Ruby: Introduction, Basics

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1 Ruby: Introduction, Basics Computer Science and Engineering College of Engineering The Ohio State University Lecture 3

2 Ruby vs Java: Similarities Imperative and object-oriented Classes and instances (ie objects) Inheritance Strongly typed Classes determine valid operations Some familiar operators Arithmetic, bitwise, comparison, logical Some familiar keywords if, then, else, while, for, class, new

3 But Ruby Looks Different Punctuation Omit: ;'s and ()'s on function calls Include: Function names ending in? and! New keywords and operators def, do..end, yield, unless ** (exp), =~ (match), <=> (spaceship) Rich core libraries Collections: Hashes, Arrays Strings and regular expressions Enumerators for iteration

4 Deeper Differences As Well Interpreted (typically) Run a program directly, without compiling Dynamically typed Objects have types, variables don't Everything is an object C.f. primitives in Java Code can be passed in to a function as a parameter Added to Java in version 8 ("lambdas")

5 Compiling Programs Program = Text file Contains easy-to-understand statements like "print", "if", "while", etc. But a computer can only execute machine instructions Instruction set architecture of the CPU A compiler translates the program (source code) into an executable (machine code) Recall "Bugs World" from CSE 2231 Examples: C, C++, Objective-C, Ada

6 Interpreting Programs An interpreter reads a program and executes it directly Advantages Platform independence Read-eval-print loop (aka REPL) Reflection Disadvantages Speed Later error detection (i.e., at run time) Examples: JavaScript, Python, Ruby

7 Combination of Both A language is not inherently compiled or interpreted A property of its implementation Sometimes a combination is used: Compile source code into an intermediate representation (byte code) Interpret the byte code Examples of combination: Java, C#

8 Ruby is (Usually) Interpretted REPL with Ruby interpreter, irb $ irb >> => 7 >> puts "hello world" hello world => nil >> def square(x) x**2 end => :square >> square -4 => 16

9 Literals Numbers (FixNum, Float, Rational, Complex) 83, 0123, 0x53, 0b , 0b101_ , e2, 12345E-2 2/3r, 4+3i Strings Delimeters " " and ' ' Interpolation of #{ } occurs (only) inside " " "Sum 6+3 is #{6+3}" is "Sum 6+3 is 9" Custom delimeter with %Q$ $ or %q$ $ Ranges 0..4 includes start and end value (0,1,2,3,4) "cab"..."cat" does not include end value Arrays and hashes (later)

10 Comments and Statements Single-line comments start with # Don't confuse it with string interpolation! Multi-line comments bracketed by =begin =end Must appear at beginning of line All statements have a value result Convention: => to indicate result "Hi #{name}" + "!" #=> "Hi Liam!"

11 Operators Arithmetic: + - * / % ** / is either or div, depending on operands Integer / (div) rounds towards -, not 0 % is modulus, not remainder 1 / 3.0 #=> / 3 #=> 0 (same as Java) -1 / 3 #=> -1 (not 0 as in Java) -1 % 3 #=> 2 (not -1 as in Java) Bitwise: ~ & ^ << >> 5 2 #=> 7 (ie 0b101 0b10) 13 ^ 6 #=> 11 (ie 0b1101 ^ 0b0110) 5 << 2 #=> 20 (ie 0b101 << 2)

12 Operators (Continued) Comparison: < > <= >= <=> Last is so-called "spaceship operator" Returns -1/0/1 iff LHS is smaller/equal/ larger than RHS "cab" <=> "da" #=> -1 "cab" <=> "ba" #=> 1 Logical: &&! and or not Words have low precedence (below =) "do_this or die" idiom needs low-binding x = crazy or raise "problem"

13 Pseudo Variables Objects self, the receiver of the current method (recall this) nil, nothingness (recall null) Booleans true, false nil also evaluates to false 0 is not false, it is true just like 1 or -4! Specials FILE, the current source file name LINE, the current line number

14 Significance in Names A variable's name affects semantics! Variable name determines its scope Local: start with lowercase letter (or _) Global: start with $ Many pre-defined global variables exist, e.g.: $/ is the input record separator (newline) $; is the default field separator (space) Instance: start Class: start with Variable name determines mutability Constant: start with uppercase, eg Size but idiom is all upper case, eg SIZE

