What is the difference between multiplying an array and looping to create one?-CodePudding

For some reason when I do the second one it works the way I want to, but when I used the first one and I try to append it appends to all lists. Specifically the line at the bottom is where the difference happens.

count = [[]] * (len(nums)  1)
count = [[] for i in range(len(nums) 1)]

        for key, val in myMap.items():
            count[val].append(key)

CodePudding user response：

With the first option, all the sublists, are pointing to the same list, because you are "cloning" the reference. So if you modify one, all will be modified:

count = [[]] * (3)
print(count)  # [[], [], []]
count[0].append(1)
print(count)  # [[1], [1], [1]]

Otherwise, making list comprenhension (looping), will create diferent sublists, because you are instantiating a new list in each iteration. So if you modify one, only that will be modified:

count = [[] for i in range(3)]
print(count)  # [[], [], []]
count[0].append(1)
print(count)  # [[1], [], []]

CodePudding user response：

First version is simply multiplying same object. There are no n different lists here. All are the same list.

count = [[]] * (len(nums)  1)

print(count)

[[], [], [], []]

We can easily see this by printing these lists` id. They are exactly same. So if you append new element let's say on the first list, it'll appear in others too. Because they are same.

print(f"first - {id(count[0])}")
print(f"second = {id(count[1])}")

count[0].append(1)
print(count)

first - 4310957440
second -4310957440
[[1], [1], [1], [1]]

On the second version, you generate new lists and they are different. You can check yourself by printing the ID values.

count = [[] for i in range(3 1)]

print(count)

[[], [], [], []]

print(f"first - {id(count[0])}")
print(f"second = {id(count[1])}")

first - 4310956160
second -4310940416