[Tutor] Rearranging a list

[Peter Otten]

Ken G. wrote:

> Assuming I have the following list and code how do I best be able
> rearrange the list as stated below:
> 
> list = [0, 0, 21, 35, 19, 42]
> 
> Using print list[2:6] resulted in the following:
> 
> 2    21
> 3    35
> 4    19
> 5    42
> 
> I would like to rearrange the list as follow:
> 
> 5    42
> 3    35
> 2    21
> 4    19
> 
> I tried using list.reverse() and print list[0,6] and it resulted in:
> 
> [42, 19, 35, 21, 0, 0] or as printed:
> 
> 0    42
> 1    19
> 2    35
> 3    21
> 4    0
> 5    0
> 
> In another words, I would desire to show that:
> 
> 5 appears 42 times
> 3 appears 35 times
> 2 appears 21 times
> 4 appears 19 times
> 
> but then I lose my original index of the numbers by reversing. How do I
> best keep the original index number to the rearrange numbers within a
> list?

Don't rearrange the original list, make a new one instead:

>>> frequencies = [0, 0, 21, 35, 19, 42]
>>> wanted_indices = [5, 3, 2, 4]
>>> [frequencies[i] for i in wanted_indices]
[42, 35, 21, 19]

Or look up the values as you print them:

>>> for i in wanted_indices:
...     print(i, "appears", frequencies[i], "times")
... 
5 appears 42 times
3 appears 35 times
2 appears 21 times
4 appears 19 times

The expression [frequencies[i] for i in wanted_indices] is called "list 
comprehension" and is a shortcut for a loop:

>>> result = []
>>> for i in wanted_indices:
...     result.append(frequencies[i])
... 
>>> result
[42, 35, 21, 19]