Thanks everyone for the useful feedback. I'll check out all three options. :-)
On Fri, Mar 22, 2013 at 12:23 PM, Pierpaolo Bernardi <[email protected]>wrote: > On Thu, Mar 21, 2013 at 11:55 PM, Tim Nelson <[email protected]> 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. >
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