I'm looking for a general solution. Basically, I thought binding multiple
id's would have achieved what this does:
> (for/list ([t '((1 2 3) (4 5 6) (7 8 9) (a b c))])
(match-define (list x y z) t)
(list x z))
'((1 3) (4 6) (7 9) (a c))
The way the documentation for `for` is worded [3], I thought if I could get
the list of lists coerced into a being a sequence of multiple values, then
the same thing could be achieved with:
> (for/list ([(x y z) (??? '((1 2 3) (4 5 6) (7 8 9) (a b c))])
(list x z))
It looks like I would need a transpose-type operation before `apply`ing
in-parallel, because this works,
> (for/list ([(x y z) (in-parallel '(1 4 7 a) '(2 5 8 b) '(3 6 9 c))])
(list x z))
'((1 3) (4 6) (7 9) (a c))
[3] http://docs.racket-lang.org/reference/for.html : "...a location is
created for each id to hold the values of each element; the sequence
produced by a seq-expr must return as many values for each iteration as
corresponding ids."
On Mon, Jul 10, 2017 at 9:46 PM, Jon Zeppieri <zeppi...@gmail.com> wrote:
> On Mon, Jul 10, 2017 at 11:40 PM, Nadeem Abdul Hamid <nad...@acm.org>
> wrote:
> > Given a list of pairs, I'm trying to iterate through the list and bind
> the
> > first and second elements of each pair to x and y, respectively, in each
> > iteration.
> >
> > For example, hash sets are a multiple-valued sequence[1], so this:
> > (for/list ([(x y) #hash((1 . 2) (3 . 4))]) x)
> > produces:
> > (list 1 3)
> >
> > i.e. here x gets bound to 1 in the first iteration and 3 in the second. I
> > would like something analogous for a list of pairs, i.e. replacing #hash
> > with ' :
> > (for/list ([(x y) '((1 . 2) (3 . 4))]) x)
> > except that lists are a by default a single-valued sequence[2].
> >
> >
>
> in-dict will work for lists of pairs, like the ones in this example
> (and it will work for hashes, as well), but it won't work for lists of
> lists, like the examples in your previous message. Are lists of pairs
> all you need to handle, or do you need something more general?
>
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