dplyr: how to reference columns by column index rather than column name using mutate?

You can try:

iris %>% head %>% mutate(sum = .[[1]] + .[[2]])

  Sepal.Length Sepal.Width Petal.Length Petal.Width Species sum
1          5.1         3.5          1.4         0.2  setosa 8.6
2          4.9         3.0          1.4         0.2  setosa 7.9
3          4.7         3.2          1.3         0.2  setosa 7.9
4          4.6         3.1          1.5         0.2  setosa 7.7
5          5.0         3.6          1.4         0.2  setosa 8.6
6          5.4         3.9          1.7         0.4  setosa 9.3

I'm a bit late to the game, but my personal strategy in cases like this is to write my own tidyverse-compliant function that will do exactly what I want. By tidyverse-compliant, I mean that the first argument of the function is a data frame and that the output is a vector that can be added to the data frame.

sum_cols <- function(x, col1, col2){
   x[[col1]] + x[[col2]]
}

iris %>%
  head %>%
  mutate(sum = sum_cols(x = ., col1 = 1, col2 = 2))