On Thu, Mar 21, 2013 at 11:55 PM, Tim Nelson <tbnel...@gmail.com> wrote: > Dear All, > > I've recently started hacking Racket again, and am loving the for/* > constructs. > Awesome stuff. However, I have two questions about for/hash. > > (1) How can I *not* add a hash entry for a given iteration? > > When I use for/list, can I abuse the nature of empty and append* (I cringe > as I write this): > > (append* (for/list ([i '(1 2 3 4 5)]) > (cond [(even? i) (list (* i 100))] > [else empty]))) > > Surely there is a better way to do this for for/list, and a way in general > for for/hash? > > (2) How can I add multiple entries (that is, >1 key-value pair) in a given > iteration? > > Again, using for/list I can cheat via append*: > > (append* (for/list ([i '(1 2 3 4 5)]) > (list (* i 100) (* i 200)))) > > What's the right way to do this in for/hash?
To both of the questions, the answer is for/fold! Cheers P. ____________________ Racket Users list: http://lists.racket-lang.org/users
