[Tutor] skip/slice more than every second?

[Steven D'Aprano]

On Tue, Sep 29, 2015 at 01:16:36PM +1000, questions anon wrote:
> a = [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
> 
> how can I show the first then skip three and show the next and so on?
> For example:
> show 1
> then skip 2,3,4
> then show 5
> then skip 6,7,8
> then show 9
> then skip 10,11,12,


Here is one method, using slicing:

py> a = [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
py> a[::4]
[1, 5, 9, 13]


The slice a[::4] is equivalent to a[0:len(a):4], that is, it starts at 
position 0, keeps going until the end, and extracts every fourth value.

Here is another method:


py> it = iter(a)
py> while True:
...     try:
...         print(next(it))
...         who_cares = next(it), next(it), next(it)
...     except StopIteration:
...         break
...
1
5
9
13

This grabs each item in turn, prints the first, then throws away three, 
then prints the next, and so on. This is a good technique for when you 
want something that doesn't fit into the fairly simple patterns 
available using slicing, e.g.:

 - print 1 item;
 - throw away 2;
 - print 3 items;
 - throw away 4;
 - print 5 items;
 - throw away 6;
 - print 7 items;
 - throw away 8; 

etc. 


But for cases where slicing does the job, it is better to use that.



-- 
Steve