Sometimes my set comes out ordered and sometimes not (Python)

问题

So I know that a set is supposed to be an unordered list. I am trying to do some coding of my own and ended up with a weird happening. My set will sometimes go in order from 1 - 100 (when using a larger number) and when I use a smaller number it will stay unordered. Why is that?

#Steps:
#1) Take a number value for total random numbers in 1-100
#2) Put those numbers into a set (which will remove duplicates)
#3) Print that set and the total number of random numbers

import random

randomnums = 0

Min = int(1)
Max = int(100)
print('How many random numbers would you like?')
numsneeded = int(input('Please enter a number.            '))
print("\n" * 25)

s = set()

while (randomnums < numsneeded):
    number = random.randint(Min, Max)
    s.add(number)
    randomnums = randomnums + 1

print s
print len(s)

If anyone has any pointers on cleaning up my code I am 100% willing to learn. Thank you for your time!

回答1:

When the documentation for set says it is an unordered collection, it only means that you can assume no specific order on the elements of the set. The set can choose what internal representation it uses to hold the data, and when you ask for the elements, they might come back in any order at all. The fact that they are sorted in some cases might mean that the set has chosen to store your elements in a sorted manner.

The set can make tradeoff decisions between performance and space depending on factors such as the number of elements in the set. For example, it could store small sets in a list, but larger sets in a tree. The most natural way to retrieve elements from a tree is in sorted order, so that's what could be happening for you.

See also Can Python's set absence of ordering be considered random order? for further info about this.

回答2:

Sets are implemented with a hash implementation. The hash of an integer is just the integer. To determine where to put the number in the table the remainder of the integer when divided by the table size is used. The table starts with a size of 8, so the numbers 0 to 7 would be placed in their own slot in order, but 8 would be placed in the 0 slot. If you add the numbers 1 to 4 and 8 into an empty set it will display as:

set([8,1,2,3,4])

What happens when 5 is added is that the table has exceeded 2/3rds full. At that point the table is increased in size to 32. When creating the new table the existing table is repopulated into the new table. Now it displays as:

set([1,2,3,4,5,8])

In your example as long as you've added enough entries to cause the table to have 128 entries, then they will all be placed in the table in their own bins in order. If you've only added enough entries that the table has 32 slots, but you are using numbers up to 100 the items won't necessarily be in order.

来源：https://stackoverflow.com/questions/23122983/sometimes-my-set-comes-out-ordered-and-sometimes-not-python