wrap.array

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

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=".", ...)

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.

See Also

*unwrap(). See aperm().

Aliases

• wrap.array
• wrap.matrix
• wrap.data.frame

Examples

# 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))