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Array slices for Common Lisp
Common Lisp
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README.org

This library is experimental. Your comments are very welcome.

Array slices for Common Lisp

This library provides the following:

  1. A user interface for taking slices (elements selected by the Cartesian product of vectors of subscripts for each axis) of array-like objects. The most important function is slice. Unless you want to define a method for this (besides what is already implemented), this is pretty much all you need from this library. See the next section for a tutorial.
  2. An extensible DSL for selecting a subset of valid subscripts. This is useful if, for example, you want to resolve column names in a data frame in your implementation of slice.
  3. A set of utility functions for traversing slices in array-like objects.

User interface

The most frequently used form is

(slice object slice1 slice2 ...)

Each slice selects a subset of subscripts along the corresponding axis. The library supports the following slice specifications:

  • a nonnegative integer selects the corresponding index, while a negative integer selects an index counting backwards from the last index:
    (slice #(0 1 2 3) 1)                  ; => 1  
    (slice #(0 1 2 3) -2)                 ; => 2    
        

    These are called singleton slices. Each singleton slice drops the dimension: vectors become atoms, matrices become vectors, etc.

  • (cons start end) selects integers $i: \text{start} \leq i < \text{end}$. When end is nil, the last index is included (cf. subseq). Each boundary is resolved according to the other rules if applicable, so you can use negative integers:
    (slice #(0 1 2 3) (cons 1 3))         ; => #(1 2)
    (slice #(0 1 2 3) (cons 1 -1))        ; => #(1 2)
        

    However, (cons start end) is invalid unless $\text{start} < \text{end}$, so you can’t use

    (slice #(0 1 2 3) (cons 2 2))         ; ERROR
        
  • t selects all subscripts:
    (slice #2A((0 1 2)
               (3 4 5))
           t 1)                           ; => #(1 4)
        
  • vectors concatenate selections (except for bit vectors):
    (slice #(0 1 2 3 4 5 6 7 8 9)
            (vector (cons 1 3) 6 (cons -2 -1)))
                                          ; => #(1 2 3 6 8 9)
    (slice #(0 1 2) #(2 2 1 0 0))         ; => #(2 2 1 0 0)
        
  • bit vectors can be used as a mask:
    (slice #(0 1 2 3 4) #*00110)          ; => #(2 3)
        

This is pretty much the core functionality — the semantics can be extended, see the next section. The following extensions are provided by the library.

  • including is convenient if you want the slice to include the end of the range:
(slice #(0 1 2 3) (including 1 2))     ; => #(1 2), cf (slice ... (cons 1 3))
  • nodrop is useful if you don’t want to drop dimensions:
    (slice #(0 1 2 3) (nodrop 2))          ; => #(2), cf (slice ... (cons 2 3))
        
  • head and tail do the obvious:
    (slice #(0 1 2 3) (head 2))            ; => #(0 1)
    (slice #(0 1 2 3) (tail 2))            ; => #(2 3)
        

All of these are trivial to implement — if there is something you are missing, you can easily extend slice. You may also want to submit a patch!

ref is a version of slice that always returns a single element, so it can only be used with singleton slices.

Slice semantics

Arguments of slice (apart from the first one) are meant to be resolved using canonical-representation, in the cl-slice-dev package. If you want to extend slice, you should define methods for canonical-representation. See the documentation for details, here I provide a simple example that extends the semantics with ordinal numbers.

(defmacro define-ordinal-slice (number)
  (check-type number (integer 0))
  `(defmethod cl-slice-dev:canonical-representation
       ((axis integer) (slice (eql ',(intern (format nil "~:@(~:r~)" number)))))
     (assert (< ,number axis))
     (cl-slice-dev:canonical-singleton ,number)))

(define-ordinal-slice 1)
(define-ordinal-slice 2)
(define-ordinal-slice 3)

(slice #(0 1 2 3 4 5) (cons 'first 'third)) ; => #(1 2)

Note the following:

  1. The value returned by canonical-representation needs to be constructed using canonical-singleton, canonical-range, or canonical-sequence. You should not use the internal representation directly as it is subject to change.
  2. You can assume that axis is an integer: this is the default. An object may define a more complex mapping (such as, for example, named rows & columns), but unless a method specialized to that is found, canonical-representation will just query its dimension (with axis-dimension) and try to find a method that works on integers.
  3. You need to make sure that the subscript is valid, hence the assertion.

Traversing slices

write this

Reporting bugs

Please report bugs using the issue tracker.

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