On 2017-03-08 17:03, Alan Gauld via Tutor wrote:
On 08/03/17 19:56, Sri Kavi wrote:
It’s about making a function that returns a list of lists, with each
list
being all of the elements that are the same as another element in the
original list.
This is one of those problems where there is probably no
definitive "correct" answer just different compromises.
My first reaction was to sort the initial list then iterate
over it creating a new sublist for each change of value.
But, it only works for homogenous lists. For mixed types
I'd probably opt for a dictionary:
def f(lst):
res = {}
for item in lst:
res.setdefault(item,[]).append(item)
return list(res.values())
But I've no idea how that performs compared to your methods.
The above works BUT
how????
Things like this can usually be broken down into their component parts
but I've been unsuccessful doing so:
def f(lst):
res = {}
for item in lst:
method_res = res.setdefault(item, [])
res.method_res.append(item)
# res.setdefault(item,[]).append(item)
return list(res.values())
res.method_res.append(item)
AttributeError: 'dict' object has no attribute 'method_res'
Python must be doing some trickery behind the scenes.
The algorithm I came up with is:
def make_sublists_of_duplicates(original_list):
frequency_counter = {}
for item in original_list:
_ = frequency_counter.setdefault(item, 0)
frequency_counter[item] += 1
res = []
for key in frequency_counter:
res.append([key for i in range(frequency_counter[key])])
return res
although I would argue that the creation of the frequency_counter should
be in its own function and then it could be used to create the list of
lists IF required. That would get around the criticism of not being able
to deal with huge input.
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