Variables

Variables are holders or buckets for values with a name. For example:

i = 3

In this, i is the name of the variable, and the value three is being placed in the variable. Later if we say i = 5 i will still be the same variable, but it will contain a different value.

Types

Variables can be different kinds of values called types. Lua has these types:

• nil - this is a special type that represents nothing or empty.
• booleans - this is a type that can only contain the values true or false
• numbers - this represents a number, which can have a decimal part. Lua does not have a separate type for integers unlike most programming languages.
• strings - this is a sequence of characters, which are letters, numbers, punctuation, etc.
• tables - these are the only type of data structures lua has. tables contain pairs of keys and values, where you lookup a value from a key. A value can also be another table.
• functions - functions perform work or calculations on input(parameters) and optionally produce output(return) values
• userdata - these are special placeholders when working with C programs and Lua so that other types can be returned to Lua
• threads - these are units of parallel execution, basically mini-programs which are part of your program as a whole but execute simultaneously

For our first few lessons we will be working mostly with numbers and strings. We will not get into threads or userdata for a while, and not into tables or writing our own functions just yet.

First Program

It is a tradition for the first program in a new computer language to print the message "Hello, World!".

-- Lines which start with -- are comments, they have no effect on the
-- program's execution

-- These are all ways to print the same message in Lua
-- print is a function which takes one or more arguments or parameters
print("Hello, World")
-- Things inside quotations in Lua are string literals, just like 4.3 is a number literal.
print("Hello,", "World")
-- The .. operator concatenates (joins in order) two strings.
print("Hello," .. "World")

Create a text file, and try one of these ways to print the message. Name it something like hello.lua.

If your program is named hello.lua, you can execute it like this:

lua hello.lua

You can also just run the command lua to enter the Lua interpreter in immediate mode to enter commands that execute as you type them.

Loops

Loops are how you repeat something in a program. For example, to do something 10 times you could write a for loop like this.

-- i is a new variable
-- i has a different value each time thru the loop, from 1 to 10
for i = 1, 10 do
    -- whatever you want to do 10 times goes here.
end

-- you could create a similar loop using a while statement instead

i = 1
while i <= 10 do
    -- whatever you want to do 10 times
    i = i + 1
do

Now try printing out the numbers 1 thru 10 using a loop.

If Statements

If statements allow you to test a condition, something which may be true or false, then execute certain parts of your code contained inside the if statement, only when the condition is true.

For example:

if age < 18 then
    print("child")
end

This will only print "child" if the variable age contains a number less than 18.

If statements can also have an else clause which will be executed whenever the if statement was false.

if age < 18 then
  print("child")
else
  print("adult")
end

If statements can also include elseif clauses, which are checked against a condition only if the primary if statement and any preceding if statements were false. You can have as many elseif clauses as you like.

if age < 1 then
  print("infant")
elseif age < 3 then
  print("toddler")
elseif age < 13 then
  print("child")
elseif age < 18 then
  print("teenager")
else
  print("adult")
end

The statements inside the if/elseif/or else clause can contain other control flow statements, such as entirely separate if block or a loop.