subsetting.r

`[.my_df` <- function (x, i, j) {
  if (missing(i) && missing(j)) return(x)
  
  # First, subset columns
  if (!missing(j)) {
    x <- .subset(x, j)
  }
  
  # Next, subset rows
  if (!missing(i)) {
    x <- lapply(x, `[`, i)
  }
  
  # TODO: handle 0 column case
  structure(x,
    class = "data.frame",
    row.names = .set_row_names(length(x[[1]]))
  )
}

class(mtcars) <- c("my_df", "data.frame")

mtcars[]
mtcars[,]

mtcars[1:5, ]
mtcars[, 1:5]

mtcars[, c("mpg", "disp", "hp")]
mtcars[mtcars$mpg < 20, ]