syntax for nested ifelse and if statements in R

Question

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:

Date	Location	Code	pH	pHDelta
22/07/01	AA	A	7.1
22/07/01	AA	B	6.8
22/07/01	AA	F	8.2
22/07/01	AB	A	7. 2
22/07/01	AB	B	7.8
22/07/01	AB	F	8.4
22/07/01	AC	A	7.5
22/07/01	AC	B	6.2
22/07/01	AC	F	8.3
22/07/01	AD	A	7.1
22/07/01	AD	B	6.8
22/07/01	AD	F	8.2
22/07/02	AA	A	7.1
22/07/02	AA	B	6.8
22/07/02	AA	F	8.2
22/07/02	AB	A	7.2
22/07/02	AB	B	7.8
22/07/02	AB	F	8.4
22/07/02	AC	A	7.5
22/07/02	AC	B	6.2
22/07/02	AC	F	8.3
22/07/02	AD	A	7.1
22/07/02	AD	B	6.8
22/07/02	AD	F	8.2

Answer 1

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