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#### 6.7.5.3 Shared Arrays

- Scheme Procedure:
**make-shared-array***oldarray mapfunc bound …* - C Function:
**scm_make_shared_array***(oldarray, mapfunc, boundlist)* Return a new array which shares the storage of

`oldarray`. Changes made through either affect the same underlying storage. The`bound`… arguments are the shape of the new array, the same as`make-array`

(see section Array Procedures).`mapfunc`translates coordinates from the new array to the`oldarray`. It’s called as`(`

with one parameter for each dimension of the new array, and should return a list of indices for`mapfunc`newidx1 …)`oldarray`, one for each dimension of`oldarray`.`mapfunc`must be affine linear, meaning that each`oldarray`index must be formed by adding integer multiples (possibly negative) of some or all of`newidx1`etc, plus a possible integer offset. The multiples and offset must be the same in each call.

One good use for a shared array is to restrict the range of some dimensions, so as to apply say

`array-for-each`

or`array-fill!`

to only part of an array. The plain`list`

function can be used for`mapfunc`in this case, making no changes to the index values. For example,(make-shared-array #2((a b c) (d e f) (g h i)) list 3 2) ⇒ #2((a b) (d e) (g h))

The new array can have fewer dimensions than

`oldarray`, for example to take a column from an array.(make-shared-array #2((a b c) (d e f) (g h i)) (lambda (i) (list i 2)) '(0 2)) ⇒ #1(c f i)

A diagonal can be taken by using the single new array index for both row and column in the old array. For example,

(make-shared-array #2((a b c) (d e f) (g h i)) (lambda (i) (list i i)) '(0 2)) ⇒ #1(a e i)

Dimensions can be increased by for instance considering portions of a one dimensional array as rows in a two dimensional array. (

`array-contents`

below can do the opposite, flattening an array.)(make-shared-array #1(a b c d e f g h i j k l) (lambda (i j) (list (+ (* i 3) j))) 4 3) ⇒ #2((a b c) (d e f) (g h i) (j k l))

By negating an index the order that elements appear can be reversed. The following just reverses the column order,

(make-shared-array #2((a b c) (d e f) (g h i)) (lambda (i j) (list i (- 2 j))) 3 3) ⇒ #2((c b a) (f e d) (i h g))

A fixed offset on indexes allows for instance a change from a 0 based to a 1 based array,

(define x #2((a b c) (d e f) (g h i))) (define y (make-shared-array x (lambda (i j) (list (1- i) (1- j))) '(1 3) '(1 3))) (array-ref x 0 0) ⇒ a (array-ref y 1 1) ⇒ a

A multiple on an index allows every Nth element of an array to be taken. The following is every third element,

(make-shared-array #1(a b c d e f g h i j k l) (lambda (i) (list (* i 3))) 4) ⇒ #1(a d g j)

The above examples can be combined to make weird and wonderful selections from an array, but it’s important to note that because

`mapfunc`must be affine linear, arbitrary permutations are not possible.In the current implementation,

`mapfunc`is not called for every access to the new array but only on some sample points to establish a base and stride for new array indices in`oldarray`data. A few sample points are enough because`mapfunc`is linear.

- Scheme Procedure:
**shared-array-increments***array* - C Function:
**scm_shared_array_increments***(array)* For each dimension, return the distance between elements in the root vector.

- Scheme Procedure:
**shared-array-offset***array* - C Function:
**scm_shared_array_offset***(array)* Return the root vector index of the first element in the array.

- Scheme Procedure:
**shared-array-root***array* - C Function:
**scm_shared_array_root***(array)* Return the root vector of a shared array.

- Scheme Procedure:
**array-contents***array [strict]* - C Function:
**scm_array_contents***(array, strict)* If

`array`may be*unrolled*into a one dimensional shared array without changing their order (last subscript changing fastest), then`array-contents`

returns that shared array, otherwise it returns`#f`

. All arrays made by`make-array`

and`make-typed-array`

may be unrolled, some arrays made by`make-shared-array`

may not be.If the optional argument

`strict`is provided, a shared array will be returned only if its elements are stored internally contiguous in memory.

- Scheme Procedure:
**transpose-array***array dim1 dim2 …* - C Function:
**scm_transpose_array***(array, dimlist)* Return an array sharing contents with

`array`, but with dimensions arranged in a different order. There must be one`dim`argument for each dimension of`array`.`dim1`,`dim2`, … should be integers between 0 and the rank of the array to be returned. Each integer in that range must appear at least once in the argument list.The values of

`dim1`,`dim2`, … correspond to dimensions in the array to be returned, and their positions in the argument list to dimensions of`array`. Several`dim`s may have the same value, in which case the returned array will have smaller rank than`array`.(transpose-array '#2((a b) (c d)) 1 0) ⇒ #2((a c) (b d)) (transpose-array '#2((a b) (c d)) 0 0) ⇒ #1(a d) (transpose-array '#3(((a b c) (d e f)) ((1 2 3) (4 5 6))) 1 1 0) ⇒ #2((a 4) (b 5) (c 6))

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