Data frame rearraging in R and dyplr (mutate)

Question

I have a data frame like so

Number  Type  Time 
4         B     10
5         B     11
5         B      9
1         B      8
8         R      7
3         R      9
4         R      5
4         R      5

Where I have grouped by Type and would like to make a new column called "Adjusted_Time" That contains the time of the Number before, the final Df would look like this

Number  Type  Time      Adjusted_Time
4         B     10           8         
5         B     11          10
6         B      9          11
3         B      8          N/A
8         R      7           9          
7         R      9           5
6         R      5           5
5         R      5          N/A

Where the N/A goes whenever there isn't a number directly below that number. I would like to use dplyr for this Right now I have

df %>% group_by(Type) %>% Mutate(Adjusted_Time = ....)

Answer 1

df %>% 
  group_by(Type) %>%
  arrange(Number) %>%
  mutate(Adjusted_Time = lag(Time)) %>%
  arrange(Adjusted_Time)

Answer 2

The function you are looking for is lag

df %>% group_by(Type) %>% arrange(Number) %>% mutate(Adjusted_Time = lag(Time))

Source: local data frame [8 x 4]
Groups: Type
  Number Type Time Adjusted_Time
1      1    B    8            NA
2      4    B   10             8
3      5    B   11            10
4      5    B    9            11
5      3    R    9            NA
6      4    R    5             9
7      4    R    5             5
8      8    R    7             5

For more information see:

http://cran.r-project.org/web/packages/dplyr/vignettes/window-functions.html