How to get the oddly indexed elements in a list in mathematica

Question

How to get the oddly indexed elements in a list? I am thinking of Select, but did not find anything returning an element's position, especially considering there are repetitive elements in the list.

Also in general, how to select those elements whose indices satisfy some certain conditions?

Answer 1

Here's a few more in addition to @belisarius's answer, which don't require computing Length[lis]:

Take[lis, {1, -1, 2}]

lis[[1 ;; -1 ;; 2]]

You can often use -1 to represent the "last" position.

Answer 2

There are a lot of ways, here are some of them:

In[2]:= a = Range[10];le = Length@a;

In[3]:= Table[a[[i]], {i, 1, le, 2}]

In[5]:= Pick[a, Table[Mod[i, 2], {i, 1, le}], 1]

In[6]:= a[[1 ;; le ;; 2]]

In general, with Pick[] (as an example) you can model any conceivable index mask.

Answer 3

For some reason the terse form of Span has been omitted from the answers.

Range[20][[;;;;2]]

{1, 3, 5, 7, 9, 11, 13, 15, 17, 19}

Quoting the documentation:

;;;;k 
from the beginning to the end in steps of k.