[Tutor] Rearranging a list

[Peter Otten]

Peter Otten wrote:

> 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]

Oops, it occurs to me that you may want the most frequent indices rather 
than specific indices. You can achieve that with

>>> def lookup_frequency(index):
...     return frequencies[index]
... 
>>> wanted_indices = sorted(range(len(frequencies)), key=lookup_frequency, 
reverse=True)
>>> wanted_indices
[5, 3, 2, 4, 0, 1]
>>> wanted_indices[:4]
[5, 3, 2, 4]

You may also want to look at the collections.Counter class:

>>> import collections
>>> c = collections.Counter()
>>> for i, freq in enumerate(frequencies):
...     c[i] = freq
... 
>>> c.most_common(4)
[(5, 42), (3, 35), (2, 21), (4, 19)]
>> for key, freq in c.most_common(4):
...     print(key, "appears", freq, "times")
... 
5 appears 42 times
3 appears 35 times
2 appears 21 times
4 appears 19 times