list ← ##.list vect ⍝ list from vector ⍵. rslt ← init (acc ##.ltrav) list ⍝ List traversal. Function [list] takes a vector as right argument and returns a list. Operator [ltrav] takes a dyadic [acc]umulating function as left operand, with an initial accumulator value of [init] as left argument. The right argument is the subject [list]. Background ---------- APL is pre-eminent when it comes to the processing of arrays. Occasionally, how- ever we encounter a task where the value of the first of a number of items det- ermines how we are to interpret any remaining ones. In this case, it might be worth considering representing the items as a list, rather than as a vector. An example might be in writing a parser for a sequence of expression tokens. Definition: A List-of-⍺-s is either Null or a Pair (Head Tail), where Head is an ⍺ and Tail is a List-of-⍺-s. More succinctly: List ⍺ ::= Null | Pair ⍺ (List ⍺) Pair is sometimes called "Cons", short for "list constructor". We use a nested 2-vector to implement a list with a special value '∘' for Null. Note that the choice of '∘' for Null is arbitrary and could just as well be any specific array value, including a 2-vector! In general, a list has many of the properties of an APL vector, except that: Separating the first from any remaining items in a list: head tail←⍵ is quicker and more elegant than the vector equivalent: head tail←(⊃⍵)(1↓⍵) However, A list has no prototypical item, so "overtake" must be coded explicitly. There is an overhead of around 40 bytes per item (80 in 64-bit APL), comp- ared with a simple vector but this overhead is diluted when compared with a nested vector. size←{⎕size'⍵'} ⍝ size in bytes. size 1e3⍴⎕a ⍝ size of 1000-vector of bytes. 1016 size list 1e3⍴⎕a ⍝ size of 1000-list of bytes. 39980 size ,¨1e3⍴⎕a ⍝ size of 1000-vector of 1-vectors. 24016 size list ,¨1e3⍴⎕a ⍝ size of 1000-list of 1-vectors. 44016 To access the ⍺'th item of a list takes ⍺ iterations, as opposed to the vector's 1. In other words, list-item-access is O(n), whereas vector-item- access is O(1). However, in both cases, access-each-item is O(n). To process all of the items in a list, a common operation, there is no equi- valent of the array's primitive operator ¨ (each). Instead, an explicit loop must be coded. ( However, in defence of the list: splitting a problem into what-to-do- with-the-head together with what-to-do-with-the-tail is a powerful de- composition technique. Proponents of the list argue that, in situations where a vector can not be treated as a whole, recurring on a list as op- posed to iterating on the vector leads to many fewer "plus-or-minus-one" problems. ) The following function counts the number of items in a list. litems←{ ⍝ No. of list items. ⍺←0 ⍝ initial count is 0. ⍵≡'∘':⍺ ⍝ null list: accumulated count. head tail←⍵ ⍝ head item and tail. (⍺+1)∇ tail ⍝ accumulate with tail. } and this function converts a list to its equivalent vector: vect←{ ⍝ Vector from list. ⍺←⍬ ⍝ initial vector is ⍬. ⍵≡'∘':⍺ ⍝ null list: accumulated vector. head tail←⍵ ⍝ head item and tail. (⍺,⊂head)∇ tail ⍝ accumulate with tail. } leading to operator [ltrav], which abstracts the essence of traversing a list: ltrav←{ ⍝ List traversal. ⍵≡'∘':⍺ ⍝ null list : accumulator. head tail←⍵ ⍝ head item and tail. (⍺ ⍺⍺ head)∇ tail ⍝ accumulated with tail. } Note that in this general case, the initial accumulator ⍺ must be supplied ex- plicitly as the operator can not know its type. Using such techniques we can show that, in some circumstances, the processing of a list can outpace the equivalent (naïvely coded) vector version: vsum←{ ⍝ vector summation. ⍺←0 ⍝ initial accumulator. 