How to call nth next value in dplyr?

user_16729175 0
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I'm sorry that I cannot explain exactly. So, I'll ask with simple example. While I'm looking at This question I use for loop to replace yes value to N/A.

stim <- c("a", "k", "y", "j", "t", "Stop", "f", "l", "b", "a", "c", "Stop")
df <- data.frame(stim)

df<- df%>%
  mutate(YesorNo = ifelse(stim == "Stop", "N/A", "yes"))
for (i in 1:dim(df)[1]){
  if (df$YesorNo[i] == "N/A") {
    if(i == 1){
    } else {
      df$YesorNo[i-1] = "no"
    }
  }
}

In for loop above, how can I call that YesorNo[i-1] in dplyr?

Similarly, for a fixed number n, how to call YesorNo[i-n]?

dplyr r
1 Answers

You may use lag and lead functions in dplyr.

library(dplyr)

df %>%
  mutate(previous_value = lag(stim), 
         next_value = lead(stim))

#   stim previous_value next_value
#1     a           <NA>          k
#2     k              a          y
#3     y              k          j
#4     j              y          t
#5     t              j       Stop
#6  Stop              t          f
#7     f           Stop          l
#8     l              f          b
#9     b              l          a
#10    a              b          c
#11    c              a       Stop
#12 Stop              c       <NA>

Note that default first and last value for lag and lead function is NA respectively.

In data.table the equivalent function is shift -

library(data.table)

setDT(df)
df[, previous_value := shift(stim, type = 'lag')]
df[, next_value := shift(stim, type = 'lead')]
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