unsort a list to get it back the way it was-CodePudding

I want to unsort a list. For you to understand:

list1 = ["Hi", "Whats up this Morning" "Hello", "Good Morning"]

new_list = sorted(list1, key=len, reverse=True)

["Whats up this Morning", "Good Morning", "Hello", "Hi"]

And know it should go back exactly in the same way it was in the beginning

["Hi", "Whats up this Morning", "Hello", "Good Morning"]

Anyone knows the answer?

CodePudding user response：

Getting to it right out of the gate, list1 never changes, so as long as you retain this object in memory you will always have a way to refer to the original list, since it's unchanged. new_list is a new object, and the absolute simplest thing you can do is keep both these objects and refer to them at will.

Taking your question from a conceptual standpoint: is there a way to keep track of the original order based on the new object? If you absolutely needed to revert to the original order based only on the new_list, you could first index the list using enumerate(). This tracks the order of your object like so: (0, 'Hi') (1, 'Whats up this Morning') (2, 'Hello') (3, 'Good Morning'). Then, you can sort by the length of the second object in each tuple (the original string) with new_list = sorted(enumerate(list1), key=lambda x: len(x[1]), reverse=True). Finally, you can extract your original list (as a tuple) with words, indices = zip(*new_list). Then reversing back to the string is quite simple, see the below example:

list1 = ["Hi", "Whats up this Morning", "Hello", "Good Morning"]
# Pack indices and sort
new_list = sorted(enumerate(list1), key=lambda x: len(x[1]), reverse=True)

# Unpack sorted strings and old indices
indices, words = zip(*new_list)
print(words)  # ('Whats up this Morning', 'Good Morning', 'Hello', 'Hi')
print(indices)  # (1, 3, 2, 0)

# Reverse the sorting based on the old indices
original_list_packed = sorted(zip(indices, words))
print(original_list_packed)  # [(0, 'Hi'), (1, 'Whats up this Morning'), (2, 'Hello'), (3, 'Good Morning')]
_, original_list = zip(*original_list_packed)
print(original_list)  # ('Hi', 'Whats up this Morning', 'Hello', 'Good Morning')

CodePudding user response：

Our list is:

list1 = ["Hi", "Whats up this Morning" "Hello", "Good Morning"]

And the new list is:

new_list = sorted(list1, key=len, reverse=True)

We sort list1and store original keys of new_list

list_keys = sorted(range(len(list1)), key=lambda k: list1[k], reverse=True)

>>> [1, 0, 2]

Ant get back the fist list:

list3 = [None] * len(list1)
for indx in range(len(list_keys)):
    list3[indx] = list1[indx]

Output

>>> list3
>>> ['Hi', 'Whats up this MorningHello', 'Good Morning']