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comp.lang.lisp

Re: looking for a more elegant solution to a simple problem

William James

2/23/2016 9:41:00 PM

Pascal Costanza wrote:

> > I was wondering if there was a more concise way of doing something using
> > built-in functions: Given a list lst and a function fn, I want to
> > construct a list which contains the results of applying F to memmbers of
> > L and only keeps the ones which return a true value. It is much simpler
> > in code:
> >
> > (member-if identity (map #'fn lst))
>
> I know about three ways to do that:
>
> (mapcan (lambda (elem)
> (when (fn elem)
> (list (fn elem))))
> list)
>
> (loop for elem in list
> when (fn elem) collect (fn elem))
>
> or check out the excellent collecting macros at
> http://www.tfeb.org/lisp/hax.html#...
>
> You might want to make sure that fn is only applied once to each
> element. You can do that with when-let in Lispworks. Or use the
> following idiom:
>
> (let ((var (fn elem)))
> (when var (list var)))
>
> Or the following macro:
>
> (defmacro whenlet ((var exp) &body body)
> `(let ((,var ,exp))
> (when ,var ,@body)))

MatzLisp (Ruby):

def func x
5 == x ? nil : x*x
end

(0..12).map{|x| func(x)}.compact
==>[0, 1, 4, 9, 16, 36, 49, 64, 81, 100, 121, 144]


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