[racket-users] Iterating through a list more than one at a time
I feel like there should be a simpler way to do this, but I haven't found
one despite much Googling and reading of docs about "for" and its
siblings. Can someone point me the right way?
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.
This will do the job, but I suspect there's a more built-in way:
(define data '(("foo") ("bar") ("baz") ("jaz") ("quux") ("glug")))
(define (step-by-n func data [num 2])
(if (null? data)
(func '())
(append (func (take data num))
(step-by-n func (drop data num) num
(step-by-n (lambda (l)
(if (null? l)
'()
(list (hash (car (first l))
(car (second l))
data)
(step-by-n (lambda (l)
(if (null? l)
'()
(list (hash (car (first l))
(list (car (second l))
(car (third l)))
data
3)
OUTPUT:
'(#hash(("foo" . "bar")) #hash(("baz" . "jaz")) #hash(("quux" . "glug")))
'(#hash(("foo" . ("bar" "baz"))) #hash(("jaz" . ("quux" "glug"
(Note that the fact that it returns the results as a list was a deliberate
choice but not an essential one.)
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Re: [racket-users] Iterating through a list more than one at a time
On Aug 4, 2016, at 3:10 PM, David Storrs 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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Re: [racket-users] Iterating through a list more than one at a time
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: /..." lines]
raco setup: docs failure: query-exec: unable to open the database file
error code: 14
SQL: "ATTACH $1 AS other"
database: #
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 wrote:
>
> On Aug 4, 2016, at 3:10 PM, David Storrs 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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Re: [racket-users] Iterating through a list more than one at a time
> I feel like there should be a simpler way to do this, but I haven't found
> one despite much Googling and reading of docs about "for" and its siblings.
> Can someone point me the right way?
>
> 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.
>
> This will do the job, but I suspect there's a more built-in way:
>
> (define data '(("foo") ("bar") ("baz") ("jaz") ("quux") ("glug")))
racket/sequence provides an `in-slice` sequence to use with `for` forms:
https://docs.racket-lang.org/reference/sequences.html#%28def._%28%28lib._racket%2Fsequence..rkt%29._in-slice%29%29
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Re: [racket-users] Iterating through a list more than one at a time
On Wed, Aug 10, 2016 at 5:50 AM, Greg Hendershott wrote:
> > I feel like there should be a simpler way to do this, but I haven't found
> > one despite much Googling and reading of docs about "for" and its
> siblings.
> > Can someone point me the right way?
> >
> > 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.
> >
> > This will do the job, but I suspect there's a more built-in way:
> >
> > (define data '(("foo") ("bar") ("baz") ("jaz") ("quux") ("glug")))
>
> racket/sequence provides an `in-slice` sequence to use with `for` forms:
>
> https://docs.racket-lang.org/reference/sequences.html#%
> 28def._%28%28lib._racket%2Fsequence..rkt%29._in-slice%29%29
>
in-slice produces lists of N elements, so I guess if I wanted to iterate
over every 2nd element I would use something like this?:
(define data '(("foo") ("bar") ("baz") ("jaz") ("quux") ("glug")))
(for ((x (~> data
in-list
(in-slice _ 2)
(map car
...do something...)
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Re: [racket-users] Iterating through a list more than one at a time
Yes. Also each slice is a sequence. So you could do all the usual
things to it. Including but not limited to using another `for` form,
applying it to afunction, or destructuring it using `match` and
friends.
In these examples I included an extra, "leftover" element to make sure
it was handled:
#lang racket
(define data '(("foo") ("bar") ("baz") ("jaz") ("quux") ("glug")
("EXTRA")))
(for/list ([slice (in-slice 2 data)])
(for/list ([x slice])
x))
(for/list ([slice (in-slice 2 data)])
(apply list slice))
(for/list ([slice (in-slice 2 data)])
(match slice
[(list x y) (list x y)]
[(list x) (list x)]))
All eval to:
'((("foo") ("bar")) (("baz") ("jaz")) (("quux") ("glug")) (("EXTRA")))
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