R - Replace the value of a specific position where there is a NA value

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Given this data set:

df <- data.frame(a = c(NA, NA, NA, NA, 10, 10, 16, 21),
                 b = c(NA, NA, NA, NA, NA, NA, 2, 4))
a b
NA NA
NA NA
NA NA
NA NA
10 NA
10 NA
16 2
21 4

Calculate the differences of values between the previous row:

df <- df%>%
  mutate(a_diff = a - lag(a, default = first(a)),
         b_diff = b - lag(b, default = first(b)))
a b a_diff b_diff
NA NA NA NA
NA NA NA NA
NA NA NA NA
NA NA NA NA
10 NA NA NA
10 NA 0 NA
16 2 6 NA
21 4 5 2

But the result I need would be the following:

a b a_diff b_diff
NA NA NA NA
NA NA NA NA
NA NA NA NA
NA NA NA NA
10 NA 10 NA
10 NA 0 NA
16 2 6 2
21 4 5 2

That is, I need to replace the last 'NA' value of column 'a_diff' with the value corresponding to the row number of column 'a'. And the same with the 'b_diff' column.

Thank you in advance for reading my question and I would appreciate any clue you can give me to solve my problem.

missing-data r
2 Answers

Maybe this is what you are looking for. Using max(which(is.na())) with your "diff" value will get the position of the last NA in that column. You can substitute with values for that particular row as below.

library(tidyverse)

df %>%
  mutate(a_diff = a - lag(a, default = first(a)),
         b_diff = b - lag(b, default = first(b))) %>%
  mutate(a_diff = ifelse(row_number() == max(which(is.na(a_diff))), a, a_diff),
         b_diff = ifelse(row_number() == max(which(is.na(b_diff))), b, b_diff))

Output

   a  b a_diff b_diff
1 NA NA     NA     NA
2 NA NA     NA     NA
3 NA NA     NA     NA
4 NA NA     NA     NA
5 10 NA     10     NA
6 10 NA      0     NA
7 16  2      6      2
8 21  4      5      2
user_34606700

Perfect use case for coalesce:

library(dplyr)

df%>%
  mutate(a_diff = a - lag(a, default = first(a)),
         b_diff = b - lag(b, default = first(b))) %>% 
  mutate(a_diff = coalesce(a_diff, a),
         b_diff = coalesce(b_diff, b))

  a  b a_diff b_diff
1 NA NA     NA     NA
2 NA NA     NA     NA
3 NA NA     NA     NA
4 NA NA     NA     NA
5 10 NA     10     NA
6 10 NA      0     NA
7 16  2      6      2
8 21  4      5      2
user_133216470
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