Stacks_PhPP_4
Below is function written for Stack ADT , the Stack ADT is implemented using a Python's list.
Which function is this?
Assume Empty is implemented.

def mystery(self): if self.is_empty(): raise Empty('Stack is empty') return self._data[-1]

Below is the STACK ADTs' constructor:
class ArrayStack: """LIFO Stack implementation using a Python list as underlying storage.""" def __init__(self): """Create an empty stack.""" self._data = [] # nonpublic list instance

Question

Stacks_PhPP_4
Below is function written for Stack ADT ,  the Stack ADT is implemented using a Python's list.
Which function is this? 
Assume Empty is implemented.

def mystery(self):       if self.is_empty():        raise Empty('Stack is empty')    return self._data[-1]

Below is the STACK ADTs' constructor:
class ArrayStack:    """LIFO Stack implementation using a Python list as underlying storage."""    def __init__(self):        """Create an empty stack."""        self._data = []  # nonpublic list instance

BlackTom AI · Accepted Answer

To begin, let's understand what mystery(self) is doing. It first checks if the stack is empty, and raises an Empty exception in that case. If not empty, it returns the last element of the underlying list, self._data[-1], which corresponds to the top of the stack in a LIFO structure since the end of the list is treated as the top.

Option 1: pop
- This option suggests the function would remove and return the top element. In a typical pop implementation, you would call something like self._data.pop() to remove and return the last item. However, mystery does not remove the element; it merely accesses and returns it. Therefore, pop is not the correct match for this function.

Option 2: top
- This aligns with the observed behavior: a function that returns the top element without removing it. The code returns self._data[-1] after confirming the stack isn't empty, which is exactly the top element. This is a strong candidate for what mystery represents.

Option 3: push
- Push would add a new element to the top of the stack, typically via self._data.append(x). That operation does not occur in mystery; there is no insertion happening, only a read of the current top value. Thus, push is incorrect for this function.

Option 4: return self._data[-1]
- While this line precisely reflects the action of returning the top element, the question asks which function is being defined. 'return self._data[-1]’ is a statement inside the function, not the function’s name. Selecting this would be naming the implementation detail rather than the function itself, making it misleading as an answer to which function is defined. Hence it is not the correct choice for the function name, even though it mirrors the behavior.

In summary, the behavior described in mystery corresponds to a function that reports the top element without removing it, which is best described by top. The other options describe different stack operations or misinterpret the role of the return statement within the function.

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Which of the following Python data structures can be used to implement a stack efficiently?

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Stacks_2 See the Stack ADT implemented using a Python's List, this ADT was discussed in lectures. The pop method's amortized run time complexity is O(____).

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