Re: Re: [Q] {a,b,c}->{{a,b},{b,c}}
- To: mathgroup at smc.vnet.net
- Subject: [mg2632] Re: [mg2600] Re: [Q] {a,b,c}->{{a,b},{b,c}}
- From: Allan Hayes <hay at haystack.demon.co.uk>
- Date: Thu, 30 Nov 1995 21:00:55 -0500
In [mg2600] Re: [Q] {a,b,c}->{{a,b},{b,c}}
"Bart van der Zwet" <B.W.v.d.Zwet at stud.tue.nl>
gave a list of responses and timings (attached).
Here are two variants that are slightly quicker than the fastest he
reports.The main improvement comes from using Thread instead of
Transpose.
lst = Range[10^4];
Timing[Do[Transpose[{Drop[lst,-1],Drop[lst,1]}],{10}]][[1]]
7.8 Second
Timing[Do[Thread[{Drop[lst,-1],Drop[lst,1]}],{10}]][[1]]
6.53333 Second
Timing[Do[Thread[{Drop[lst,-1],Rest[lst]}],{10}]][[1]]
6.5 Second
Allan Hayes
hay at haystack.demon.co.uk
Begin forwarded message:
>From: "Bart van der Zwet" <B.W.v.d.Zwet at stud.tue.nl>
>Subject: [mg2600] Re: [Q] {a,b,c}->{{a,b},{b,c}}
Hello Mma-users,
I while ago I posted a question here.
I wanted to know how I could make {{a,b},{b,c}} out of {a,b,c} for
instance,
but then for any list.
I received a number of different solutions that I shall mention in this
mail. I've let them all solve the problem for the list : Range[10^4] and
I include the timing to each solution.
1:
Partition[list,2,1]
2.64 Second
2:
Map[ Module[ {qux = absolutelyNothing},
(If[ qux === absolutelyNothing,
qux = #; Sequence @@ {},
{qux, qux = #} ])& ], list ]
9.51 Second
3:
Transpose[ { Drop[ list, -1 ], Drop[ list, 1 ] } ]
1.81 Second
4:
r[ {x__} ] := Map[ Pattern[ #, Blank[] ]&, {x} ] ->
Table[ { {x}[[i]], {x}[[i+1]] }, {i, 1, Length[ {x} ]
- 1 } ]
x /. r[x]
33.94 Second
P.S.
Not all responses are included, some didn't do exactly what I wanted or I
couldn't get the answer with them. Some minor changes have been made in a
solution.
P.P.S.
Thank you all for replying to my question, it's been of great help.
Yours,
Bart van der Zwet
Student Mathematics
Eindhoven University of Technology
The Netherlands
e-mail : B.W.v.d.Zwet at stud.tue.nl