Subtracting 1 in a column after a specific row in R

July 7, 2023

I have a dataframe that looks like this:

example <- data.frame(
  `Event Full` = c('SocReframeNeg',
                   'SocReframeNeg',
                   'test',
                   'SocReframeNeg',
                   'SocReframeNeg',
                   'SocImmerseNeg',
                   'SocImmerseNeg'),
  Num = c(1,2,3,4,5,6,7)
)

I want to subtract 1 from Num for all rows after ‘test’, and then remove the row with ‘test’ from the dataframe.

This is the output:

solution <- data.frame(
  `Event Full` = c('SocReframeNeg',
                   'SocReframeNeg',
                   'SocReframeNeg',
                   'SocReframeNeg',
                   'SocImmerseNeg',
                   'SocImmerseNeg'),
  Num = c(1,2,3,4,5,6)
)

Does anyone know an efficient way to solve this?

Thank you!

>Solution :

You can do it like this:

i_test <- which(example[["Event.Full"]] == "test")
example$Num[i_test:nrow(example)] <- example$Num[i_test:nrow(example)] - 1
example <- example[-i_test, ]
example
##      Event.Full Num
## 1 SocReframeNeg   1
## 2 SocReframeNeg   2
## 4 SocReframeNeg   3
## 5 SocReframeNeg   4
## 6 SocImmerseNeg   5
## 7 SocImmerseNeg   6

Note that data.frame() has replaced the space in the column name "Event Full" by a dot. If you don’t want this, you have to call data.frame() with check.names = FALSE like this:

example <- data.frame(
  `Event Full` = c('SocReframeNeg',
                   'SocReframeNeg',
                   'test',
                   'SocReframeNeg',
                   'SocReframeNeg',
                   'SocImmerseNeg',
                   'SocImmerseNeg'),
  Num = c(1,2,3,4,5,6,7),
  check.names = FALSE
)