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Sunday Okechukwu
720 PointsModifying a list in reverse order
Guys I want to know why the code below could not modify the list in reverse order I am getting [8, 7, 6, 5, 5, 6, 7, 8] when I run the code instead of [8,7,6,5,4,3,2,1]. Please kindly assist
x = [1, 2, 3, 4, 5, 6, 7, 8]
for num in range(len(x)):
x[num] = x[-(num + 1)]
print(x)
[MOD: added ```python formatting -cf]
1 Answer
Chris Freeman
Treehouse Moderator 68,426 PointsHey Sunday Okechukwu,, good question! The issue centers on the list x being modified in-place. By the time the loop gets to the later values, the original numbers have been overwritten. Here is a modified execution of your code:
>>> x = list(range(1, 9))
>>> x
[1, 2, 3, 4, 5, 6, 7, 8]
>>> for num in range(len(x)):
... print(f"setting x[{num}] equal to x[{-(num + 1)}]")
... x[num] = x[-(num + 1)]
... print(f"new x: {x}")
...
setting x[0] equal to x[-1]
new x: [8, 2, 3, 4, 5, 6, 7, 8]
setting x[1] equal to x[-2]
new x: [8, 7, 3, 4, 5, 6, 7, 8]
setting x[2] equal to x[-3]
new x: [8, 7, 6, 4, 5, 6, 7, 8]
setting x[3] equal to x[-4]
new x: [8, 7, 6, 5, 5, 6, 7, 8]
setting x[4] equal to x[-5]
new x: [8, 7, 6, 5, 5, 6, 7, 8]
setting x[5] equal to x[-6]
new x: [8, 7, 6, 5, 5, 6, 7, 8]
setting x[6] equal to x[-7]
new x: [8, 7, 6, 5, 5, 6, 7, 8]
setting x[7] equal to x[-8]
new x: [8, 7, 6, 5, 5, 6, 7, 8]
Notice how after x is 3, the values with x do not change since the list is now symmetrical.
There are two ways around this. Use a copy of the list to hold the original values:
>>> x = list(range(1, 9))
>>> x
[1, 2, 3, 4, 5, 6, 7, 8]
>>> for idx, num in enumerate(x[:], start=1):
... print(f"setting x[{-idx}] equal to {num}")
... x[-idx] = num
... print(f"new x: {x}")
...
setting x[-1] equal to 1
new x: [1, 2, 3, 4, 5, 6, 7, 1]
setting x[-2] equal to 2
new x: [1, 2, 3, 4, 5, 6, 2, 1]
setting x[-3] equal to 3
new x: [1, 2, 3, 4, 5, 3, 2, 1]
setting x[-4] equal to 4
new x: [1, 2, 3, 4, 4, 3, 2, 1]
setting x[-5] equal to 5
new x: [1, 2, 3, 5, 4, 3, 2, 1]
setting x[-6] equal to 6
new x: [1, 2, 6, 5, 4, 3, 2, 1]
setting x[-7] equal to 7
new x: [1, 7, 6, 5, 4, 3, 2, 1]
setting x[-8] equal to 8
new x: [8, 7, 6, 5, 4, 3, 2, 1]
Or, you may use a slice to directly invert a list using a negative step:
>>> x = list(range(1, 9))
>>> x
[1, 2, 3, 4, 5, 6, 7, 8]
>>> x = x[::-1]
>>> x
[8, 7, 6, 5, 4, 3, 2, 1]
Post back if you need more help. Good luck!!