R QUIRKS

Weird and unexpected corners of R.


1. matrix element names

  vec <- setNames(1:9, letters[1:9])  # vector with names()
  mat <- matrix(vec, 3, 3)            # names() are not preserved
  names(mat) <- letters[1:9]          # now mat elements has names()

  mat["b"]                            # works
  mat <- mat[,]                       # names are lost after any subsetting
  mat["b"]                            # error


2. assignment syntax for turning elements into NA

  vec <- 1:9
  is.na(vec) <- c(3,5)                # turns 3rd and 5th elements into NA


3. factor level interactions with ":"

  f1 <- factor(c("young", "old", "young"))
  f2 <- factor(c("male", "female", "female"))

  f1:f2                               # same as interaction(f1, f2)


4. selecting elements from a nested list without stacking brackets

  l <- list(1, 2, list(3, 4, list(5)))

  l[[c(3,3,1)]]                       # returns "5"


5. assignment to operators

  # function to tranform 'x' so that sum of 'x' and 'y' is equal to 'value'
  `+<-` <- function(x, y, value) value - y

  x <- 1
  y <- 4

  x + y                               # 5
  x + y <- 10                         # transform x so that x + y = 10
  x + y                               # 10
  x                                   # 6
  y                                   # 4

  # can also be used with built-int and custom %% operators
  `%in%<-` <- function(x, y, value) {x[x %in% y] <- value; x}

  vec <- letters[1:4]
  vec %in% c("a", "c")                # TRUE, FALSE, TRUE, FALSE
  vec %in% c("a", "c") <- "o"         # turns 1st and 3rd elements of vec to "o"