如何更改 data.frame 中的单元格值

Question

Let say I have below data.frame

library(dplyr)
DF = data.frame('A' = 1:4, 'B' = letters[1:4])

Now I want to change a specific value -

DF[DF[, 'B'] == 'a', 'A'] <- 9999

How can I use dplyr::pipe function to achieve the same?

Answer 1

dplyr::mutate() and dplyr::replace() work well for this task.

DF %>% 
  mutate(A = replace(A, B == "a", 9999))

mutate() creates a variable A, so after execution, it will overwrite the existing variable A, then replace() replaces the value in A with 9999 whenever B equals "a".

Finally you want to store the result, so the full code looks like this:

DF <- 
 DF %>% 
 mutate(A = replace(A, B == "a", 9999))

Answer 2

Update: in new version of dplyr use mutate(across(...)) rather than mutate_at :

DF %>%
  mutate(across(A, replace, B == 'a', 9999))

     A B
1 9999 a
2    2 b
3    3 c
4    4 d

Here's how you can accomplish the above using a pipe ( %>% ):

DF %>%
  mutate_at("A", ~replace(., B == 'a', 9999))

     A B
1 9999 a
2    2 b
3    3 c
4    4 d

Explanation: you are mutating "at" the variable called "A", so whatever function you call on it will mutate it in place (rather than creating a new variable). You can use ~ instead of the typical syntax function(x) to call an anonymous function that will automatically take . as its argument. Wherever you place . in the function it will operate on the column "A". You use the replace function on . (column A ), to replace the value 9999 everywhere that the statement B == 'a' is TRUE .