dplyr: lead() and lag() wrong when used with group_by()

Question

I want to find the lead() and lag() element in each group, but had some wrong results.

For example, data is like this:

library(dplyr)
df = data.frame         


        

Answer 1

It seems you have to pass additional argument to lag and lead functions. When I run your function without arrange, but with order_by added, everything seems to be ok.

df %>%
group_by(name) %>%
mutate(next.score = lead(score, order_by=name),
before.score = lag(score, order_by=name))

Output:

  name score next.score before.score
1   Al   100         60           NA
2  Jen    80        100           NA
3   Al    60         80          100
4  Jen   100         60           80
5   Al    80         NA           60
6  Jen    60         NA          100

My sessionInfo():

R version 3.1.1 (2014-07-10)
Platform: x86_64-w64-mingw32/x64 (64-bit)

locale:
[1] LC_COLLATE=Polish_Poland.1250  LC_CTYPE=Polish_Poland.1250        LC_MONETARY=Polish_Poland.1250
[4] LC_NUMERIC=C                   LC_TIME=Polish_Poland.1250    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] dplyr_0.4.1

loaded via a namespace (and not attached):
[1] assertthat_0.1  DBI_0.3.1       lazyeval_0.1.10 magrittr_1.5                parallel_3.1.1  Rcpp_0.11.5    
[7] tools_3.1.1