Python循环不会给出预期的输出

Question

I am trying to find the missing elements between arr1 and arr2 but not sure what's the issue with the code, why it's not working. Please suggest.

def miss2(arr1, arr2):
    arr3=arr1
    for i in arr1:
        # print(i)
        for j in arr2:
            # print(i,j)
            if i == j:
                arr3.remove(j)

    print(arr3)

arr1=[1,2,3,4]
arr2=[1,2]

miss2(arr1,arr2)

result: [2, 3, 4] instead of [3, 4]

Answer 1

Objects in Python are stored by reference ,which means you didn't assign the value of arr1 to arr3 , but a pointer to the object.You can use is operator to test if two objects have the same address in memory.

Sequences can be copied by slicing so you can use this to copy a list:

arr3 = arr1[:]

Also you can use

arr3 = list(arr1)

Or you can use copy() module:

from copy import copy
arr3 = copy(arr1)

By the way,you can try this:

print [i for i in arr1 if i not in arr2]

Answer 2

McGrady is right. Here is an article tells you more stuff about the reference problem. Is Python call-by-value or call-by-reference?

And you can use "set"(consider the math concept) data structure instead of "list", here:

x = set([1,2,3,4])
y = set([1,2])
x - y