Exercise 37: Symbol Review
It's time to review the symbols and Ruby words you know and to try to pick up a few more for the next few lessons. I have written out all the Ruby symbols and keywords that are important to know.
In this lesson take each keyword and first try to write out what it does from memory. Next, search online for it and see what it really does. This may be difficult because some of these are difficult to search for, but try anyway.
If you get one of these wrong from memory, make an index card with the correct definition and try to "correct" your memory.
Finally, use each of these in a small Ruby program, or as many as you can get done. The goal is to find out what the symbol does, make sure you got it right, correct it if you did not, then use it to lock it in.
Keywords
| Keyword | Description | Example |
|---|---|---|
| BEGIN | Run this block when the script starts. | BEGIN { puts "hi" } |
| END | Run this block when the script is done. | END { puts "hi" } |
| alias | Create another name for a function. | alias X Y |
| and | Logical and, but lower priority than &&. | puts "Hello" and "Goodbye" |
| begin | Start a block, usually for exceptions. | begin end |
| break | Break out of a loop right now. | while true; break; end |
| case | Case style conditional, like an if. | case X; when Y; else; end |
| class | Define a new class. | class X; end |
| def | Define a new function. | def X(); end |
| defined? | Is this class/function/etc. defined already? | defined? Class == "constant" |
| do | Create a block that maybe takes a parameter. | (0..5).each do |x| puts x end |
| else | Else conditional. | if X; else; end |
| elsif | Else if conditional | if X; elsif Y; else; end |
| end | Ends blocks, functions, classes, everything. | begin end # many others |
| ensure | Run this code whether an exception happens or not. | begin ensure end |
| for | For loop syntax. The .each syntax is preferred. | for X in Y; end |
| if | If conditional. | if X; end |
| in | In part of for-loops. | for X in Y; end |
| module | Define a new module. | module X; end |
| next | Skip to the next element of a .each iterator. | (0..5).each {|y| next } |
| not | Logical not. But use ! instead. | not true == false |
| or | Logical or. | puts "Hello" or "Goodbye" |
| redo | Rerun a code block exactly the same. | (0..5).each {|i| redo if i > 2} |
| rescue | Run this code if an exception happens. | begin rescue X; end |
| retry | In a rescue clause, says to try the block again. | (0..5).each {|i| retry if i > 2} |
| return | Returns a value from a function. Mostly optional. | return X |
| self | The current object, class, or module. | defined? self == "self" |
| super | The parent class of this class. | super |
| then | Can be used with if optionally. | if true then puts "hi" end |
| undef | Remove a function definition from a class. | undef X |
| unless | Inverse of if. | unless false then puts "not" end |
| until | Inverse of while, execute block as long as false. | until false; end |
| when | Part of case conditionals. | case X; when Y; else; end |
| while | While loop. | while true; end |
| yield | Pause and transfer control to the code block. | yield |
Data Types
For data types, write out what makes up each one. For example, with strings, write out how you create a string. For numbers, write out a few numbers.
| Type | Description | Example |
|---|---|---|
| true | True boolean value. | true or false == true |
| false | False boolean value. | false and true == false |
| nil | Represents "nothing" or "no value". | x = nil |
| strings | Stores textual information. | x = "hello" |
| numbers | Stores integers. | i = 100 |
| floats | Stores decimals. | i = 10.389 |
| arrays | Stores a list of things. | j = [1,2,3,4] |
| hashes | Stores a key=value mapping of things. | e = {'x' => 1, 'y' => 2} |
String Escape Sequences
For string escape sequences, use them in strings to make sure they do what you think they do.
| Escape | Description |
|---|---|
| \\ | Backslash |
| \' | Single-quote |
| \" | Double-quote |
| \a | Bell |
| \b | Backspace |
| \f | Formfeed |
| \n | Newline |
| \r | Carriage |
| \t | Tab |
| \v | Vertical tab |
Operators
Some of these may be unfamiliar to you, but look them up anyway. Find out what they do, and if you still can't figure it out, save it for later.
| Operator | Description | Example |
|---|---|---|
| + | Add | 2 + 4 == 6 |
| - | Subtract | 2 - 4 == -2 |
| * | Multiply | 2 * 4 == 8 |
| ** | Power of | 2 ** 4 == 16 |
| / | Divide | 2 / 4.0 == 0.5 |
| % | Modulus | 2 % 4 == 2 |
| > | Greater than | 4 > 4 == false |
| . | Dot access | "1".to_i == 1 |
| :: | Colon access | Module::Class |
| [] | List brackets | [1,2,3,4] |
| ! | Not | !true == false |
| < | Less than | 4 < 4 == false |
| > | Greater than | 4 < 4 == false |
| >= | Greater than equal | 4 >= 4 == true |
| <= | Less than equal | 4 <= 4 == true |
| <=> | Comparison | 4 <=> 4 == 0 |
| == | Equal | 4 == 4 == true |
| === | Equality | 4 === 4 == true |
| != | Not equal | 4 != 4 == false |
| && | Logical and (higher precedence) | true && false == false |
| || | Logical or (higher precedence) | true || false == true |
| .. | Range inclusive | (0..3).to_a == [0, 1, 2, 3] |
| ... | Range non-inclusive | (0...3).to_a == [0, 1, 2] |
| @ | Object scope | @var ; @@classvar |
| @@ | Class scope | @var ; @@classvar |
| $ | Global scope | $stdin |
Spend about a week on this, but if you finish faster that's great. The point is to try to get coverage on all these symbols and make sure they are locked in your head. What's also important is to find out what you do not know so you can fix it later.
Reading Code
Now find some Ruby code to read. You should be reading any Ruby code you can and trying to steal ideas that you find. You actually should have enough knowledge to be able to read but maybe not understand what the code does. What this lesson teaches is how to apply things you have learned to understand other people's code.
First, print out the code you want to understand. Yes, print it out, because your eyes and brain are more used to reading paper than computer screens. Make sure you print a few pages at a time.
Second, go through your printout and take notes on the following:
- Functions and what they do.
- Where each variable is first given a value.
- Any variables with the same names in different parts of the program. These may be trouble later.
- Any if-statements without else clauses. Are they right?
- Any while-loops that might not end.
- Any parts of code that you can't understand for whatever reason.
Third, once you have all of this marked up, try to explain it to yourself by writing comments as you go. Explain the functions, how they are used, what variables are involved and anything you can to figure this code out.
Lastly, on all of the difficult parts, trace the values of each variable line by line, function by function. In fact, do another printout, and write in the margin the value of each variable that you need to "trace."
Once you have a good idea of what the code does, go back to the computer and read it again to see if you find new things. Keep finding more code and doing this until you do not need the printouts anymore.
Study Drills
- Find out what a "flow chart" is and draw a few.
- If you find errors in code you are reading, try to fix them, and send the author your changes.
- Another technique for when you are not using paper is to put # comments with your notes in the code. Sometimes, these could become the actual comments to help the next person.
Common Student Questions
- How would I search for these things online?
- Simply put "ruby" before anything you want to find. For example, to find yield search for ruby yield.