CODEWITHSUNDEEP
sortingcommon-lisp
JT93
JT93

Reputation: 511

Create a new Sort in LISP

I want to define a new sorting method in LISP that sticks to this formula.

2 > 3 > 4 > 5 > 6 > 7 > 8 > 9 > 10 > J > Q > K > A

As seen in Poker.

So for example I called it on this: 

(mysortmethod '((3 H)(2 H)(J D)(8 C)(5 C)))

I would get this:

((2 H)(3 H)(5 C)(8 C)(J D))

as the sorted list where the 2nd element in each bracket is ignored.

I have absolutely no idea how to do this. Could someone point me in the right direction or perhaps explain to me a method?

Upvotes: 0

Views: 251

Answers (1)

Joshua Taylor
Joshua Taylor

Reputation: 85823

This is a pretty standard type of Lisp exercise. First you need a convenient way to figure out the ordering. One easy way to is to keep a sequence of the pips (if you have lots, maybe a hash table mapping pips to positions would be better) and a function that compares their positions:

(defconstant +pips+ #(2 3 4 5 6 7 8 9 10 J Q K A))

(defun pip< (pip1 pip2)
  (< (position pip1 +pips+)
     (position pip2 +pips+)))

Then you can use the standard sort function (remember, it's destructive, so save the result, and don't call it with quoted data, since you shouldn't modify literal data), passing pip< as the predicate and first as the key, because you use first to get the pip from each card (since you're representing cards as lists of a pip and a suit):

CL-USER> (let ((hand (copy-tree '((3 H)(2 H)(J D)(8 C)(5 C)))))
           (sort hand 'pip< :key 'first))
;=> ((2 H) (3 H) (5 C) (8 C) (J D))

Upvotes: 4

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