(a) A stack named FruitStack, implemented using list, contains records of some fruits. Each record is represented as a dictionary with keys Name', Origin', Price', and Expiry'. A sample record is given here :
``python
{'Name':'Apple','Origin':'France','Price':120,
'Expiry':'12-08-2025'}
`
Write the following user-defined functions in Python to perform the specified operations on FruitStack :
(i) push_fruit(FruitStack, Fruit): This function takes the stack FruitStack and a new record Fruit as arguments and pushes the record stored in Fruit onto FruitStack if the Price is less than 100.
(ii) pop_fruit(FruitStack): This function pops the topmost record from the stack and returns it. If the stack is already empty, the function should display "UNDERFLOW".
(iii) display(FruitStack): This function displays all the elements of the stack starting from the topmost element. If the stack is empty, the function should display EMPTY STACK'.
OR
(b) Write a Python program to accept 10 integers from the user. If the entered number is a three-digit even integer, push it onto a stack. After all inputs are taken, pop all the three-digit even integers from the stack and display them. For example, if the user enters 12, 31, 320, 457, 6, 92, 924, 220, 1, 218, then the stack should contain :
320, 924, 220, 218
and the output of the program should be :
218 220 924 320
(a) A stack named FruitStack, implemented using list, contains records of some fruits. Each record is represented as a dictionary with keys Name', Origin', Price', and Expiry'. A sample record is given here :
``python
{'Name':'Apple','Origin':'France','Price':120,
'Expiry':'12-08-2025'}
`
Write the following user-defined functions in Python to perform the specified operations on FruitStack :
(i) push_fruit(FruitStack, Fruit): This function takes the stack FruitStack and a new record Fruit as arguments and pushes the record stored in Fruit onto FruitStack if the Price is less than 100.
(ii) pop_fruit(FruitStack): This function pops the topmost record from the stack and returns it. If the stack is already empty, the function should display "UNDERFLOW".
(iii) display(FruitStack): This function displays all the elements of the stack starting from the topmost element. If the stack is empty, the function should display EMPTY STACK'.
OR
(b) Write a Python program to accept 10 integers from the user. If the entered number is a three-digit even integer, push it onto a stack. After all inputs are taken, pop all the three-digit even integers from the stack and display them. For example, if the user enters 12, 31, 320, 457, 6, 92, 924, 220, 1, 218, then the stack should contain :
320, 924, 220, 218
and the output of the program should be :
218 220 924 320
Official answer
Two independent OR options are answered in full below — a student attempts only one.
Part (a) — Stack of Fruit records
``python
def push_fruit(FruitStack, Fruit):
if Fruit['Price'] < 100:
FruitStack.append(Fruit)
def pop_fruit(FruitStack):
if FruitStack == []:
print("UNDERFLOW")
else:
return FruitStack.pop()
def display(FruitStack):
if FruitStack == []:
print("EMPTY STACK")
else:
top = len(FruitStack) - 1
while top >= 0:
print(FruitStack[top])
top -= 1
`
Part (b) — Three-digit even integers on a stack
`python
Stack = []
for i in range(10):
num = int(input("Enter a number: "))
if len(str(num)) == 3 and num % 2 == 0:
Stack.append(num)
while Stack:
print(Stack.pop(), end=' ')
``
Marking scheme
- 1.Part (a): 1 mark for push_fruit() with the Price<100 condition; 1 mark for pop_fruit() with the UNDERFLOW check; 1 mark for display() printing top-to-bottom with the EMPTY STACK check.
- 2.Part (b): 1 mark for correctly reading 10 integers and testing three-digit + even (len(str(num))==3 and num%2==0); 1 mark for pushing qualifying numbers onto the stack; 1 mark for popping and printing all stack elements in LIFO order.
- 3.Either OR option, correctly and completely coded, earns the full 3 marks.
Hint
Stack push = append(), pop = list.pop(); always check for an empty list before popping or displaying.
Quick oral answer
A stack follows LIFO; in Python we push with list.append() and pop with list.pop(), always testing 'if stack == []' first to catch underflow before removing an element.
Common mistakes
- 1.Forgetting to check for UNDERFLOW/EMPTY STACK before popping or displaying an empty list, causing an IndexError in real code.
- 2.In display(), iterating the list front-to-back instead of from the last index backwards, which prints the stack in the wrong (non-LIFO) order.
- 3.In part (b), checking num>=100 and num<=999 with the wrong operator, or forgetting the even-number condition entirely.
Analysis and explanation
Both parts test the LIFO (Last-In-First-Out) behaviour of a stack implemented on a Python list.
Concept
append()pushes an item onto the top of the list-stack;pop()removes and returns the topmost (last) item — this is exactly LIFO.- An empty list stack (
[]) must be checked before every pop/display to avoid an UNDERFLOW error.
Part (a) — condition-guarded push
push_fruitonly appends when Price < 100, so costlier fruit is silently rejected.display()must print from the LAST index backwards, since the top of the stack is the last appended element, not the first.
Part (b) — filtering while stacking
len(str(num)) == 3is a quick built-in way to test "three-digit" without arithmetic range checks.- Because
pop()always removes the last-pushed number first, the popped/output order is naturally the reverse of the input order — 218, 220, 924, 320 for the given sample.
Exam trap
- Forgetting the UNDERFLOW/EMPTY STACK messages costs marks even if the push logic itself is correct.
Interesting facts
A Python list already provides fast append/pop-from-end operations, which is exactly why list.append() and list.pop() are the standard way to build a stack without importing any extra module.
The collections.deque class is often preferred over a plain list for very large stacks because it avoids the occasional memory reallocation that a growing list can incur.
The stack data structure is named after a physical stack of plates — you can only add or remove from the top, never from the middle.
Question details
Q33 of 48 questions
- Board
- CBSE
- Class
- Class 12
- Subject
- Computer Science
- Year
- 2026
- Section
- Section C
- Marks
- 3
- Type
- Short Answer
- Chapter
- Data Structures using Python — Stack
- Topic
- Stack Implementation using List — PUSH, POP and Display Operations
- Bloom's level
- apply
- Syllabus topic
- Unit 1: Computational Thinking and Programming – II — Data Structures: Stack (LIFO) implemented using a List; user-defined PUSH/POP/Traversal functions
- Author
- CBSE
- Reviewed by
- Padhantu Academic Team
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Frequently asked questions
Why does display() print from the last index backwards?
Because the top of a stack is the last element pushed (highest index in the list); printing top-to-bottom means starting from len(FruitStack)-1 down to 0.
What does len(str(num))==3 check?
Converting the integer to a string and checking its length is a quick way to confirm it has exactly three digits, equivalent to checking 100 <= num <= 999.
What happens if pop_fruit() is called on an empty stack?
It prints 'UNDERFLOW' instead of raising an error, because the function explicitly checks 'if FruitStack == []' before calling pop().