Exchange the values between two columns based on a condition using R

I have got the following df. I want if the value in the dm column is less than 20000, then that value should go to the nd column. Similarly, if the value in the nd column is greater then 20000 then that value should go to the dm column

structure(list(id = c(1, 2, 3), nd = c(NA, 20076, NA), dm = c(10113, 
NA, 10188)), class = "data.frame", row.names = c(NA, -3L))

I want my final df to look like this

structure(list(id = c(1, 2, 3), nd = c(10113, NA, 10188), dm = c(NA, 
20076, NA)), class = "data.frame", row.names = c(NA, -3L))

Thank you

ifelse is your friend for this.

base R

transform(df,
  nd = ifelse(dm < 20000, dm, nd), 
  dm = ifelse(nd > 20000, nd, dm)
)
#   id    nd    dm
# 1  1 10113    NA
# 2  2    NA 20076
# 3  3 10188    NA

Note that this works in base R because unlike dplyr::mutate, the calculation for the dm= (second) expression (and beyond) does not see the change from the previous expressions, so the nd that it sees is the original, unchanged nd.

We can also use the temporary-variable trick illustrated in the dplyr example below:

df |>
  transform(
    nd2 = ifelse(dm < 20000, dm, nd),
    dm2 = ifelse(nd > 20000, nd, dm)
  ) |>
  subset(select = -c(nd, dm))

and then rename nd2 to nd (etc).

dplyr

Because mutate "sees" the changes immediately, we need to store into other variables and then reassign.

library(dplyr)
df %>%
  mutate(
    nd2 = ifelse(dm < 20000, dm, nd),
    dm2 = ifelse(nd > 20000, nd, dm)
  ) %>%
  select(-nd, -dm) %>%
  rename(nd=nd2, dm=dm2)
#   id    nd    dm
# 1  1 10113    NA
# 2  2    NA 20076
# 3  3 10188    NA

Another base R option using apply:

as.data.frame(t(apply(df, 1, function(x) {
  if(x[2] > 20000 | x[3] < 20000) x[c(1, 3, 2)] else x})))
#>   id    dm    nd
#> 1  1 10113    NA
#> 2  2    NA 20076
#> 3  3 10188    NA

^{Created on 2023-02-18 with reprex v2.0.2}