How can I use one column to determine where I get the value for another column?

Question

I\'m trying to use one column to determine which column to use as the value for another column It looks something like this:

     X   Y  Z   Target
1    a   b           


        

Answer 1

library(tidyverse)

df <-setNames(data.frame(cbind(matrix(letters[1:9],3,3,byrow=T), c("X", "Y", "Z"))), c("X", "Y", "Z", "Target"))

df

df %>% 
  gather(key="ID", value="TargetValue", X:Z) %>%
  filter(ID==Target) %>%
  select(Target, TargetValue) %>%
  left_join(df, by="Target")