Re: Passing function to mapcar

[Stephen Berman <stephen.berman@gmx.net>]

On Sun, 01 Sep 2024 23:44:03 +0000 Heime <heimeborgia@protonmail.com> wrote:

> I am using mapcar to apply the lambda function taking values
> in rgb-list as argument c.
>
> (mapcar (lambda (c)
>           (min 255 (max 0 (round (* c factor)))))
>         rgb-list)
>
> I think that the function used for mapcar should have only a
> single argument.  Is this correct ?

Yes.

> What can one do if I want to call a function with more than
> a single argument ?
>
> (defun myfunc (c factor)
>   (min 255 (max 0 (round (* c factor)))))
>
> (mapcar myfunc(c factor) rgb-list)

If you redefine `myfunc' to make `factor' the first argument and `c' the
second, you can use apply-partially; maybe that does what you want:

(let ((rgb-list '(37 157 217))
      (factor 0.7))
  (mapcar (apply-partially #'myfunc factor) rgb-list))
=>
(26 110 152)

(If `factor' is always a single number you don't need to redefine
`myfunc', since multiplication is commutative, but then the argument `c'
is assigned the value of `factor' in the call to apply-partially and the
argument `factor' gets the respective values of `rgb-list', so
conceptually it's better to switch the arguments.)

Steve Berman