20.9 Application: reimplementing base::subset()

base::subset() works like dplyr::filter(): it selects rows of a data frame given an expression.

What do we need?

• Quote the expression to filter
• Figure out which rows in the data frame pass the filter
• Subset the data frame

subset2 <- function(data, rows) {
  rows <- enquo(rows)
  rows_val <- eval_tidy(rows, data)
  stopifnot(is.logical(rows_val))

  data[rows_val, , drop = FALSE]
}

sample_df <- data.frame(a = 1:5, b = 5:1, c = c(5, 3, 2, 4, 1))

# Shorthand for sample_df[sample_df$b == sample_df$c, ]
subset2(sample_df, b == c)
#>   a b c
#> 1 1 5 5
#> 5 5 1 1