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Find the highest and lowest mark from the selected "points"


sathalex
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Hi,

 

Easily you can do that by retrieving the coordinates of each point then sort them with their Maximum Y coordinate, then finally the first and the last items in the list would be the Highest & Lowest.

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If the points have been compiled as a list then

(apply 'max (mapcar 'caddr pt_list))
(apply 'min (mapcar 'caddr pt_list))

 

There are several other metheods available as well. -David

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Sorting should be avoided when trying to determine the extrema of a point set, as the set need only be processed once. This may therefore be performed with O(n) efficiency, whereas sorting is generally on the order of O(n log n).

 

I would therefore suggest something like:

(defun maxmin ( lst / mni mxa zco zmi zmx )
   (setq mni (car lst) zmi (caddr mni) mxa mni zmx zmi)
   (foreach  itm (cdr lst)
       (setq zco (caddr itm))
       (cond
           (   (< zmx zco)
               (setq mxa itm zmx zco)
           )
           (   (< zco zmi)
               (setq mni itm zmi zco)
           )
       )
   )
   (list mni mxa)
)

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Sorting should be avoided when trying to determine the extrema of a point set, as the set need only be processed once. This may therefore be performed with O(n) efficiency, whereas sorting is generally on the order of O(n log n).

 

I would therefore suggest something like:

(defun maxmin ( lst / mni mxa zco zmi zmx )
   (setq mni (car lst) zmi (caddr mni) mxa mni zmx zmi)
   (foreach  itm (cdr lst)
       (setq zco (caddr itm))
       (cond
           (   (< zmx zco)
               (setq mxa itm zmx zco)
           )
           (   (< zco zmi)
               (setq mni itm zmi zco)
           )
       )
   )
   (list mni mxa)
)

 

At the command prompt Error: too few arguments (((

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Awesome (effective) function Lee,

Just figured out:

Why don't write something more global using the same algorithm, like accepting function and list arguments (just like vl-sort), i.e.:

 

(foo (lambda (r x) (< r x)) '(3 8 9 1 5 6 2 7) )
-> 1

 

where lambda is a test function and takes two arguments for comparing:

r - item to be returned

x - comparsion item

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Thanks Grrr - as for a generic function, consider the following:

(defun extremum ( cmp lst / rtn )
   (setq rtn (car lst))
   (foreach itm (cdr lst)
       (if (apply cmp (list itm rtn)) (setq rtn itm))
   )
   rtn
)

_$ (extremum '< '(3 8 9 1 5 6 2 7))
1

_$ (extremum '(lambda ( a b ) (< (caddr a) (caddr b))) '((1.2 5.7 8.3) (9.4 2.6 0.3) (5.7 6.6 7.2)))
(9.4 2.6 0.3)

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Or recursively:

(defun extremum ( cmp lst )
   (   (lambda ( foo ) (if (cdr lst) (foo (car lst) (extremum cmp (cdr lst))) (car lst)))
       (lambda ( a b ) (if (apply cmp (list a b)) a b))
   )
)

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Thanks Lee!

It looks quite useful, for example finding the nearest/furthest distance between point list and origin:

_$ (extremum '(lambda ( a b ) (< (distance '(0. 0. 0.) a)(distance '(0. 0. 0.) b))) '((1.2 5.7 8.3) (9.4 2.6 0.3) (5.7 6.6 7.2)))
(9.4 2.6 0.3)

More expanded idea for usage would be constructing assoc list of (distN . enameN) to find the nearest/furthest entity.

 

I see it as a combination between vl-some and vl-sort (like doing something like: (vl-some 'min lst) ).

 

I'd say good job! :)

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Thanks Grrr. :)

 

In practice however, the function could be optimised, as the result of the comparison function when applied to the current extremum would be known and would therefore not need to be recalculated for each item in the list (this is demonstrated by the 'zmx' & 'zmi' variables used in my earlier function which returns the points with minimum & maximum z-coordinate values). This optimisation is not possible when an arbitrary comparison function is used.

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