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

wrap.array {R.utils} | R Documentation

## 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

`x` | 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` `vector`s specifying the dimensions to be joined in corresponding new dimension. One element may equal `NA` to indicate that that dimension should be a join of all non-specified (remaining) dimensions. Default is to wrap everything into a `vector`.
`sep` | A `character` pasting joined dimension names.
`...` | Not used.

### Details

If the indices 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`.

### 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.

### Author(s)

Henrik Bengtsson

### See Also

`*unwrap()`. See aperm().

### 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':\n")
    print(x)

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

    cat("\nReshape 'x' by swapping dimensions 2 and 3, i.e. aperm(x, perm=c(1,3,2)):\n")
    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("\nWrap 'x' to a matrix 'y' by keeping dimension 1 and joining the others:\n")
    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("\nUnwrap matrix 'y' back to array 'x':\n")
    z <- unwrap(y)
    print(z)
    stopifnot(identical(z,x))

    cat("\nWrap a matrix 'y' to a vector and back again:\n")
    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("\nWrap and unwrap a randomly sized and shaped array 'x2':\n")
    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("\nArray 'x2':\n")
    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("\nRandom 'map':\n")
    print(map)

    cat("\nArray 'y2':\n")
    y2 <- wrap(x2, map=map)
    print(y2)

    cat("\nArray 'x2':\n")
    z2 <- unwrap(y2)
    print(z2)

    stopifnot(identical(z2,x2))

[Package R.utils version 2.7.0 Index]