## Group the elements of a list

Q&A's, tips, howto's
cameyo
Posts: 102
Joined: Sun Mar 27, 2011 3:07 pm

### Group the elements of a list

I use this function to group the elements of a list:

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``````(define (take n lst) (slice lst 0 n))
(define (drop n lst) (slice lst n))

(define (group-by-num n lst)
(if (null? lst) '()
(cons (take n lst) (group-by-num n (drop n lst)))
)
)

(setq lst '(0 1 2 3 4 5 6 7 8 9))

(group-by-num 2 lst)
;-> ((0 1) (2 3) (4 5) (6 7) (8 9))

(group-by-num 3 lst)
;-> ((0 1 2) (3 4 5) (6 7 8) (9))

(setq lst '(1 2 3 4 5 6 7 8 9 10 11 12))

(group-by-num 2 (group-by-num 2 lst))
;-> (((1 2) (3 4)) ((5 6) (7 8)) ((9 10) (11 12)))``````

fdb
Posts: 49
Joined: Sat Nov 09, 2013 8:49 pm

### Re: Group the elements of a list

HI, cameyo, also here you can use a standard function: explode, see below:

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``````> (setq lst (sequence 0 9))
(0 1 2 3 4 5 6 7 8 9)
> (explode lst 2)
((0 1) (2 3) (4 5) (6 7) (8 9))
> (explode lst 3)
((0 1 2) (3 4 5) (6 7 8) (9))

