在python中打印排序和未排序的列表

Question

I have a code which generates a list of numbers and then sorts it. The sorting function works but I need to print the unsorted list after sorting. That's why I put the print function to print the unsorted but instead it prints the sorted list. I thought I stored the unsorted list by assigning the unsorted list to a variable and then try to print it in the end. Any ideas why it prints the sorted list instead of the unsorted? I didn't put the GenerateNumbers() function as what is does is give the "numbers" variable a list of unsorted numbers.I'm using insertion sort.

def InsertionSort(sort_list):
    print("Sorting numbers...")
    for i in range(0, len(sort_list)-1):
       for j in range(i+1,0,-1):
         if(sort_list[j] < sort_list[j-1]):
             temp = sort_list[j-1]
             sort_list[j-1] = sort_list[j]
             sort_list[j] = temp
             print(sort_list)
          else:
             break
    print("Finished sorting.")
    print()
    return sort_list

numbers = GenerateNumbers()
unsorted = numbers
sort_list = numbers

sorted_numbers = InsertionSort(sort_list)
print("unsorted list:", numbers)
print("sorted list:", sorted_numbers)

Answer 1

You have three assignment commands

numbers = GenerateNumbers()
unsorted = numbers
sort_list = numbers

You think that you have three different copies of the list, but you do not. Since a list is a mutable type, Python has made three different names for the same list. Hence your confusion. Changing the contents one of those names changes them all immediately. You need to make copies of the list. There are several ways to do this. The two easiest ways are

numbers = GenerateNumbers()
unsorted = numbers[:]
sort_list = numbers[:]

and

numbers = GenerateNumbers()
unsorted = list(numbers)
sort_list = list(numbers)

There are other ways, such as deepcopy() , which are useful for more complicated data structures, but one of these two will suffice for this situation.

Answer 2

When you're calling

unsorted = numbers
sort_list = numbers

you're not actually copying the list, you're just copying the reference to the list. unsorder and sort_list both point to the same list in the memory.

To copy the list instead you can do:

sort_list = list(numbers)

Now sort_list points to a new list in the memory and you can change sort_list without changing numbers .

Answer 3

Your function sorts the list in place and hence sorted_numbers is sort_list , further, the assignment sort_list = numbers is simply a renaming, it does not create a new list and hence sort_list will be "the same" as "unsorted". Thus, when you sort sort_list , unsorted will also become sorted.

You can solve this by calling list on the numbers, which will cause python to create a copy.

numbers = GenerateNumbers()
unsorted = list(numbers)  # creates a copy
sort_list = numbers

sorted_numbers = InsertionSort(sort_list)
print("unsorted list:", numbers)
print("sorted list:", sorted_numbers)