Execute dplyr operation only if column exists

Question

Drawing on the discussion on conditional dplyr evaluation I would like conditionally execute a step in pipeline depending on whether the reference column exists in the passed da

Answer 1

Avoid this trap:

On a busy day, one might do like the following:

library(dplyr)
df <- data.frame(A = 1:3, B = letters[1:3], stringsAsFactors = F)
> df %>% mutate( C = ifelse("D" %in% colnames(.), D, B)) 
# Notice the values on "C" colum. No error thrown, but the logic and result is wrong
  A B C
1 1 a a
2 2 b a
3 3 c a

Why? Because "D" %in% colnames(.) returns only one value of TRUE or FALSE, and therefore ifelse operates only once. Then the value is broadcasted to the whole column!

Correct way:

> df %>% mutate( C = if("D" %in% colnames(.)) D else B)
  A B C
1 1 a a
2 2 b b
3 3 c c