> (setq lst (sequence 1 12))
(1 2 3 4 5 6 7 8 9 10 11 12)
> (explode (explode lst 2)2)
(((1 2) (3 4)) ((5 6) (7 8)) ((9 10) (11 12)))
``````

rickyboy
Posts: 595
Joined: Fri Apr 08, 2005 7:13 pm
Location: Front Royal, Virginia

### Re: Group the elements of a list

Right on, fdb! Lutz wins this round! :)
(λx. x x) (λx. x x)

cameyo
Posts: 102
Joined: Sun Mar 27, 2011 3:07 pm

### Re: Group the elements of a list

damn :-)

ralph.ronnquist
Posts: 216
Joined: Mon Jun 02, 2014 1:40 am
Location: Melbourne, Australia

### Re: Group the elements of a list

How about "transitive closure", then? I.e. given a list of pairs that notionally would, say, represent links in a graph, determine the lists of transitively connected "nodes", or in other words, join all sub-lists that share some element (transitively).

A recursive solution could be something like this:

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``````(define (trans s (x s) (f (and s (curry intersect (first s)))))
(if s (trans (rest s) (cons (unique (flat (filter f x))) (clean f x))) x))

> (trans '((13 1) (9 19) (4 13) (4 12) (15 8) (3 15) (7 5) (9 4) (11 0) (0 5)))
((7 5 0 11) (9 19 4 13 1 12) (15 8 3))``````
Perhaps there is something faster.

cameyo
Posts: 102
Joined: Sun Mar 27, 2011 3:07 pm

### Re: Group the elements of a list

Thanks to all. I have learned a lot.

rickyboy
Posts: 595
Joined: Fri Apr 08, 2005 7:13 pm
Location: Front Royal, Virginia

### Re: Group the elements of a list

ralph.ronnquist wrote:A recursive solution could be something like this:

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``````(define (trans s (x s) (f (and s (curry intersect (first s)))))
(if s (trans (rest s) (cons (unique (flat (filter f x))) (clean f x))) x))``````
Perhaps there is something faster.
Looks good, Ralph! I don't think I could do it faster. I prefer your code anyway because it's understandable.

This is what I see. The input s is a list of sets where each member set relates each of its members to one another. For instance, if one of the members of s is (1 2 3) each of 1, 2 and 3 are related to any other. In math terms, if the input s describes a (symmetric) relation R, then it is the case that 1R2, 2R1, 1R3, 3R1, 2R3 and 3R2 are all true.

So, for instance, the first member of your example input (13 1) implies both 13R1 and 1R13 (when example input describes R). This is because, I don't see the input to trans as any different kind of "stuff" as its eventual output -- they are both relation "descriptions" -- except that the output is guaranteed to describe a transitive relation.

Now, when looking at the input set instead as a set of "links" of a graph (as you described them), then your function trans has to be assuming that all the "links" it finds in the input are bi-directional, or in other words, the "links" describe non-directed edges. Here I note that this assumption does not go unmentioned in your posting, because your posting said "join all sub-lists that share some element (transitively)" which to me says "the links are non-directed edges." Just an observation in case someone else didn't catch that.

The recursive step of trans conses the partial transitive relation descriptor member (set) containing link (first s) (by absorption/subsumption), namely (unique (flat (filter f acc))), to the subset of partial transitive relation descriptor members in x which are mutually exclusive to link (first s), namely (clean f acc). (Whew! <wipes-brow/>)

When I use your trans to compute transitive closures in the future, I will pair it with something like the following which generates a predicate for it.

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``````(define (make-symmetric-relation S)
(letex ([S] S)
(fn (x y)
(exists (fn (s) (and (member x s) (member y s)))
'[S]))))``````
Here's a terrible test that shows it in action.

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``````(define (test-trans input x y)
(let (R     (make-symmetric-relation input)
Rt    (make-symmetric-relation (trans input))
yesno (fn (x) (if x 'yes 'no)))
(list ;; is (x,y) in the original relation?
(yesno (R x y))
;; is (x,y) in the transitive closure?
(yesno (Rt x y)))))``````
For instance, (8 15) is in the original relation; so, it will also be in the transitive closure. (9 13) is not in the original relation, but it is in the transitive closure. (9 15) is in neither.

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``````> (define input
'((13 1) (9 19) (4 13) (4 12) (15 8) (3 15) (7 5) (9 4) (11 0) (0 5)))
> (test-trans input 8 15)
(yes yes)
> (test-trans input 9 13)
(no yes)
> (test-trans input 9 15)
(no no)``````
(λx. x x) (λx. x x)

ralph.ronnquist
Posts: 216
Joined: Mon Jun 02, 2014 1:40 am
Location: Melbourne, Australia

### Re: Group the elements of a list

Thanks. Yes, as you say, the trans function really treats its input list s as a collection of equivalence classes, and combines those that overlap into the smallest collection of classes.

The similar function for non-reflexive relations (or directed arcs) would rather concern transitive reachability, from one element to those that are reachable when following the articulated relation (links) in the forward direction only. I came up with the following for that, which is two functions: one that determines the individual reach from a given element, and an outer function that makes the map of all those for all the elements:

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``````(define (reach s n (f (fn (x) (= n (x 0)))))
(cons n (if s (flat (map (curry reach (clean f s))
(map (curry nth 1) (filter f s)))))))

(define (reachability s)
(map (fn (x) (reach s x)) (sort (unique (flat s)))))

> (reachability '((13 1) (9 19) (4 13) (4 12) (15 8) (3 15) (7 5) (9 4) (11 0) (0 5)))
((0 5) (1) (3 15 8) (4 13 1 12) (5) (7 5) (8)
(9 19 4 13 1 12) (11 0 5) (12) (13  1) (15 8) (19))

``````

The "reachability map" in each sub list tells which elements are reachable from the first according to the original directed relation. So then it's a matter of inflating the map into pair-wise associations to make the directed transitive closure, e.g.:

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``````(define (transD s)
(flat (map (fn (x) (if (1 x) (map (curry list (x 0)) (1 x)) '())) (reachability s)) 1))

