## Cluster

### May 9, 2014

`Cluster`

calls for some kind of dictionary structure to accumulate signature/item pairs. You could use a hash table or balanced tree, but instead we choose a simple (unbalanced) binary tree, on the theory that if the input is not presented in order by signature (if it is, a simple scan through the input in order would be sufficient) then it is unlikely that the tree will become too unbalanced; besides, it gives us a chance to write a simple binary tree `insert`

and `in-order`

traversal. Here’s our `cluster`

:

`(define (cluster proc lt? lst)`

(define (insert key value tree)

(cond ((null? tree)

(list key (list value) '() '()))

((lt? key (car tree))

(let ((left (insert key value (caddr tree))))

(list (car tree) (cadr tree) left (cadddr tree))))

((lt? (car tree) key)

(let ((right (insert key value (cadddr tree))))

(list (car tree) (cadr tree) (caddr tree) right)))

(else

(let ((new (cons value (cadr tree))))

(list key new (caddr tree) (cadddr tree))))))

(define (in-order tree)

(if (null? tree) '()

(append (in-order (caddr tree))

(list (cadr tree))

(in-order (cadddr tree)))))

(let loop ((lst lst) (tree '()))

(if (null? lst) (in-order tree)

(loop (cdr lst) (insert (proc (car lst)) (car lst) tree)))))

The tree is represented as a recursive four-element list, with signature in the `car`

, list of values in the `cadr`

, left child in the `caddr`

, and right child in the `cadddr`

. Since the data structure is recursive, so is the code that does insertion, which begins at the root of the tree. If a tree is null, the signature must not appear in the tree, so a new node is built. Otherwise, the signature must already exist in the tree, so the existing node is updated, either by recursively inserting in the left sub-tree if the new signature is less than the current signature, or in the right sub-tree if the new signature is greater than the current signature, or by consing the input element to the current node if the two signatures are equal. The main loop of the function `cdr`

s through the input, inserts each signature/item pair into the tree, and writes the tree when the input is exhausted. Here are some examples:

`> (define x '("this" "is" "a" "fun" "and" "useful" "program"))`

> (cluster string-length < x)

(("a") ("is") ("and" "fun") ("this") ("useful") ("program"))

> (cluster (lambda (x) (string-ref x 0)) char<? x)

(("and" "a") ("fun") ("is") ("program") ("this") ("useful"))

> (define (anagram s) (list->string (sort char<? (string->list s))))

> (define dict '("pots" "time" "spot" "pans" "item" "tops"))

> (cluster anagram string<? dict)

(("pans") ("item" "time") ("tops" "spot" "pots"))

You can run the program at http://programmingpraxis.codepad.org/tNQg34dH.

Simple-minded grouping of consecutive items, the requested effect achieved by composing with sort:

(include "sort-okeefe.scm")

(define (group-by sign < items)

(if (null? items)

items

(let group ((g (list (car items)))

(s (sign (car items)))

(items (cdr items)))

(if (null? items)

(list (reverse g))

(let ((t (sign (car items))))

(if (or (< s t) (< t s))

(cons (reverse g)

(group (list (car items)) t (cdr items)))

(group (cons (car items) g) s (cdr items))))))))

`(define (cluster sign < items)`

(group-by sign < (sort items (lambda (i j) (< (sign i) (sign j))))))

Test:

(define test-data

(list "oh" "one" "two" "three" "four" "five" "seven" "ate" "nine"))

`(write (cluster string-length < test-data)) (newline)`

Test result:

(("oh") ("one" "two" "ate") ("four" "five" "nine") ("three" "seven"))

(The predicate signature needs to have a pair of betas for input.)

Added a beta. Thanks.

Here’s a Haskell solution: tag the input list with the signature, sort, then run through the list gathering up the sublists. I tried to do something with foldl for this, but didn’t result in any nice simplifications.

Nice solution. I’m still finding my way around the standard library so lots of new stuff there for me. I was wondering about converting from the Bool function from an Ord function, but I thought being Haskell, we’d probably get one of them anyway.

Had another think about fold and came up with the following (I’m not sure what the pros and cons of using foldr or foldl are – here foldr works nicely to keep everything in the right order):

Clojure:

@chmllr:

NIce, but will group-by preserve the correct ordering by signatures? If not, then you can move the sort to the end.

@matthew, I don’t know why exactly, but I observed in Clojure, that the sets are always iterated as LIFO queues (maybe the maps are based on Java’s LinkedHashMaps). Now, since sort-by is implemented with the reduce function starting with an empty set, I always get the correct order. However, you’re right: this is a weak assumption to rely on.

I thought again and I came up with even a shorter solution, without any implicit assumptions:

(defn cluster [proc lt? lst]

(partition-by proc (sort-by proc lt? lst)))

@christian: Very nice (of course, we call proc twice for each element, but that’s just one of those tradeoffs).

Here’s something similar in Haskell, p is partition-by:

@matthew: just for curiosity: I didn’t touch Haskell for years, but would this pattern matching based idea work, to implement partition-by? (I think it’s the same what you do above, but without if-the-else and with TCO)

partition-by sig lst = h [] [] (map (\ x -> [(sig x) x]) lst)

where

h tmp acc [] = map reverse acc

h tmp acc [[s:x]:[s:y]:xs] = h [x:tmp] acc [[s:y]:xs]

h tmp acc [[s:x]:[q:y]:xs] = h [] [[x:tmp]:acc] [[q:y]:xs]

PS: oops, one case was missing :-)

@chmllr

That would be nice, but Haskell doesn’t seem to like having the same variable twice in a pattern match:

We could use a guard expression here (I would have done this but I wanted to bind the result of sig first, maybe there’s some other way of doing this).

Writing list generating functions as “f a b c = x : f a’ b’ c'” without TCO I suppose seems more natural in Haskell since it will work with lazy lists (which partition-by could do).