python - Infinite Loop When Iterating Over a List - Stack Overflow

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Explanation of the Main Problem

Problem Details: The goal is to iterate through a list of numbers and print only positive numbers. However, the provided code enters an infinite loop and does not terminate, even after processing all the elements. The problematic code is as follows:

numbers = [5, -3, 7, -1, 10]
i = 0

while i > len(numbers):  # Incorrect loop condition
    if numbers[i] < 0:
        print(f"Skipping negative number: {numbers[i]}")
    else:
        print(f"Processing positive number: {numbers[i]}")

Attempts to Solve the Problem:

• Verified that the list contains both positive and negative numbers.

• Checked the loop condition to ensure it avoids running indefinitely.

• Tried adding a break statement or modifying the list, though these changes do not address the core issue.

Expected Behavior:

• The loop iterates through the list of numbers.

• Skips negative numbers while processing positive numbers.

• Terminates after processing all elements in the list.

Explanation of the Main Problem

Problem Details: The goal is to iterate through a list of numbers and print only positive numbers. However, the provided code enters an infinite loop and does not terminate, even after processing all the elements. The problematic code is as follows:

numbers = [5, -3, 7, -1, 10]
i = 0

while i > len(numbers):  # Incorrect loop condition
    if numbers[i] < 0:
        print(f"Skipping negative number: {numbers[i]}")
    else:
        print(f"Processing positive number: {numbers[i]}")

Attempts to Solve the Problem:

• Verified that the list contains both positive and negative numbers.

• Checked the loop condition to ensure it avoids running indefinitely.

• Tried adding a break statement or modifying the list, though these changes do not address the core issue.

Expected Behavior:

• The loop iterates through the list of numbers.

• Skips negative numbers while processing positive numbers.

• Terminates after processing all elements in the list.

• python
• loops
• while-loop
• iteration

Share Improve this question Follow edited Nov 19, 2024 at 22:09 Bergi 666k161161 gold badges1k1k silver badges1.5k1.5k bronze badges asked Nov 19, 2024 at 21:09 user28382400user28382400 311 silver badge22 bronze badges 2

• Unless your assignment requires the use of a while loop specifically, you would never actually write code like this. Iterate over the list with a for loop: for x in numbers: if x < 0: ... else : .... – chepner Commented Nov 19, 2024 at 22:11
• What makes you think the loop is infinite? What is the actual output you're getting? – Barmar Commented Nov 19, 2024 at 22:29

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2 Answers 2

Sorted by: Reset to default 2

The loop isn't technically infinite because it's never entered in the first place. i is 0 and len(numbers) is 5. Since 0 is not greater than 5, the condition is never met and the loop is never entered.

As an aside... You'd know if the loop was infinite because execution of the program would never stop. Since execution immediately stops, that's usually an indicator that the execution is not infinitely continuing. Using a debugger is also a great way to observe such behaviors.

Flip the condition:

while i < len(numbers):

Now it's an infinite loop, because a loop will continue as long as the condition is true. And in this case, when it is true, under what circumstances would it ever be false? i is never modified in the loop. Update the value of i in the loop:

i += 1

Problem Explanation:

The code you wrote is meant to go through a list of numbers and print only the positive ones. However, it's not working correctly. The main problem is with the loop condition:

while i > len(numbers):

This condition is wrong because the loop won't start at all. Here's why: you start with i as 0, but 0 is not greater than the length of the list (which is 5), so the loop never runs. This causes the program to act strangely or just stop.

What You Need to Fix:

1. Change the loop condition: The condition should check if i is still within the list’s boundaries (i.e., if i is less than the length of the list). This way, the loop can properly go through each number in the list.

2. Add an index increment: You also need to make sure that i increases after each iteration, so the loop moves to the next number. Without this, the loop would keep processing the same number forever.

Here’s the Updated Code

numbers = [5, -3, 7, -1, 10]
i = 0

while i < len(numbers):  # The loop runs as long as 'i' is within the list
    if numbers[i] < 0:
        print(f"Skipping negative number: {numbers[i]}")
    else:
        print(f"Processing positive number: {numbers[i]}")
    i += 1  # Move to the next number

What’s Happening:

• The loop checks each number in the list, one at a time.
• If the number is negative, it skips it and shows a message.
• If the number is positive, it processes it and shows a message.
• Once all numbers are checked, the loop stops.

Output:

Processing positive number: 5
Skipping negative number: -3
Processing positive number: 7
Skipping negative number: -1
Processing positive number: 10

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