How to use mutate() +across() only for specific rows

May 6, 2022

The data:

df <- tribble(
  ~name, ~val.I, ~val.V, ~`val.%`,
  "Peter", 123, 12.4, 14,
  "Peter in %", 111, 532, 57,
  "Harald", 2222, 3333, 444,
  "Harald in %", 22, 15, 203,
)

I would like to mutate (function: divide the value by 100) that contain .I and .V, but only for rows that contain the string in % in the column name. I know an easy for-loop solution, but could not figure out it with dplyr. Any hints much appreciated! 🙂

>Solution :

A dplyr option with mutate and across using matches for the specific columns. You can use the following code:

library(dplyr)

df %>% 
  mutate(across(matches(".I|.V"), ~ if_else(row_number() %in% grep("in %", name), ./100, .)))

Output:

# A tibble: 4 × 4
  name          val.I   val.V `val.%`
  <chr>         <dbl>   <dbl>   <dbl>
1 Peter        123      12.4       14
2 Peter in %     1.11    5.32      57
3 Harald      2222    3333        444
4 Harald in %    0.22    0.15     203