How to get every other item in a list

Question

I have a list in Mathematica, and I am trying to obtain every other number in the list and store it as a new list.

Currently I have

ReadList["file",Number]

which reads out the entire list, { x1, x2, x3, x4, ... }; I just want to pick out every other number and store it in a new list, e.g. { x1, x3, x5, ... }.

How to do that?

Answer 1

Try:

  yourlist = {a, b, c, d, e, f, g, h};
 (* use Span: search for  Span or ;; in Documentation Center *)
 everyotheritemlist = yourlist[[1 ;; -1 ;; 2]];
 (* or use Take *)
 Take[yourlist, {1, -1, 2}]

Both give:

 {a,c,e,g}    

Answer 2

For tasks like that there are always dozens of creative ways to do it in Mathematica. kguler already gave you the canonical ways, but here's another one:

Partition[yourlist, 2]\[Transpose][[1]]

(*
==> {a, c, e, g}
*)

By the way: There's a dedicated Mathematica Stackexchange site at https://mathematica.stackexchange.com/. The Mathematica community is more and more moving in that direction, so you may want to join us there as well.

Answer 3

One more way:

First /@ ReadList["test.dat", {Number, Number}]