Split a data frame into overlapping dataframes

Question

I'm trying to write a function that behaves as follows, but it is proving very difficult:

DF <- data.frame(x = seq(1,10), y = rep(c('a','b','c','d','e'),2))
> DF
    x y
1   1 a
2   2 b
3   3 c
4   4 d
5   5 e
6   6 a
7   7 b
8   8 c
9   9 d
10 10 e

>OverLapSplit(DF,nsplits=2,overlap=2)
[[1]]
  x y
1 1 a
2 2 b
3 3 c
4 4 d
5 5 e
6 6 a

[[2]]
   x y
1  5 a
2  6 b
3  7 c
4  8 d
5  9 e
6 10 a

>OverLapSplit(DF,nsplits=1)
[[1]]
    x y
1   1 a
2   2 b
3   3 c
4   4 d
5   5 e
6   6 a
7   7 b
8   8 c
9   9 d
10 10 e

>OverLapSplit(DF,nsplits=2,overlap=4)
[[1]]
  x y
1 1 a
2 2 b
3 3 c
4 4 d
5 5 e
6 6 a
7 7 b

[[2]]
   x y
1  4 e
2  5 a
3  6 b
4  7 c
5  8 d
6  9 e
7 10 a

>OverLapSplit(DF,nsplits=5,overlap=1)
[[1]]
  x y
1 1 a
2 2 b
3 3 c

[[2]]
  x y
1 3 c
2 4 d
3 5 e

[[3]]
  x y
1 5 e
2 6 a
3 7 b

[[4]]
  x y
1 7 b
2 8 c
3 9 d

[[5]]
   x y
1  8 d
2  9 e
3 10 f

I haven't thought a lot about what would happen if you tried something like OverLapSplit(DF,nsplits=2,overlap=1)

Maybe the following:

[[1]]
  x y
1 1 a
2 2 b
3 3 c
4 4 d
5 5 e

[[2]]
   x y
1  5 a
2  6 b
3  7 c
4  8 d
5  9 e
6 10 a

Thanks!

Answer 1

Try something like :

OverlapSplit <- function(x,nsplit=1,overlap=2){
    nrows <- NROW(x)
    nperdf <- ceiling( (nrows + overlap*nsplit) / (nsplit+1) )
    start <- seq(1, nsplit*(nperdf-overlap)+1, by= nperdf-overlap )

    if( start[nsplit+1] + nperdf != nrows )
        warning("Returning an incomplete dataframe.")

    lapply(start, function(i) x[c(i:(i+nperdf-1)),])
}

with nsplit the number of splits! (nsplit=1 returns 2 dataframes). This will render an incomplete last dataframe in case the overlap splits don't really fit in the dataframe, and issues a warning.

> OverlapSplit(DF,nsplit=3,overlap=2)
[[1]]
  x y
1 1 a
2 2 b
3 3 c
4 4 d

[[2]]
  x y
3 3 c
4 4 d
5 5 e
6 6 a

[[3]]
  x y
5 5 e
6 6 a
7 7 b
8 8 c

[[4]]
    x y
7   7 b
8   8 c
9   9 d
10 10 e

And one with a warning

> OverlapSplit(DF,nsplit=1,overlap=1)
[[1]]
  x y
1 1 a
2 2 b
3 3 c
4 4 d
5 5 e
6 6 a

[[2]]
    x    y
6   6    a
7   7    b
8   8    c
9   9    d
10 10    e
NA NA <NA>

Warning message:
In OverlapSplit(DF, nsplit = 1, overlap = 1) :
  Returning an incomplete dataframe.

Answer 2

This uses the shingle idea from Lattice graphics and so leverages code from package lattice to generate the intervals and then uses a loop to break the original DF into the correct subsets.

I wasn't exactly sure what is meant by overlap = 1 - I presume you meant overlap by 1 sample/observation. If so, the code below does this.

OverlapSplit <- function(x, nsplits = 1, overlap = 0) {
    stopifnot(require(lattice))
    N <- seq_len(nr <- nrow(x))
    interv <- co.intervals(N, nsplits, overlap / nr)
    out <- vector(mode = "list", length = nrow(interv))
    for(i in seq_along(out)) {
        out[[i]] <- x[interv[i,1] < N & N < interv[i,2], , drop = FALSE]
    }
    out
}

Which gives:

> OverlapSplit(DF, 2, 2)
[[1]]
  x y
1 1 a
2 2 b
3 3 c
4 4 d
5 5 e
6 6 a

[[2]]
    x y
5   5 e
6   6 a
7   7 b
8   8 c
9   9 d
10 10 e

> OverlapSplit(DF)
[[1]]
    x y
1   1 a
2   2 b
3   3 c
4   4 d
5   5 e
6   6 a
7   7 b
8   8 c
9   9 d
10 10 e

> OverlapSplit(DF, 4, 1)
[[1]]
  x y
1 1 a
2 2 b
3 3 c

[[2]]
  x y
3 3 c
4 4 d
5 5 e

[[3]]
  x y
6 6 a
7 7 b
8 8 c

[[4]]
    x y
8   8 c
9   9 d
10 10 e