15 Basic Statements: Conditionals Classic structure if (boolean_condition) [then]... else... end But usually omit ( )'s and "then" keyword if x < 10 puts "small" end "If" keyword also a statement modifier x = x + 1 if x < LIMIT Good for single-line body Good when statement execution is common case Good for positive conditions

16 Variations on Conditionals Unless: equivalent to "if not " unless size >= 10 puts "small" end Can also be a statement modifier x = x + 1 unless x >= LIMIT Same idiom: single-line body, execution is common case, and positive condition Do not use "else" with "unless" Do not use negation in condition Do not use else with statement modifiers

17 Pitfalls with Conditionals Keyword "elsif" instead of "else if" if x < 10 puts "small" elsif x < 20 puts "medium" else puts "large" end If's do not create nested lexical scope if x < 10 y = x end puts y #y is defined, but could be nil puts z #NameError: undefined local var z

18 Case Statements are General [variable = ] case expression when nil statements execute if the expr was nil when value # e.g. 0, 'start' statements execute if expr equals value when type # e.g. String statements execute if expr resulted in Type when /regexp/ # e.g. /[aeiou]/ statements execute if expr matches regexp when min..max statements execute if the expr is in range else statements end

19 Basic Iteration: While and Until Classic structure while boolean_condition [do] end Can also be used as a statement modifier work while awake Until: equivalent to "while not " until i > count end Can also be a used as a statement modifier Pitfall: Modified block executes at least once sleep while dark #may not sleep at all begin i = i + 1 end while i < MAX #always increments i at least once

20 Functions Definition: keyword def def foo (x, y) x + y end Notice: no types in signature No types for parameters No types for return value All functions return something Value of last statement implicitly returned Convention: Omit explicit return statement def foo(x, y) x + y # last statement executed end

21 Functions Dot notation for method call Math::PI.rationalize() #rec vr Math::PI Convention: Omit () s in definition of functions with no parameters def launch() def launch end #bad end #good Parens can be omitted in calls too! Math::PI.rationalize puts Hello World Convention: omit for keyword-like calls attr_reader :name, :age Note: needed when chaining foo(13).equal? value

22 Summary Ruby is a general-purpose, imperative, object-oriented language Ruby is (usually) interpreted REPL Familiar flow-of-control and syntax Some new constructs (e.g., unless, until) Terse (e.g., optional parentheses, optional semicolons, statement modifiers)

23 Primitive vs Reference Types Recall Java type dichotomy: Primitive: int, float, double, boolean, Reference: String, Set, NaturalNumber, A variable is a "slot" in memory Primitive: the slot holds the value itself Reference: the slot holds a pointer to the value (an object) a 34 d width: 12 height: 15 color: "blue"

24 Object Value vs Reference Value Variable of reference type has both: Reference value: value of the slot itself Object value: value of object it points to (corresponding to its mathematical value) Variable of primitive type has just one Value of the slot itself, corresponding to its mathematical value a 34 d width: 12 height: 15 color: "blue"

25 Two Kinds of Equality Question: "Is x equal to y?" A question about the mathematical value of the variables x and y In Java, depending on the type of x and y we either need to: Compare the values of the slots x == y // for primitive types Compare the values of the objects x.equals(y) // for non-primitive types

26 Ruby: "Everything is an Object" In Ruby, every variable maps to an object Integers, floats, strings, sets, arrays, Benefit: A more consistent mental model References are everywhere Every variable has both a reference value and an object value Equality of mathematical value always means comparing object values Ruby terminology: Reference value is called the "object id" The 4- or 8-byte number stored in the slot Unique identifier for corresponding object msg = "hi" msg.object_id #=>

27 Everything is an Object a 34 d width: 12 height: 15 color: "blue" done msg true "shark" list <1,2,8,2>

28 Operational Detail: Immediates For small integers, the mathematical value is encoded in the reference value! LSB of reference value is 1 Remaining bits encode value, 2's complement x = 0 x.object_id #=> 1 (0b ) y = 6 y.object_id #=> 13 (0b ) Benefit: Performance No change to model (everything is an object) Known as an "immediate" value Other immediates: true, false, nil, symbols

