syntax for nested ifelse and if statements in R

I am trying to nest if and/or ifelse statements together in R and I can’t quite get the syntax correct. I would like to perform some basic arithmetic: Referencing a similar dataset below, if the date is the same and if the location code is the same, I would like to subtract the pH values of codes A and B from the corresponding pH value for code F and enter the result into pHDelta. Or written out, A – F for a given date and location.

Thank you!

I am not certain the daset that I created below will appear correctly, please pardon me if it does not.

My dataset is similar to the following:

>Solution :

We can use a group by approach – grouped by ‘Date’, ‘Location’, subset the ‘pH’ where ‘Code’ value is "F" (assuming only a single "F" per Location) and then subtract from the ‘pH’ column

library(dplyr)
df1 <- df1 %>%
     group_by(Date, Location) %>%
     mutate(phDelta = pH - pH[Code == "F"][1]) %>%
     ungroup