7 Additional Methods

7.1 Introduction

Until now we have standard methods like print(), summary(), and plot(). However, we can add more common (and not so common) methods such as:

• replacement: "[<-.toss"
• extraction: "[.toss"
• testing: is.toss()
• addition: "+.toss"

7.2 Replacement Method

Replacement functions are those calls like x[1] <- 3. The function behind this expression is the replacement "[<-"() function. We can also create a replacement function for a given class using the notation "[<-.class", where class is the name of the class:

"[<-.toss" <- function(x, i, value) {
  if (value != x$coin$sides[1] & value != x$coin$sides[2]) {
    stop(sprintf('\nreplacing value must be %s or %s',
                 x$coin$sides[1], x$coin$sides[2]))
  }
  x$tosses[i] <- value
  make_toss(x$coin, x$tosses)
}

Test it:

set.seed(3752)
dime1 <- coin(c("roosevelt", "torch"), prob = c(0.48, 0.52))
b <- toss(dime1, times = 5)
b$tosses
#> [1] "roosevelt" "roosevelt" "roosevelt" "torch"     "torch"

# replacement
b[1] <- "torch"
b$tosses
#> [1] "torch"     "roosevelt" "roosevelt" "torch"     "torch"

What about replacing out of the original range?

# replacement in sixth position (weird!)
b[6] <- "torch"
b
#> object "toss"
#> sides: "roosevelt", "torch" 
#> prob: "0.48", "0.52" 
#> total tosses: 6 
#> num of roosevelt: 2 
#> num of torch: 4

Or something like this?

# replacement
b[10] <- "torch"
b
#> object "toss"
#> sides: "roosevelt", "torch" 
#> prob: "0.48", "0.52" 
#> total tosses: 10 
#> num of roosevelt: NA 
#> num of torch: NA

Because, in general, it does not make sense to replace if index is out of the original length, we can add a stop() condition:

"[<-.toss" <- function(x, i, value) {
  if (value != x$coin$sides[1] & value != x$coin$sides[2]) {
    stop(sprintf('\nreplacing value must be %s or %s', 
                 x$coin$sides[1], x$coin$sides[2]))
  }
  if (i > x$total) {
    stop("\nindex out of bounds")
  }
  x$tosses[i] <- value
  make_toss(x$coin, x$tosses)
}

Now we cannot replace if index is out of the original length:

set.seed(3752)
b <- toss(dime1, times = 5)
b$tosses
#> [1] "roosevelt" "roosevelt" "roosevelt" "torch"     "torch"

# replacement
b[10] <- "torch"
#> Error in `[<-.toss`(`*tmp*`, 10, value = "torch"): 
#> index out of bounds

7.3 Extraction Method

What if you want to know what is the value of toss in position 3? You could type something like this:

b$tosses[3]
#> [1] "roosevelt"

Or you could create an extraction method that allows you to type x[3]. The function behind this expression is the extraction "["() function. We can also create a extraction function for a given class.

"[.toss" <- function(x, i) {
  x$tosses[i]
}

Test it:

set.seed(3752)
b <- toss(dime1, times = 5)
b$tosses
#> [1] "roosevelt" "roosevelt" "roosevelt" "torch"     "torch"
b[1]
#> [1] "roosevelt"

7.4 Is "toss"

Another common type of function for an object of a given class is is.class()-like functions: e.g. is.list(), is.numeric(), is.matrix(). This type of functions allow you to test or check whether an object is of a given class.

We can create our own is.toss() to check whether a given R object is of class "coss". Here’s how to define such a function:

is.toss <- function(x) {
  inherits(x, "toss")
}

is.toss(b)
#> [1] TRUE
is.toss(c("heads", "tails"))
#> [1] FALSE

7.5 Addition Method

R comes with generic Math methods (see ?Math). Among these generic methods we can find the "+" operator. This means that we can define our own plus method for objects of class "toss". The idea is to be able to call a command like this:

# toss object
b <- toss(dime1, times = 5)

# add 5 more flips
b + 5

Here’s one implementation of "+.toss()" in which the first argument is an object of class "toss", and the second argument is a single positive number that will play the role of additional tosses:

"+.toss" <- function(obj, incr) {
  if (length(incr) != 1 | incr <= 0) {
    stop("\ninvalid increament (must be positive)")
  }
  more_flips <- toss(obj$coin, times = incr)
  make_toss(obj$coin, c(obj$tosses, more_flips$tosses))
}

Remember that "+" is a binary operator, which means that writing a "+" method requires a function with two arguments. Let’s try it:

# add four more tosses
mycoin <- coin()
seven <- toss(mycoin, times = 7)
seven
#> object "toss"
#> sides: "heads", "tails" 
#> prob: "0.5", "0.5" 
#> total tosses: 7 
#> num of heads: 4 
#> num of tails: 3

Let’s add a couple of more tosses to seven:

# two more flips
seven + 2
#> object "toss"
#> sides: "heads", "tails" 
#> prob: "0.5", "0.5" 
#> total tosses: 9 
#> num of heads: 4 
#> num of tails: 5

# three more flips
seven + 3
#> object "toss"
#> sides: "heads", "tails" 
#> prob: "0.5", "0.5" 
#> total tosses: 10 
#> num of heads: 6 
#> num of tails: 4

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