29 Objects Have Methods Familiar "." operator to invoke (instance) methods list = [6, 15, 3, -2] list.size #=> 4 Since numbers are objects, they have methods too! 3.to_s #=> "3" 3.odd? #=> true 3.lcm 5 #=> #=> 8 3.class #=> FixNum 3.methods #=> [:to_s, :inspect, :+, ]

30 Pitfall: Equality Operator Reference value is still useful sometimes "Do these variables refer to the same object?" So we still need 2 methods: x == y x.equal? y Ruby semantics are the opposite of Java! == is object value equality.equal? is reference value equality Example s1, s2 = "hi", "hi" s1 == s2 #=> true (obj values equal) s1.equal? s2 #=> false (ref vals differ)

31 Assignment (Just Like Java) Assignment copies the reference value Result: Both variables point to the same object (ie an "alias") Parameter passing works this way too a b <5, 1> <3, 4>

32 Assignment (Just Like Java) Assignment copies the reference value Result: Both variables point to the same object (ie an "alias") Parameter passing works this way too a b a b a = b; <5, 1> <3, 4> <5, 1> <3, 4>

33 Assignment (Just Like Java) Assignment copies the reference value Result: Both variables point to the same object (ie an "alias") Parameter passing works this way too a b a b a = b; <5, 1> <3, 4> <5, 1> <3, 4>

34 Aliasing Mutable Objects When aliases exist, a statement can change a variable's object value without mentioning that variable x = [3, 4] y = x # x and y are aliases y[0] = 13 # changes x as well! Question: What about numbers? i = 34 j = i # i and j are aliases j = j + 1 # does this increment i too?

35 Immutability Recall in Java strings are immutable No method changes the value of a string A method like concat returns a new instance Benefit: Aliasing immutable objects is safe Immutability is used in Ruby too Numbers, true, false, nil, symbols list = [3, 4] list[0] = 13 # changes list's object value # list points to same object n = 34 n = n + 1 # changes n's reference value # n points to different object Pitfall: Unlike Java, strings in Ruby are mutable

36 Assignment Operators Arithmetic contraction += -= *= /= %= **= Pitfall: no ++ or -- operators (use += 1) Logical contraction = &&= Idiom: = for initializing potentially nil variables Pitfall (minor): x = y not quite equivalent to x = x y Better to think of it as x x = y Usually amounts to the same thing Parallel assignment x, y, z = y, 10, radius

37 Declared vs Dynamic Types In Java, types are associated with both Variables ("declared" / "static" type), and Objects ("dynamic" / "run-time" type) Queue line = new Queue1L(); Recall: Programming to the interface Compiler uses declared type for checks line.inc(); //error no such method line = new Set1L(); //error wrong type boolean isempty (Set s) { } if isempty(line) //error arg type

38 Statically Typed Language line Queue <1, 2, 8, 2> Queue1L msg String d Shape "hello" String width: 12 height: 15 color: "blue" Rectangle

39 Dynamically Typed Language line <1, 2, 8, 2> Queue1L msg "hello" String d width: 12 height: 15 color: "blue" Rectangle

40 Dynamically Typed Language Equivalent definitions: No static types Dynamic types only Variables do not have type, objects do

41 Function Signatures Statically typed String parse(char[] s, int i) { return e;} out = parse(t, x); Parameter types (i.e. s and i) are declared Return type (i.e. of parse) is declared The compiler checks conformance of (Declared) types of arguments (t, x) (Declared) type of return expression (e) (Declared) type of expression using parse (out) Dynamically typed def parse(s, i) end out = parse t, x You are on your own!

42 Type Can Change at Run-time Statically Typed //a is undeclared String a; //a is null string a = "hi; //compile-time err a = "hi"; a = 3; //compile-time err a.push(); //compile-time err Dynamically Typed //a is undefined a = a //a is nil a = "hi //load-time error a = "hi" a = 3 //a is now a number a.crazy //run-time error

43 Changing Dynamic Type line <1, 2, 8, 2> Queue1L msg "hello" String

44 Changing Dynamic Type msg, line = line, msg line <1, 2, 8, 2> Queue1L msg "hello" String

45 Changing Dynamic Type msg, line = line, msg line line <1, 2, 8, 2> Queue1L <1, 2, 8, 2> Queue1L msg msg "hello" String "hello" String

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