[SOLVED] CS Evaluate (each is true or false): 3 == 3

Evaluate (each is true or false): 3 == 3
3.equal? 3
hello == hello hello.equal? hello
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Objects and Dynamic Types
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Primitive vs Reference Types
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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)
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Object Value vs Reference Value
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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
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Two Kinds of Equality
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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

Ruby: Everything is an Object
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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
Comparison of mathematical values is always comparison of object value
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 = shark
msg.object_id #=> 1544150170

Everything is an Object
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1544150170

Operational Detail: Immediates
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For small integers, the mathematical value is
encoded in the reference value!
LSB of reference value is 1
Remaining bits encode value, 2s complement
x.object_id #=> 1 (0b00000001)
y.object_id #=> 13 (0b00001101)
Benefit: Performance
No change to model (everything is an object) Known as an immediate value
Other immediates: true, false, nil, symbols

Objects Have Methods
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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 #=> 15
3.+ 5 #=> 8
3.class #=> Integer
3.methods #=> [:to_s, :inspect, :+, ]

Pitfall: Equality Operator
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Reference value is still useful sometimes Do these variables refer to the same object?
So we still need 2 methods:
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)

Evaluate (each is true or false): 3 == 3
3.equal? 3
hello == hello hello.equal? hello
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Assignment (Just Like Java)
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Assignment copies the reference value Result: Both variables point to the same object (ie an alias)
Parameter passing works this way too

Assignment (Just Like Java)
Computer Science and Engineering The Ohio State University
Assignment copies the reference value Result: Both variables point to the same object (ie an alias)
Parameter passing works this way too

Assignment (Just Like Java)
Computer Science and Engineering The Ohio State University
Assignment copies the reference value Result: Both variables point to the same object (ie an alias)
Parameter passing works this way too

Aliasing Mutable Objects
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When aliases exist, a statement can
change a variables 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?
j = i # i and j are aliases
j = j + 1 # does this increment i too?

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 lists object value
# list points to same object
n = n + 1 # changes ns reference value
# n points to different object Pitfall: Unlike Java, strings in Ruby are
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Assignment Operators
Parallel assignment
x, y, z = y, 10, radius
+= -= *= /= %= **=
Pitfall: no ++ or operators (use += 1)
Idiom: ||= for initializing potentially nil
Pitfall (minor):
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Arithmetic contraction
Logical contraction
x ||= y not quite equivalent to x = x || y Better to think of it as x || x = y
Usually amounts to the same thing

Declared vs Dynamic Types
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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

Statically Typed Language
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<1, 2, 8, 2>
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Dynamically Typed Language
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<1, 2, 8, 2>
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Dynamically Typed Language
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Equivalent definitions:
No static types
Dynamic types only
Variables do not have type, objects do

Object-oriented
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References are everywhere
Assignment copies reference value (alias) Primitives (immediates) are objects too == vs .equal? are flipped

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