Wraps rows into new rows, optionally including the source row. https://stackoverflow.com/q/65118797/1270695

row_wrap.R

row_wrap <- function(input, ...) {
  UseMethod("row_wrap")
}

row_wrap.data.frame <- function(input, ncols, row_ind = FALSE) {
  if (ncol(input) %% ncols != 0) stop("Number of columns not divisible by desired output")
  data.frame(row_wrap.matrix(input, ncols, row_ind))
}

row_wrap.matrix <- function(input, ncols, row_ind = FALSE) {
  if (ncol(input) %% ncols != 0) stop("Number of columns not divisible by desired output")
  out <- matrix(t(input), ncol = ncols, byrow = TRUE, 
                dimnames = list(NULL, colnames(input)[sequence(ncols)]))
  if (isTRUE(row_ind)) {
    cbind(row_ind = rep(1:nrow(input), each = ncol(input)/ncols), out)
  } else {
    out
  }
}

# sample data
set.seed(1)
M <- m <- matrix(sample(c(-5:5, NA), 24, TRUE), nrow = 2)
colnames(m) <- paste0("V", sequence(ncol(m)))
df <- data.frame(m)
df
##   V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12
## 1  3  1 -4  5  5 -5 -1  0  1  -1   3  -1
## 2 -2 -5  1 -4 -3 -1  4  4  3  -1   3  -1

row_wrap(M, 2)
##       [,1] [,2]
##  [1,]    3    1
##  [2,]   -4    5
##  [3,]    5   -5
##  [4,]   -1    0
##  [5,]    1   -1
##  [6,]    3   -1
##  [7,]   -2   -5
##  [8,]    1   -4
##  [9,]   -3   -1
## [10,]    4    4
## [11,]    3   -1
## [12,]    3   -1

row_wrap(m, 3, TRUE)
##      row_ind V1 V2 V3
## [1,]       1  3  1 -4
## [2,]       1  5  5 -5
## [3,]       1 -1  0  1
## [4,]       1 -1  3 -1
## [5,]       2 -2 -5  1
## [6,]       2 -4 -3 -1
## [7,]       2  4  4  3
## [8,]       2 -1  3 -1

row_wrap(df, 6)
##   V1 V2 V3 V4 V5 V6
## 1  3  1 -4  5  5 -5
## 2 -1  0  1 -1  3 -1
## 3 -2 -5  1 -4 -3 -1
## 4  4  4  3 -1  3 -1