Slide along data frame rows and compare rows with next rows - r

Question

I guess something similar should have been asked before, however I could only find an answer for python and SQL. So please notify me in the comments when this was also asked for R!
Data
Let's say we have a dataframe like this:
set.seed(1); df <- data.frame( position = 1:20,value = sample(seq(1,100), 20))
# In cause you do not get the same dataframe see the comment by #Ian Campbell - thanks!
position value
1 1 27
2 2 37
3 3 57
4 4 89
5 5 20
6 6 86
7 7 97
8 8 62
9 9 58
10 10 6
11 11 19
12 12 16
13 13 61
14 14 34
15 15 67
16 16 43
17 17 88
18 18 83
19 19 32
20 20 63
Goal
I'm interested in calculating the average value for n positions and subtract this from the average value of the next n positions, let's say n=5 for now.
What I tried
I now used this method, however when I apply this to a bigger dataframe it takes a huge amount of time, and hence wonder if there is a faster method for this.
calc <- function( pos ) {
this.five <- df %>% slice(pos:(pos+4))
next.five <- df %>% slice((pos+5):(pos+9))
differ = mean(this.five$value)- mean(next.five$value)
data.frame(dif= differ)
}
df %>%
group_by(position) %>%
do(calc(.$position))
That produces the following table:
position dif
<int> <dbl>
1 1 -15.8
2 2 9.40
3 3 37.6
4 4 38.8
5 5 37.4
6 6 22.4
7 7 4.20
8 8 -26.4
9 9 -31
10 10 -35.4
11 11 -22.4
12 12 -22.3
13 13 -0.733
14 14 15.5
15 15 -0.400
16 16 NaN
17 17 NaN
18 18 NaN
19 19 NaN
20 20 NaN

Answer 1

I suspect a data.table approach may be faster.
library(data.table)
setDT(df)
df[,c("roll.position","rollmean") := lapply(.SD,frollmean,n=5,fill=NA, align = "left")]
df[, result := rollmean[.I] - rollmean[.I + 5]]
df[,.(position,value,rollmean,result)]
# position value rollmean result
# 1: 1 27 46.0 -15.8
# 2: 2 37 57.8 9.4
# 3: 3 57 69.8 37.6
# 4: 4 89 70.8 38.8
# 5: 5 20 64.6 37.4
# 6: 6 86 61.8 22.4
# 7: 7 97 48.4 4.2
# 8: 8 62 32.2 -26.4
# 9: 9 58 32.0 -31.0
#10: 10 6 27.2 -35.4
#11: 11 19 39.4 -22.4
#12: 12 16 44.2 NA
#13: 13 61 58.6 NA
#14: 14 34 63.0 NA
#15: 15 67 62.6 NA
#16: 16 43 61.8 NA
#17: 17 88 NA NA
#18: 18 83 NA NA
#19: 19 32 NA NA
#20: 20 63 NA NA
Data
RNGkind(sample.kind = "Rounding")
set.seed(1); df <- data.frame( position = 1:20,value = sample(seq(1,100), 20))
RNGkind(sample.kind = "default")