Intro to object-oriented programming

I gave this talk at our graduate student computing club. It was an informal introduction to object-oriented programming in R, focusing on S4, with a brief comparison to Python classes. Examples on github: https://github.com/alyssafrazee/cards

Transcript

1. Object-oriented
   programming
   Alyssa Frazee
   computing club, Dec 4 2014
   

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2. Definition
   ● “object” = user-defined data structure
   ● “attribute” = piece of data associated with the
   object
   ● “method” = procedure/function associated
   with the object
   I got this from Wikipedia (and remembering
   stuff from when I took CS101 in 2008)
   

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3. What’s the point?
   It’s convenient to be able to create your own
   way of storing data.
   Famous R examples:
   lm
   SummarizedExperiment
   fmridata
   Note: some people are anti-OOP: http://c2.
   com/cgi/wiki?ArgumentsAgainstOop
   

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4. R has 3 or 4 OO systems
   ● S3
   ● S4
   ● Reference classes
   ● Base types (“not quite OO”)
   http://adv-r.had.co.nz/OO-essentials.html
   

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5. R has 3 or 4 OO systems
   ● S3
   ● S4
   ● Reference classes
   ● Base types (“not quite OO”)
   http://adv-r.had.co.nz/OO-essentials.html
   

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6. S3
   

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7. S4 example
   #' Card
   #'
   #' A playing card
   #'
   #' @slot suit Is this card a spade, club, diamond, or heart?
   #' @slot number The card’s value. Can be 2-10, ‘J’, ‘Q’, ‘K’, or ‘A’
   #’ @slot trump Is the card a trump card? Boolean.
   #'
   #' @exportClass Card
   setClass("Card",
   representation(
   suit = "character", number = "numeric", trump = "logical")
   )
   AllClass.R
   

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8. Options for defining methods:
   ● Just write a function
   ○ probably the easiest choice if the function name is
   unique.
   

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9. S4 example
   #' find_pair
   #'
   #' Find the card with the same value and color as the given card
   #'
   #' @param card Object of class “Card”
   #' @export
   find_pair = function(card){
   new_suit = switch(card@suit,
   diamonds = ‘hearts’, hearts = ‘diamonds’,
   spades = ‘clubs’, clubs = ‘spades’)
   return(new(‘Card’, suit = new_suit, number = card@number,
   trump=FALSE))
   }
   find_pair.R
   

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10. Options for defining methods:
    ● Just write a function
    ○ probably the easiest choice if the function name is
    unique.
    ● Extend or create a generic function, then
    create a way for that function to work with
    your class of object.
    ○ required if you need to make a base function like
    “plot” work with your object
    ○ optional if function name is unique -- consider
    whether the function might be useful for existing or
    future classes of objects
    

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11. S4 example
    #' @name cardplot
    #' @export
    #' @docType methods
    #' @rdname cardplot
    #' @importFrom graphics plot
    setGeneric("plot", function(x,y,...) standardGeneric("plot"))
    Card-plot-method.R
    

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12. S4 example
    #' make an awesome plot of a card!
    #' @name cardplot
    #' @exportMethod plot
    #' @docType methods
    #' @rdname cardplot
    #' @aliases cardplot,ballgown-method
    #' @param x a Card object
    setMethod("plot", "Card", function(x){
    plot(0, 0, type='n', xaxt='n', yaxt='n', xlab='', ylab='', main='My Card!')
    if(x@number > 10){
    s = ifelse(x@number==11, 'Jack',
    ifelse(x@number==12, 'Queen',
    ifelse(x@number==13, 'King', 'Ace')))
    }else{
    s = x@number
    }
    text(0, 0, paste(s, 'of', x@suit), cex=2)
    })
    Card-plot-method.R
    (continued)
    

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13. *Key Insight* about R methods
    *Functions* have different types of *objects*
    associated with them. (**not** the other way
    around!)
    (“generic function OO” vs. “message-passing
    OO”)
    

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14. An example in Python
    class Card(object):
    def __init__(self, suit, number, trump=False):
    self.suit = suit
    self.number = number
    self.trump = trump
    def repr(self): ## like R’s show
    return str(self.number)+‘ of ’+self.suit
    def plot(self):
    ## plotting code, whatever whatever
    

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15. message-passing OO
    mycard.plot()
    

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16. An example in Python
    class Card(object):
    def __init__(self, suit, number, trump=False):
    self.suit = suit
    self.number = number
    self.trump = trump
    def __cmp__(self, other): ## THIS is very cool
    if self.number > other.number:
    return 1
    elif other.number > self.number:
    return -1
    else:
    return 0
    

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17. Python
    classes
    

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18. Syntax Note
    Many languages (includng Python) use the “.”
    operator to access attributes and methods of
    different objects. (e.g. mycard.suit)
    (The fact that R uses “.” like any other
    character makes it weird).
    

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19. Much more fun to be had!
    ● inheritance!
    ○ but things get complicated VERY quickly
    ● S3 and S4 classes working together;
    documentation, etc
    ● questions? discussion?
    ● R example on github: https://github.
    com/alyssafrazee/cards
    

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