Discussion 10

Scheme Data Abstractions

Antonio Kam
anto [at] berkeley [dot] edu

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teaching.rouxl.es

Slides by Antonio Kam (anto@)

Announcements

  • Scheme Project has started 🥳 💀
    • I liked this project quite a bit, but it is fairly technical
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Slides by Antonio Kam (anto@)

Results from last section

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Notes from last section

  • Once again, pretty much nothing
  • Please fill the forms in with something so I have something to say next time lol
  • I'm gonna fill this slide by saying we're super close to being done with this semester 🤯 - you all got this!
Slides by Antonio Kam (anto@)

Temperature Check 🌡️

  • Scheme
  • Scheme Lists
  • Scheme Data Abstractions
Slides by Antonio Kam (anto@)

What are data abstractions

  • In Python, we used classes to describe certain objects
  • Classes don't exist in scheme 🙀
  • As a result, data abstractions help to fill in this gap
    • If we want something in scheme to describe objects, we use data abstractions!
Slides by Antonio Kam (anto@)

Data abstractions

  • Constructors
    • Similar to __init__ in our classes
    • Function that builds our abstract data type
  • Selectors
    • Functions that receive information from the data type
    • Similar to getting an instance variable value (t.label, s.first)
Slides by Antonio Kam (anto@)

Example Data Abstraction (Python)

Constructor:

def couple(first, second):
    return [first, second]

Selectors:

def first(couple):
    return couple[0]

def second(couple):
    return couple[1]

c = couple(1, 2)
c1, c2 = first(c), second(c) # c1 = 1, c2 = 2
Slides by Antonio Kam (anto@)

Example Data Abstraction (Scheme)

Constructor:

(define (couple first second) (list first second))

Selectors:

(define (first couple) (car couple))
(define (second couple) (car (cdr couple)))

(define c (couple 1 2))
(first c)
(second c)
Slides by Antonio Kam (anto@)

Idea

  • The idea of data abstractions here is that we don't think of the couple data abstraction as a list with two elements, but rather simply as the descriptors
  • This is similar to real life where you don't think of driving a car as all its individual parts, but just as a large abstraction
  • Data abstractions effectively hide the implementation from users - all people need to use are the constructors and selectors
Slides by Antonio Kam (anto@)

Violating Data Abstraction Barriers

  • One thing you cannot do when it comes to data abstractions is violate the data abstraction barriers.
  • This basically means that you must use the constructors and selectors when dealing with data abstractions
    • This is because you do not necessarily know how the data abstractions are implemented
Slides by Antonio Kam (anto@)

Example Data Abstraction (Python)

Constructor:

def couple(first, second):
    return {"first": first, "second": second}

Selectors:

def first(couple):
    return couple["first"]

def second(couple):
    return couple["second"]

c = couple(1, 2)
c1, c2 = first(c), second(c) # c1 = 1, c2 = 2
Slides by Antonio Kam (anto@)

City

  • Let's say we have a definition for a city data abstraction
  • We just need the documentation for what the names mean, but we don't need to know how it's implemented:
(make-city name lat lon)

(get-name city)
(get-lat city)
(get-lon city)
  • All we need to know is that we can use these definitions
Slides by Antonio Kam (anto@)

Worksheet!

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Attendance
links.rouxl.es/disc

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Scheme Trees

  • We've made the Tree class in Python before
  • Scheme doesn't have classes!
  • Let's make an Abstract Data Type instead 👀
Slides by Antonio Kam (anto@)
  • Constructor:
    • (tree label branches)
    • branches is a list
  • Selectors:
    • (label t) - returns the label of the tree
    • (branches t) - returns a list of the branches of the tree
(define t 
        (tree 5 
            (list (tree 4 nil) (tree 7 nil))
        ))

(label t) -> 5
(label (car (branches t))) -> 4
Slides by Antonio Kam (anto@)

map in Scheme

  • (map) is Scheme's way of doing 'iteration'
  • Scheme doesn't have for loops, so we can't do for b in t.branches
  • We get around that with map
  • Very similar to list comprehensions
  • (map <proc> <lst>)
    • proc (procedure) must be a 1-argument function (important for later!)
    • Calls proc on each element in lst and returns a new list with that
Slides by Antonio Kam (anto@)

map in Scheme

(define (double x) (* x 2))
(define lst '(1 2 3 4))
(define new-lst (map double lst)) -> (2 4 6 8)

What if you pass in a 2 argument function?

Slides by Antonio Kam (anto@)

map in Scheme

(define (double x) (* x 2))
(define lst '(1 2 3 4))
(define new-lst (map double lst)) -> (2 4 6 8)

What if you pass in a 2 argument function?

Let's say I want to add 3 to every element

(define (add x y) (+ x y))
(define lst '(1 2 3 4))
(define new-lst (map (lambda (z) (add z 3)) lst)) -> (4 5 6 7)
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Worksheet!

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Slides by Antonio Kam (anto@)

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Thanks for coming! 🥳

Please give me feedback on what to improve!

Slides by Antonio Kam (anto@)

Notice how with the bottom part of the code you didn't need to know anything about how it was implemented (similar to OOP)