Python Coding Challenge - Question with Answer (ID -050126)

 

Step-by-step explanation

1. Initial list

arr = [1, 2, 3]

arr is a list with three integers.

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2. Loop execution

for i in arr: 
    i = i * 2

• The loop goes through each element of arr.

• i is a temporary variable that receives the value of each element — not the reference to the list element.

Iteration by iteration:

Iteration	i before	i = i * 2	arr
1	1	2	[1, 2, 3]
2	2	4	[1, 2, 3]
3	3	6	[1, 2, 3]

👉 i changes, but arr does not change.

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3. Final print

print(arr)

Since the list was never modified, the output is:

[1, 2, 3]

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Why doesn’t the list change?

Because:

• i is a copy of the value, not the element inside the list.

• Reassigning i does not update arr.

This is equivalent to:

x = 1
x = x * 2 # changes x only, not the original source

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✅ Correct way to modify the list

If you want to update the list, use the index:

arr = [1, 2, 3]
for idx in range(len(arr)):
    arr[idx] = arr[idx] * 2
print(arr)

Output:

[2, 4, 6]

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Key takeaway

Looping as for i in arr gives you values — not positions.
To change the list, loop over indices or use list comprehension.

Example:

arr = [x * 2 for x in arr]

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Book:  Probability and Statistics using Python

Summary

Code	Modifies list?
for i in arr: i *= 2	❌ No
for i in range(len(arr))	✅ Yes
arr = [x*2 for x in arr]	✅ Yes