python 3 : values of a list change when it enters a for loop

Question

Challenge : to find the minimum and maximum sums which can be obtained out of four elements from the list containing 5 elements.

Approach followed : Sort the list in descending and ascending order and store them into two different variables. Finding the sum of first 4 elements in both of the lists. One sum would be the minimum most and the second one would be the maximum most.

Code :

arr = [2,1,3,4,5]
arr.sort()
asc = arr
print(asc[0],asc[1],asc[2],asc[3])
arr.sort(reverse = True)
des = arr
print(des[0],des[1],des[2],des[3])
maxi = 0
mini = 0
for j in range(4) :
    mini = mini + asc[j]
    print(mini, asc[j])
    maxi = maxi + des[j]
    print(maxi,des[j])
print(mini, maxi)

Few print statements are introduced here for debugging purpose. As visible in the code, bot the sorted versions are printed before entering into the for loop and after entering into the loop. Its clearly visible as seen in the output that, the list which should be holding the elements in ascending order is having the elements in the descending order.

Output :

11 12 13 14 - list in the ascending order
15 14 13 12 - list in the descending order

15 15 - round 0
15 15

29 14 - round 1
29 14

42 13 - round 2
42 13

54 12 - round 3
54 12

54 54 - final output

why the elements present in one particular list change their order when they enter into the for loop ??

see this [link](https://stackoverflow.com/questions/17246693/what-exactly-is-the-difference-between-shallow-copy-deepcopy-and-normal-assignm) — Rohit-Pandey, Dec 29 '18 at 07:57

Sergey Pugach · Answer 1 · 2018-12-29T07:57:33.873

When you are doing asc = arr and des = arr a new list is not created. asc, des and arr are linked to one list object so whey you change any of them all variables will be changed as it's single object.

In [1]: a = [1, 2]

In [2]: b = a

In [3]: id(a), id(b)
Out[3]: (140601802913048, 140601802913048)

In [4]: b = a[:]

In [5]: id(a), id(b)
Out[5]: (140601802913048, 140601819243872)

If you want to have a copy of list do

asc = arr[:]
des = arr[:]

Austin · Answer 2 · 2018-12-29T07:41:32.810

You need to do:

asc = arr.copy()
# or asc = arr[:]

Or else, when arr is sorted reverse, asc also changes. asc is a pointer to the array and when arr changes, asc changes. Better, you create a copy of arr, so changes won't reflect back.

The whole code you have written can be condensed into these two lines:

arr = [2,1,3,4,5]

print(sum(sorted(arr)[:4]))  # 10
print(sum(sorted(arr, reverse=True)[:4]))  # 14

# Or print(sum(sorted(arr)[-4:])) instead of the last print.

python 3 : values of a list change when it enters a for loop

2 Answers2