if does not evaluate

James Moughan moughanj at tcd.ie
Sat Jun 12 07:00:32 CEST 2004


Jim Newton <jimka at rdrop.com> wrote in message news:<2it0kiFqud3kU1 at uni-berlin.de>...
> > (define has-element (cond list) 
> >     (equal () (member-if #'cond list))))
> > 
> > Well, it's definitely short.  It doesn't exactly translate well,
> > though; search the list for a sub-list that starts with an element
> > which evaluates to true under cond.  Evaluate to the equality of that
> > list with the null list.  Hmmm.
> > 
> 
> Not sure what you mean by "does not translate well?"  

See the later part of my post. :)

>Although maybe
> i misunderstand, but it looks like you are naming the function the
> opposite of what it does?
> 
> (define has-not-element ( cond list)
>     (null (member-if #'cond list)))
> 
> Are you really trying to find out whether the condition fails
> for every element of the list?
> 

Well, I am, but I intended to put a not in front of it!  (not (null
(member-if #'cond list))) in effect.

> By the way, how do you find out in Python if there is
> a member of a list that matches a condition?  It is something
> i often and never know the best way to do it?  I can
> use a list compression to find ALL the matches, but what
> if i only want to know whether or not there is a match?
> 
> -jim

In Lisp style- write a function to do it.



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