Taking value of previous row away from current row in R

Question

I have a data frame formatted like so:

GameId	Eval
1	1.00
1	1.50
1	0.50
2	-1.00
2	2.00
2	1.00
3	1.50
3	-0.50

I would like a new column which accounts for differences between the current row and the previous, but resets at the start of the next GameId so that it looks like so:

GameId	Eval	Diff
1	1.00	NA
1	1.50	0.50
1	0.50	-1.00
2	-1.00	NA
2	2.00	3.00
2	1.00	-2.00
3	1.50	NA
3	-0.50	-2.00

Answer 1

In base R you might do :

data <- data.frame(
      GameId = c(1L, 1L, 1L, 2L, 2L, 2L, 3L, 3L),
        Eval = c(1, 1.5, 0.5, -1, 2, 1, 1.5, -0.5)
)

data$Diff <- with(data, ave(Eval, GameId, FUN = function(x) c(NA, diff(x))))
data
#>   GameId Eval Diff
#> 1      1  1.0   NA
#> 2      1  1.5  0.5
#> 3      1  0.5 -1.0
#> 4      2 -1.0   NA
#> 5      2  2.0  3.0
#> 6      2  1.0 -1.0
#> 7      3  1.5   NA
#> 8      3 -0.5 -2.0

Or using {dplyr}

library(dplyr)
data %>% 
  group_by(GameId) %>% 
  mutate(Diff = c(NA, diff(Eval))) %>% 
  ungroup()
#> # A tibble: 8 × 3
#>   GameId  Eval  Diff
#>    <int> <dbl> <dbl>
#> 1      1   1    NA  
#> 2      1   1.5   0.5
#> 3      1   0.5  -1  
#> 4      2  -1    NA  
#> 5      2   2     3  
#> 6      2   1    -1  
#> 7      3   1.5  NA  
#> 8      3  -0.5  -2