Great, thank you. I was thinking maybe there was some construct that could
do this built-in, the way 'map' can double as 'zip'.
I looked through sugar and it looks very convenient; I'll be using it in
future. Note that when I installed it, I got the following:
[...snip lots of "raco setup: 0 skipping: <pkgs>/..." lines]
raco setup: docs failure: query-exec: unable to open the database file
error code: 14
SQL: "ATTACH $1 AS other"
database: #<path:/Users/dstorrs/Library/Racket/6.3/doc/docindex.sqlite>
mode: 'read-only
file permissions: (write read)
raco setup: --- installing collections ---
raco setup: --- post-installing collections ---
raco pkg install: packages installed, although setup reported errors
Doesn't seem to have mattered and it might just be something weird about
permissions on my system. I figured I'd mention it, though.
Dave
On Thu, Aug 4, 2016 at 5:04 PM, Matthew Butterick <m...@mbtype.com> wrote:
>
> On Aug 4, 2016, at 3:10 PM, David Storrs <david.sto...@gmail.com> wrote:
> > I'd like to be able to iterate over a list N elements at a time -- e.g.,
> grab elements 1 and 2, do something with them, grab 3 and 4, etc.
>
> Your technique seems fine. You could use `split-at` if you wanted to save
> a list operation, otherwise it's the standard recursion pattern:
>
> #lang racket
> (define data '(("foo") ("bar") ("baz") ("jaz") ("quux") ("glug")))
>
> (define (step-by-n func xs [num 2])
> (if (null? xs)
> null
> (let-values ([(head tail) (split-at xs num)])
> (cons (func head) (step-by-n func tail num)))))
>
> (step-by-n (λ (xx) (apply hash (map car xx))) data)
>
> (step-by-n (λ (xxx) (hash (caar xxx) (map car (cdr xxx)))) data 3)
>
>
> I wrote a utility function called `slice-at` to handle this problem. [1]
>
>
> [1] http://docs.racket-lang.org/sugar/#%28def._%28%28lib._
> sugar%2Flist..rkt%29._slice-at%29%29
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