0=⍴⍵:⍺ ⍝ null vector: finished. head tail←(⊃⍵)(1↓⍵) ⍝ first and remaining items. (⍺+head)∇ tail ⍝ accumulated sum. } lsum←{ ⍝ list summation. ⍺←0 ⍝ initial accumulator. ⍵≡'∘':⍺ ⍝ null list: finished. head tail←⍵ ⍝ first and remaining items. (⍺+head)∇ tail ⍝ accumulated sum. } cmpx'vsum⍳1e4' 'lsum list⍳1e4' vsum⍳1e4 2.2E¯1 0% ⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕ lsum list⍳1e4 6.5E¯2 -72% ⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕⎕ The reason why vsum is so much slower is that (1↓⍵), which is executed ⍴⍵ times, makes a copy of all but the first item of the vector ⍵. NB: Of course, there are better ways to sum a numeric vector! Technical note: The coding for list is simply: list←{ ⍝ list from vector ⍵. ↑{⍺ ⍵}/⍵,'∘' ⍝ with '∘' as null. } As {⍺ ⍵} is an idiom, the list function is relatively fast. Naming leading items -------------------- We can name more than one item at the head of the list by using "structure as- signment" (head tail) ← ⍵ ⍝ name first item and remainder. (head (next tail)) ← ⍵ ⍝ name first two items and remainder. (a (b (c (d (e ... ))))) ← ⍵ ⍝ name first umpteen items and remainder. A rather contrived example of the use of this technique might be to remove adj- acent duplicate values from a list: rmdups←{ ⍝ remove adjacent duplicates. ⍺←'∘' ⍝ null accumulator. (a(b tail))←⍵ ⍝ a and b are first two items. b≡'∘':'∘'{⍺ ⍵}⍨ltrav a ⍺ ⍝ b null: list-reversed accumulator. a≡b:⍺ ∇ b tail ⍝ two items match: drop first one. a ⍺ ∇ b tail ⍝ accumulate first, continue. } vect rmdups list 'Mississippi' ⍝ removal of adjacent duplicates. Misisipi Handy functions on lists ------------------------ Here are some list utility functions: rev←{⍺←'∘' ⋄ ⍵≡'∘':⍺ ⋄ hd r←⍵ ⋄ hd ⍺ ∇ tl} ⍝ reversed list: (⌽⍵),⍺ cat←{⍺≡'∘':⍵ ⋄ hd tl←⍺ ⋄ hd(tl ∇ ⍵)} ⍝ list catenation: ⍺,⍵ pop←{⍺=0:⊂⍵ ⋄ hd tl←⍵ ⋄ ((⍺-1)∇ tl),⊂hd} ⍝ ⍺ items and tail from list ⍵ Examples: ltrav←{ ⍝ List traversal. ⍵≡'∘':⍺ ⍝ null list : accumulator. head tail←⍵ ⍝ head item and tail. (⍺ ⍺⍺ head)∇ tail ⍝ accumulated with tail. } 0 {⍺+1} ltrav list'hello' ⍝ length of list. 5 length←0∘( {⍺+1} ltrav ) ⍝ named list-length function. length list ⎕a 26 vect←⍬∘( {⍺,⊂⍵} ltrav ) ⍝ vector from list. vect list 'hello' ⍝ round-trip vector. hello revl←'∘'∘( {⍺ ⍵}⍨ ltrav ) ⍝ reverse of list. vect revl list 'hello' ⍝ vector reverse. olleh ⍝ A more substantial example is this little "lambda expression" parser: parse←{ ⍝ Right-to-left lambda expr parser. ↑{ ⍝ ⎕←⍺'│'⍵ ⍝ (uncomment to see trace). (toks a)(b(c accs))←⍺ ⍵ ⍝ a│b c : a│b c 3-item window. '⊣'≡a:b ⍝ ⊣│* : * finished '()'≡a c:toks ∇ b accs ⍝ (│* ) : │* · '(→'≡a b:⍺ ∇ c accs ⍝ (│→ * : (│*· · '→→'≡a b:toks ∇ ⍵ ⍝ →│→ * : │→ * '→'≡b:toks ∇ b(('→'a c)accs) ⍝ v│→ b : │→ v→b lambda node. '→)'≡a c:toks ∇ a ⍵ ⍝ →│* ) : │→ * ) a∊'→(':⍺ ∇('@'b c)accs ⍝ (│f a : (│fa apply node. toks ∇ a ⍵ ⍝ *│* · : │* * }/↑{⍺ ⍵}⍨/⌽'⊣(',⍵,')' ⍝ parse of token list ⍵. } ⍝ Notice that, apart from the last line, there are very few APL primitive funct- ⍝ ions in the code; most of the action takes place using structure construction ⍝ and deconstruction by assembling and naming parts of the ⍺ and ⍵ lists. )copy min trees ⍝ borrow Min's expression tree display. trees parse '(t→ttt)(fx→f(fx))+0' ⍝ parse tree for lambda expression. ┌─@┐ ┌──@┐ 0 ┌────@─┐ + ┌→──┐ ┌→─┐ t ┌─@┐ f ┌→─┐ ┌@┐ t x ┌@─┐ t t f ┌@┐ f x See also: alists parse lisp joy Back to: contents Back to: Workspaces