c++ - What container really mimics std::vector in Haskell? -

-

the problem

i'm looking container used save partial results of n - 1 problems in order calculate nth one. means size of container, @ end, n.

each element, i, of container depends on @ least 2 , 4 previous results.

the container have provide:

  • constant time insertions @ either beginning or end (one of two, not both)
  • constant time indexing in middle

or alternatively (given o(n) initialization):

  • constant time single element edits
  • constant time indexing in middle

what std::vector , why relevant

for of don't know c++, std::vector dynamically sized array. perfect fit problem because able to:

  • reserve space @ construction
  • offer constant time indexing in middle
  • offer constant time insertion @ end (with reserved space)

therefore problem solvable in o(n) complexity, in c++.

why data.vector not std::vector

data.vector, data.array, provide similar functionality std::vector, not quite same. both, of course, offer constant time indexing in middle, offer neither constant time modification ((//) example @ least o(n)) nor constant time insertion @ either beginning of end.

conclusion

what container mimics std::vector in haskell? alternatively, best shot?

from reddit comes suggestion use data.vector.constructn:

o(n) construct vector n elements repeatedly applying generator function constructed part of vector.

 constructn 3 f = let = f <> ; b = f <a> ; c = f <a,b> in f <a,b,c>

for example:

λ import qualified data.vector v λ v.constructn 10 v.length fromlist [0,1,2,3,4,5,6,7,8,9] λ v.constructn 10 $ (1+) . v.sum fromlist [1,2,4,8,16,32,64,128,256,512] λ v.constructn 10 $ \v -> let n = v.length v in if n <= 1 1 else (v v.! (n - 1)) + (v v.! (n - 2)) fromlist [1,1,2,3,5,8,13,21,34,55]

this seems qualify solve problem you've described above.


Comments

Post a Comment

Popular posts from this blog

database - VFP Grid + SQL server 2008 - grid not showing correctly -

-
i having problem, have grid called "gridbasica" , this: thisform.gridbase.recordsource = "sgviemp" sgviemp cursor made executing select sqlserver command sqlexec. when want modify registry, go form , the update there. so, when come form grid, grid isnt grid anymore, big white rectangle. reading around said: delete recordsource , asingt again. thisform.gridbase.recordsource = "" *--i sqlexe here thisform.gridbase.recordsource = "sgviemp" i , works, but, when click button modify again cursos updated modify form doesnt load fields did first time. could me out that? thanks in advance. --------------- edit ------------------ i have been doing recommended: lnselect = select(0) n = sqlexec(thisform.conexion,"select emp_ccodigo codigo, emp_cnombre 'nombre o razon social', emp_cnumrif 'r.i.f' sgviempr","sgviemp1") set echo on suspend select sgviemp zap in sgviemp *-- browse *--zaps correct
Read more

jquery - Set jPicker field to empty value -

-
im using jpicker color selector in form. have clear value set window/icon 'no color selected' state. i've tried following: $(':input[name="color"]').val(''); //access value directly. (does not affect jpicker) 2: $.jpicker.list[0].color.active.val('hex', '', this); //set color black , input value 000000, instead of empty string 3: $.jpicker.list[0].color.active.val(null, '', this); //does nothing none of 3 methods seems behave properly. in advance. this works me :) $.jpicker.list[0].color.active.val('v', null); here example: http://jsfiddle.net/3up7n/
Read more

.htaccess - htaccess convert request to clean url and add slash at the end of the url -

-
ok , searched more 4 days , more 20 posts here in stach or in websites without kind of success first here htaccess file rewriteengine on rewriterule ^([a-za-z0-9_]+)/?$ index.php?page=$1 [nc] rewriterule ^([a-za-z0-9_]+)\/([a-za-z0-9_]+)\/?$ index.php?page=$1&action=$2 [nc] i tried many codes 1 worked me can call request , works me www.example.com/pagename => www.example.com/index.php?page=pagename here questions , please provide me information "in detail" understand code well 1- how convert request automatically www.example.com/index.php?page=pagename www.example.com/pagename 2- if tried add slash @ end shows me page without images or styling mean www.example.com/pagename => works www.example.com/pagename/ => doesn't work that's :) for first part need 301 rule this: rewritecond %{the_request} /index\.php\?page=([^\s&]+) [nc] rewriterule ^ /%1? [r=301,l] for second part use absolute path in css, js, images files r
Read more