Asp Forum - Re: association list --updating

William James

5/7/2015 7:22:00 AM

Mark McConnell wrote:

> Ken Tilton wrote:
> >
> > Jeremy Medows wrote:
> > >
> > > Hi,
> > > I am pretty new to LISP and I can't seem to find the answer to this
> > > question in any of my books.
> > >
> > > I have the following association list:
> > >
> > > ((A.1)(B.33)(C.45)(D.9)(E.1))
> > > How can I use the mapping functions and my function inc -- which
> > > is just (defun inc (x) (+ x 1)) to increment each of the second values
> > > in each dotted pair?
> > >
> > > I would like my output to look like:
> > > ((A.2)(B.34)(C.46)(D.10)(E.2))
> > >
> > > Jeremy
> >
> > You are on the right track: use a mapping function. #'mapcar is the most
> > famous, but it would make a *new* assoc. Since you are destructively
> > modifying assoc *elements*, there is no point in that (making a new list
> > would not leave the original unchanged), so you can use #'mapc to
> > iterate over the assoc (which after all is just a list).
>
> Here is another way to explain it. Say z is your association list.
> When you add 1 to each number, you probably don't want to make
> a whole new list; you probably just want to go into z, rip out
> the 1 and replace it with a 2, rip out the 33 and replace it
> with a 34, etc. To do this:
>
> (mapc #'(lambda (pair) (incf (cdr pair)))
> z)
>
> Here (incf ...) takes the ..., adds 1 to it, and stores it back
> _into the same location_ where it was.

Gauche Scheme:

(define al '((A . 1)(B . 33)(C . 45)(D . 9)(E . 1)))

(for-each (^x (inc! (cdr x))) al)

al
===>
((A . 2) (B . 34) (C . 46) (D . 10) (E . 2))

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comp.lang.lisp

Re: association list --updating

William James

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