Weird while loop causes infinite loop

Question

This is a small code snippet which is causing my program to crash due to an infinite loop

while not stack.is_empty():
    if operators[stack.peek()] >= operators[character]:
        result += stack.pop()

where stack is a Stack object and operators is a dictionary. The below code however does not cause an infinite loop

while not stack.is_empty() and operators[stack.peek()] >= operators[character]:
    result += stack.pop()

My question is: Aren't these code snippets basically the same thing ? Why is one causing an infinite loop while the other isn't?

Thanks

Rohan Jhunjhunwala · Accepted Answer · 2015-09-07 11:44:37Z

3

The first one keeps looping through and peeking at the stack and checking whether a condition is true, but continuing. In the second one it cuts off after the condition is false

answered Sep 7, 2015 at 11:44

Rohan Jhunjhunwala

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1 Comment

xtofl Over a year ago

...which means you could make them equivalent by adding an else: break in the first one.

Barun · Accepted Answer · 2015-09-07 12:06:38Z

2

while not stack.is_empty():
    if operators[stack.peek()] >= operators[character]:
        result += stack.pop()

Here, the while loop runs until stack is empty and you pop only for >=, it leaves some elements in the stack for < condition. So in order for the stack to become empty, stack.size() == number of times operators[stack.peek()] >= operators[character] is true

while not stack.is_empty() and operators[stack.peek()] `>=` operators[character]:
    result += stack.pop()

Here, you are restricting the while loop by only allowing it to continue only when >= condition is statisfied along with stack not empty.

answered Sep 7, 2015 at 12:06

Barun

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