Why is the loop running infinitely?

Question

In the following code snippet, I intended to exit the while loop when the value of m becomes 0 or less than 0. But my while loop is still running even after the value of m finally decrements to 0. What is the reason for this infinite loop? Does it have anything to do with my else statement? If it does, why so?

NOTE: The reason I need the while loop here is because I need the for loop to run afresh when it terminates but the value of m is greater than 0.

Here is my code:

n, m = input().split()
n = int(n)
m = int(m)

while m>=0:
    for i in range(n+1):
        if m >= i:
            m -= i
        else:
            break
   
        
print(m)

"while loop is still running even after the value of `m` finally decrements to 0" - this makes sense because the code will loop `while` `m` is greater _or equal_ to zero. — ForceBru, Jun 13 '22 at 16:15
As long as the initial value of `m` is >= 0, the loop will never end. The inner loop will never make `m` negative, so the outer loop will never exit. Note that the `break` statement only applies to the inner loop. — Tom Karzes, Jun 13 '22 at 16:18
@ForceBru you are very right. But now that I have changed the condition to while m<0, the program is not even entering the inner for loop. Why is that? — JackPot, Jun 13 '22 at 16:20
See https://stackoverflow.com/q/189645/6744133 for breaking out of the outer while loop — Oli, Jun 13 '22 at 16:20
@JackPot, "I have changed the condition to `while m<0`" - why? This condition is the opposite of what you have in the post, so it makes sense for it to do the opposite of what the original code did... — ForceBru, Jun 13 '22 at 16:25
What are you really attempting to do here? It seems like you want to repeatedly decrement `m` so long as it's larger than `i`, at which point you want to reset `i` to 0 and keep reducing `m` until it hits zero. But to what end? Because this feels very much like you're trying to do something simple in a complicated. What is your expected input/output for these numbers? — Nathaniel Ford, Jun 13 '22 at 16:25
@Nathaniel Ford https://codeforces.com/problemset/problem/92/A I am attempting to solve this problem from codeforces. — JackPot, Jun 13 '22 at 16:26

Nathaniel Ford · Accepted Answer · 2022-06-13T16:41:00.303

This is solved more clearly by recursion. Imagine a function f that determines how many chips are left to the presenter:

def f(chips: int, walruses: int, position: int = 1) -> int:
    # Base case, there are fewer chips than the current position number
    if chips < position:
        return chips
    else:  # Iterative case; decrement chips and recalculate position
        chips = chips - position
        position = position + 1 if position < walruses else 1
        return f(chips, walruses, position)

walrus_count, starting_chips = input().split()
print(f(int(starting_chips), int(walrus_count)))

That said, if you really want to use a while loop:

def f(chips: int, walruses:int) -> int:
    position = 1
    # Quit if we don't have enough chips
    while chips > position:
        # Decrement chips
        chips = chips - position
        # Reset position if we've gone around the circle
        position = 1 if position >= walruses else position + 1
    return chips

The problem you are having is that you're nesting loops.

while m >= 0: # Loop until you run out of chips -> but that is not what the problem calls for
    for i in range(0, n+1):  # i is position, and you're looking to loop once you run out of walruses. Alas, you're zero-indexing here!
        if m >= i:  # Check to see if there are enough chips, which is a half-overlap of the outer loop
            m -= i  # Decrement chips, correct
        else:
            break  # Breaks out of for-loop, but not while-loop
     # No extra check here to see if you need to break out of while loop. Need to add one, or fix the while loop check.

You need to convert the input to `int`. – Barmar Jun 13 '22 at 16:35 — Barmar, Jun 13 '22 at 16:35

Why is the loop running infinitely?

1 Answers1