends with ]]:

--[[
  Anything in here is ignored.
  Useful for long explanations
  or for temporarily turning off
  a block of code.
]]
print("Still runs.")

Use comments to explain why the code does something, not what each line does. Good code already says what.

Making and reading errors

Now break the program on purpose. In 01-hello.lua, remove the last quote so the line reads:

print("Hello, world!)

Save and run it. Lua refuses to run the file and prints an error like this:

lua: exercises/07/01-hello.lua:1: unfinished string near
'"Hello, world!)'
stack traceback:
        [C]: in ?

Read errors in this order:

1. The file name and line number (01-hello.lua:1). This is where the problem is. Look here first.
2. The short message (unfinished string near ...). This is what went wrong. "Unfinished string" means Lua saw the opening quote and started a string but never found the closing one.
3. The snippet after near shows the code that confused Lua.

Errors are not punishments. They tell you exactly which line to look at and hint at what is wrong.

A note on whitespace and case

Lua does not care how many spaces or blank lines are in your file. All three of these run the same:

print("Hello, world!")

    print(   "Hello, world!"   )

print("Hello, world!")

Lua does care about capital letters. print is a different word from Print and PRINT. The built-in name is lower case.

Homework

The homework files are in exercises/07/homework/. Open each starter in VS Code, read the comment at the top, and finish the file so it runs and produces the expected output.

"The best way to predict the future is to invent it."
                                            — Alan Kay

All good now.

Stuck or finished? Open the homework solutions page for a walkthrough.