CBSE 2025, Set 4, Q30 (b)

Question

Write the following user-defined functions in Python :

**(i) `push_trail(N, myStack)` :** Here `N` and `myStack` are lists, and `myStack` represents a stack. The function should push the last 5 elements from the list `N` onto the stack `myStack`. For example, if the list `N` is `[1, 2, 3, 4, 5, 6, 7]`, then the function `push_trail()` should push the elements `3, 4, 5, 6, 7` onto the stack. Therefore the value of stack will be `[3, 4, 5, 6, 7]`. Assume that `N` contains at least 5 elements.

**(ii) `pop_one(myStack)` :** The function should pop an element from the stack `myStack`, and return this element. If the stack is empty, then the function should display the message `'Stack Underflow'`, and return `None`.

**(iii) `display_all(myStack)` :** The function should display all the elements of the stack `myStack`, without deleting them. If the stack is empty, the function should display the message `'Empty Stack'`.

Edvaya · Accepted Answer

The correct answer is Subjective. Here is the complete Python implementation for the required stack functions: ```python # (i) Function to push the last 5 elements of list N onto myStack def push_trail(N, myStack): # N[-5:] slices the last 5 elements from the list N for item in N[-5:]: myStack.append(item) # (ii) Function to pop an element or handle underflow def pop_one(myStack): if len(myStack) == 0: print("Stack Underflow") return None else: return myStack.pop() # (iii) Function to display all stack elements without deleting them def display_all(myStack): if len(myStack) == 0: print("Empty Stack") else: # Displaying from Top to Bottom (LIFO visual representation) for i in range(len(myStack) - 1, -1, -1): print(myStack[i]) # --- Example Usage (Not required in the exam, but helpful for testing) --- # N = [1, 2, 3, 4, 5, 6, 7] # myStack = [] # push_trail(N, myStack) # display_all(myStack) # print("Popped:", pop_one(myStack)) ``` **Explanation:** 1. **`push_trail`:** We utilize Python list slicing `N[-5:]` which effectively grabs the last 5 elements of the list `N`. We then iterate over this slice and use `.append()` to push each element to the top of `myStack`. 2. **`pop_one`:** We first verify if the list is empty by checking its length. If it is `0`, we print the exact required string `"Stack Underflow"` and `return None`. Otherwise, we just call the built-in `.pop()` method which inherently removes and returns the top element. 3. **`display_all`:** After verifying the stack isn't empty, a `for` loop with `range(len(myStack) - 1, -1, -1)` is used to iterate through the stack backwards. This accurately displays the stack elements from Top to Bottom without actually removing (popping) them from memory.

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Computer Science > Computational Thinking And Programming 2 > Data Structures

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