Section Summary: Getting Started
Activities 1.1 – 1.6. Use this to review, or to fill in something you missed.
1. Programs and the Workspace
A program is a list of instructions carried out in order, and the computer does exactly what the list says. You copy a template folder for every new program rather than editing the original, and you never edit conf.lua.
- Program
- A list of instructions a computer carries out in order.
- Template
- A starting folder you copy once for each new program. There are two: console and game.
- Terminal
- The panel where output appears and where you type input.
print- Shows a line of text in the terminal.
- Syntax error
- The program is not valid Lua, so nothing runs at all. Nothing is printed.
2. Drawing
love.draw is called by the engine about sixty times a second. Coordinates start at the top left and y grows downward. Rectangles are placed by their top left corner; circles by their center.
- Coordinate
- A pair of numbers giving a position.
(0, 0)is the top left of the window; the window is 800 by 600. love.graphics.rectangle- Takes a mode, then x, y, width and height. The x and y are the corner.
love.graphics.circle- Takes a mode, then x, y and radius. The x and y are the center.
love.graphics.setColor- Takes red, green and blue as numbers from 0 to 1, not 0 to 255. It stays set until you change it.
The 0-to-255 mistake is silent. Values above 1 are clamped to 1, so a color meant to be gray comes out white and everything drawn after it does too.
3. Variables
A variable is a name holding a value. = is an instruction, not a claim: work out the right side, then put the answer in the name on the left. That is why score = score + 10 makes sense.
- Variable
- A name that holds a value.
local- Makes a new variable. Used once per variable, when it is created.
- Assignment
- Putting a value into a variable. Written
=in Lua and←on the exam. - Reassignment
- Putting a different value into a variable that already exists. No
local. - Hardcoded
- A raw number typed straight into the program instead of named.
4. Input, and the Three Kinds of Wrong
io.read() always hands back a string, whatever the person typed. With 16 typed at a prompt and stored in age:
| Expression | What happens |
|---|---|
age * 2 | 32 — it silently works |
age == 16 | false — it silently lies |
age > 10 | an error — it crashes |
The fix is tonumber at the moment you collect the value. The silent lie is the dangerous one, because nothing tells you.
io.write- Prints without moving to a new line, so a prompt and the typed answer sit together.
io.read- Waits for a line to be typed and hands it back as a string.
tonumber- Turns a string into a number, or hands back
nilif it cannot. - Type
- What kind of value something is. So far: number, string, boolean.
5. Arithmetic
Normal order of operations, in Lua and on the exam. / never truncates — 17 / 5 is 3.4, not 3.
%- Remainder.
17 % 5is 2. WrittenMODon the exam, with the same precedence as*and/. math.floor- Rounds down to a whole number. What you use when you want division to truncate.
- Precedence
*and/before+and-. Parentheses beat everything.- Concatenation
..joins two pieces of text. Not the same as adding.
6. Exam Notation From This Part
| Lua | Exam notation | |
|---|---|---|
| Make a variable | local x = 400 | x ← 400 |
| Change it | x = x + 50 | x ← x + 50 |
| Show a value | print(x) | DISPLAY(x) |
| Read a value | io.read() | INPUT() |
| Remainder | a % b | a MOD b |
Two things to remember about DISPLAY: it is how the exam prints, and it puts a space after what it shows. TwoDISPLAY calls in a row give you space-separated output on one line, not two lines.
There is no local on the exam. The arrow makes the variable the first time and changes it every time after.