wrap.array: Reshape an array or a matrix by permuting and/or joining dimensions

Description

Reshape an array or a matrix by permuting and/or joining dimensions. A useful application of this is to reshape a multidimensional array to a matrix, which then can be saved to file using for instance write.table().

Usage

## S3 method for class 'array':
wrap(x, map=list(NA), sep=".", ...)

Arguments

An array or a matrix.

map

A list of length equal to the number of dimensions in the reshaped array. Each element should be an integer

sep

A character pasting joined dimension names.

...

Not used.

Value

Returns an array of length(map) dimensions, where the first dimension is of size prod(map[[1]]), the second prod(map[[2]]), and so on.

Details

If the indicies in unlist(map) is in a non-increasing order, aperm() will be called, which requires reshuffling of array elements in memory. In all other cases, the reshaping of the array does not require this, but only fast modifications of attributes dim and dimnames.

Examples

Run this code

# Create a 3x2x3 array
dim <- c(3,2,3)
ndim <- length(dim)
dimnames <- list()
for (kk in 1:ndim)
  dimnames[[kk]] <- sprintf("%s%d", letters[kk], 1:dim[kk])
x <- 1:prod(dim)
x <- array(x, dim=dim, dimnames=dimnames)


cat("Array 'x':
")
print(x)


cat("Reshape 'x' to its identity:
")
y <- wrap(x, map=list(1, 2, 3))
print(y)
# Assert correctness of reshaping
stopifnot(identical(y, x))


cat("Reshape 'x' by swapping dimensions 2 and 3, i.e. aperm(x, perm=c(1,3,2)):
")
y <- wrap(x, map=list(1, 3, 2))
print(y)
# Assert correctness of reshaping
stopifnot(identical(y, aperm(x, perm=c(1,3,2))))


cat("Wrap 'x' to a matrix 'y' by keeping dimension 1 and joining the others:
")
y <- wrap(x, map=list(1, NA))
print(y)
# Assert correctness of reshaping
for (aa in dimnames(x)[[1]]) {
  for (bb in dimnames(x)[[2]]) {
    for (cc in dimnames(x)[[3]]) {
      tt <- paste(bb, cc, sep=".")
      stopifnot(identical(y[aa,tt], x[aa,bb,cc]))
    }
  }
}


cat("Unwrap matrix 'y' back to array 'x':
")
z <- unwrap(y)
print(z)
stopifnot(identical(z,x))


cat("Wrap a matrix 'y' to a vector and back again:
")
x <- matrix(1:8, nrow=2, dimnames=list(letters[1:2], 1:4))
y <- wrap(x)
z <- unwrap(y)
print(z)
stopifnot(identical(z,x))


cat("Wrap and unwrap a randomly sized and shaped array 'x2':
")
maxdim <- 5
dim <- sample(1:maxdim, size=sample(2:maxdim))
ndim <- length(dim)
dimnames <- list()
for (kk in 1:ndim)
  dimnames[[kk]] <- sprintf("%s%d", letters[kk], 1:dim[kk])
x2 <- 1:prod(dim)
x2 <- array(x, dim=dim, dimnames=dimnames)

cat("Array 'x2':
")
print(x)

# Number of dimensions of wrapped array
ndim2 <- sample(1:(ndim-1), size=1)

# Create a random map for joining dimensions
splits <- NULL;
if (ndim > 2)
  splits <- sort(sample(2:(ndim-1), size=ndim2-1))
splits <- c(0, splits, ndim);
map <- list();
for (kk in 1:ndim2)
  map[[kk]] <- (splits[kk]+1):splits[kk+1];

cat("Random 'map':
")
print(map)

cat("Array 'y2':
")
y2 <- wrap(x2, map=map)
print(y2)

cat("Array 'x2':
")
z2 <- unwrap(y2)
print(z2)

stopifnot(identical(z2,x2))

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