Create a new Timepoint column based on a condition in R

Question

I have a dataset that has a subject ID and different timepoints for each subject. I have the data filtered on few filters and want to create a new timepoint based on the existing timepoints for the subjects. The new point should be the difference between the First visit for the subject and the current visit. How can I do this in R ?

Sample data with expected result

ID	Original_timepoint	New_timepoint
A	1	1
A	6	6
A	18	18
A	36	36
B	24	1
B	48	24

Answer 1

We can standardize by taking the diff and append 1 grouped by 'ID'. From the input data, some of the IDs are already standardized. So, we may use an if/else condition to skip those IDs

library(dplyr)
df1 %>%
    group_by(ID) %>%
    mutate(New_timepoint = if(first(Original_timepoint)  > 1) 
          c(1, diff(Original_timepoint)) else Original_timepoint) %>%
    ungroup

-ouptut

# A tibble: 6 x 3
  ID    Original_timepoint New_timepoint
  <chr>              <int>         <dbl>
1 A                      1             1
2 A                      6             6
3 A                     18            18
4 A                     36            36
5 B                     24             1
6 B                     48            24

data

df1 <- structure(list(ID = c("A", "A", "A", "A", "B", "B"), 
Original_timepoint = c(1L, 
6L, 18L, 36L, 24L, 48L), New_timepoint = c(1L, 6L, 18L, 36L, 
1L, 24L)), class = "data.frame", row.names = c(NA, -6L))