> (transD '((13 1) (9 19) (4 13) (4 12) (15 8) (3 15) (7 5) (9 4) (11 0) (0 5)))
((0 5) (3 15) (3 8) (4 13) (4 1) (4 12) (7 5) (9 19) (9 4) (9 13) (9 1) (9 12) (11  0) (11 5)  (13 1)  (15 8))
; i.e. the input plus implied reach: (3 8) (4 1) (9 13) (9 1) (9 12) (11 5)
``````
Then, how do you go the other way? I.e. how do you reduce into the smallest number of pairs, or at least find a sub list so that implied relationships are omitted from the list?

newlisp is fun :)

rickyboy
Posts: 595
Joined: Fri Apr 08, 2005 7:13 pm
Location: Front Royal, Virginia

### Re: Group the elements of a list

ralph.ronnquist wrote:Then, how do you go the other way? I.e. how do you reduce into the smallest number of pairs, or at least find a sub list so that implied relationships are omitted from the list?
Well, I perceive that you know the answer already, so I thank you for letting us share in the fun. :)

Here is a function untransD which removes the implied relationships. It does so by considering each edge edge in s which can be viewed as the pair (src dst) (although dst is not needed here). The clean fn answers the question "Is edge implied?" That will be true when the reach of src, after we remove edge from s, is the same is the reach of src under s.

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``````(define (untransD s)
(clean (fn (edge)
(let (src (edge 0)
remove (fn () (apply replace (args))))
(= (reach s src)
(reach (remove edge s) src))))
s))``````
Aside. For people not familiar with newLISP, note the remove function (defined in the let bindings). It appears as if it is only doing what the intrinsic replace does; so, why not just say (replace edge s) instead of (remove edge s)? The reason for not doing that is subtle. The replace primitive is destructive, and we don't want s to change during the runtime of untransD. Defining remove as we've done here turns it into a non-destructive removal function (because of the calling model of newLISP: a copy gets passed the a function, not a reference).

But perhaps from a (software engineering) contracts point-of-view, we shouldn't rely on the order of the outputs of the reach calls (i.e., its stability). Even though we can see the code of reach, we can also "play it safe" by assuming that we cannot see the implementation and thus replace the usage of = with the usage of another equality predicate where the order doesn't matter. There may be a better way to do this, but here's one.

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``````(define (set-equal? A B)
(= (sort A) (sort B)))``````
The primitive sort is also destructive; however, we don't need A and B (which are copies themselves) for anything else in the scope of this function (after we are done "smashing" them :). Happily, we can reuse set-equal? in our testing.

First, let's recall what running transD on the example input (input) does.

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``````> input
((13 1) (9 19) (4 13) (4 12) (15 8) (3 15) (7 5)
(9 4) (11 0) (0 5))
> (transD input)
((0 5) (3 15) (3 8) (4 13) (4 1) (4 12) (7 5)
(9 19) (9 4) (9 13) (9 1) (9 12) (11 0)
(11 5) (13 1) (15 8))``````
Now, let's see untransD in action.

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``````> (untransD (transD input))
((0 5) (3 15) (4 13) (4 12) (7 5) (9 19) (9 4)
(11 0) (13 1) (15 8))``````
Well, the order is different, but it sorta looks a lot like input (by "eyeballing" it). So, how can we test this a little better? It seems that we should be able to say that transD and untransD are inverses of each other. Let's try that.

First, note that the example input itself is devoid of implied relationships.

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``````> (set-equal? input (untransD input))
true``````
That means the following identity should hold.

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``````> (set-equal? input (untransD (transD input)))
true``````
ralph.ronnquist wrote:newlisp is fun :)
Indeed! :)
(λx. x x) (λx. x x)

rickyboy
Posts: 595
Joined: Fri Apr 08, 2005 7:13 pm
Location: Front Royal, Virginia

### Re: Group the elements of a list

While I was exploring all the code, I believe that I found a bug.

The following identity should hold: the reachability of the transitive closure of input is the same as the reachability of input.

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``````>
(set-equal? (reachability input)
(reachability (transD input)))

nil``````
Hmmm. What's going on?

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``````> (reachability (transD input))
((0 5) (1) (3 15 8 8) (4 13 1 1 12) (5) (7 5)
(8) (9 19 4 13 1 1 12 13 1 1 12) (11 0 5 5)
(12) (13 1) (15 8) (19))``````
Ok, looks like some of the reaches don't have unique elements. Here's one in particular.

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``````> (reach (transD input) 9)
(9 19 4 13 1 1 12 13 1 1 12)``````
Looks like we need a unique in the reach function.

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``````(define (reach s n (f (fn (x) (= n (x 0)))))
(cons n (if s (unique (flat (map (curry reach (clean f s))
(map (curry nth 1) (filter f s))))))))``````
Now, it works.

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``````> (reach (transD input) 9)
(9 19 4 13 1 12)``````
And the identity in fact holds, as expected.

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``````>
(set-equal? (reachability input)
(reachability (transD input)))

true``````
(λx. x x) (λx. x x)

cameyo
Posts: 102
Joined: Sun Mar 27, 2011 3:07 pm

### Re: Group the elements of a list

I need more time to understand all this...