Push and Pop Operations on a Stack Using a Python List (Class 12 Computer Science)

Understand Push and Pop Operations on a Stack Using a List

def push(stack, item):
    stack.append(item)

def pop(stack):
    if stack == []:
        print("Underflow")
        return None
    return stack.pop()

def peek(stack):
    if stack == []:
        return None
    return stack[-1]

def display(stack):
    for i in range(len(stack) - 1, -1, -1):
        print(stack[i])

Push and Pop Operations on a Stack Using a List: key facts

Push and pop in Python

Push	stack.append(item)
Pop	stack.pop()
Peek	stack[-1]
Empty test	stack == [] or len(stack) == 0

The top is the last element of the list.

Underflow

• •pop on an empty stack
• •Python raises IndexError: pop from empty list
• •Check before popping and print Underflow

A board function returns None after printing Underflow.

Overflow

• •Only for a stack with a size limit
• •Test len(stack) == limit before append
• •A plain list never overflows

State the limit in the question before coding it.

Return value of pop

• •pop() removes the item and returns it
• •Capture it: x = stack.pop()
• •Without capturing, the value is lost

Display functions must not pop unless asked.

How to answer Push and Pop Operations on a Stack Using a List questions

1. 1Name the stack list as a parameter of every function, so the same list is shared by push, pop and display.
2. 2Push: one line, append(item). Add a size test only if the question gives a limit.
3. 3Pop: test for empty first; if empty print "Underflow" and return None; otherwise return stack.pop().
4. 4Display: loop from the last index to 0 so the top prints first, or loop over a reversed copy.
5. 5Do not use insert(0, ...) or pop(0): that works on the other end and breaks LIFO thinking.
6. 6Finish by writing a short call sequence and the exact output.

Push and Pop Operations on a Stack Using a List in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Tricks to remember Push and Pop Operations on a Stack Using a List

Short memory aids. Say each one aloud once, then use it on a question the same day.

• A-P-P-L-E: append is push

  append = push, pop = pop, [-1] = peek.

  Use it when: Mapping stack operations to list code.

• Check before you pop

  Ask "is there anything on the pile?" before taking from it.

  Use it when: Preventing IndexError and underflow.

• Display from the top

  The first thing shown should be the first thing that would be popped.

  Use it when: Writing display with range(len-1, -1, -1).

• pop() returns, [-1] only looks

  pop takes the top away; stack[-1] just looks at it.

  Use it when: Peek versus pop questions.

Push and Pop Operations on a Stack Using a List: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

st = []
st.append(4)
st.append(7)
st.append(1)
print(st.pop(), st[-1], len(st))

tk = [101, 102, 103]
tk.append(104)
x = tk.pop()
y = tk.pop()
tk.append(x + y)
print(tk, tk[-1])

Where students lose marks in Push and Pop Operations on a Stack Using a List

The slips examiners see most often, and what to write instead.

• ✗Mistake: Using pop(0) as the stack pop

  pop(0) removes the bottom item, which is queue behaviour.

  Wrong working

  return stack.pop(0)

  Right working

  return stack.pop()

  ✓Do this: The top is the last index, so use pop() or pop(-1).

  Usually costs: 1 mark

• ✗Mistake: No empty check before pop

  On an empty list pop raises IndexError and the program stops.

  Wrong working

  def pop(stack):
  return stack.pop()

  Right working

  def pop(stack):
  if stack == []:
  print("Underflow")
  return None
  return stack.pop()

  ✓Do this: Write the test first; marking schemes award it separately.

  Usually costs: 1 mark

• ✗Mistake: Printing the stack inside pop

  The question says return the popped item; a print loses it for later use.

  Wrong working

  def pop(stack):
  print(stack.pop())

  Right working

  def pop(stack):
  return stack.pop()

  ✓Do this: Return the value; print it in the calling code.

  Usually costs: 0.5 mark

Practice questions on Push and Pop Operations on a Stack Using a List

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQBoard level

   Which statement reads the top of a list stack st without removing it?
   (a) st.pop() (b) st[0] (c) st[-1] (d) st.top()

   Hint

   Last index is the top.

   Answer

   (c) st[-1]

2. Q2 · 2 marks · Short answerBoard level

   What error does st.pop() raise when the list st is empty, and what is the condition called?

   Hint

   Two words: the Python error and the stack term.

   Answer

   IndexError (pop from empty list); the condition is underflow.

3. Q3 · 3 marks · Short answerHarder board level

   Write a function peek(stack) that returns the top item without removing it, or prints "Empty" and returns None when the stack is empty. Show the result of peek on [8, 9, 2] and on [].

   Hint

   Test emptiness first.

   Answer

   def peek(stack):
       if stack == []:
           print("Empty")
           return None
       return stack[-1]

   peek([8, 9, 2]) returns 2 (list unchanged); peek([]) prints Empty and returns None.

4. Q4 · 3 marks · Short answerHarder board level

   Write the output:

   st = [2, 4]
   for n in range(3):
       st.append(st[-1] + n)
   print(st)

   Hint

   Each step uses the current top.

   Answer

   n = 0: top 4, append 4. n = 1: top 4, append 5. n = 2: top 5, append 7. Output: [2, 4, 4, 5, 7].

5. Q5 · 2 marks · Short answerHOTS / competency

   A student writes display(stack) as: for x in stack: print(x). The stack is [1, 2, 3] with top 3. (i) What does it print? (ii) Rewrite it so that it prints in pop order without changing the stack.

   Hint

   Pop order starts at the top.

   Answer

   (i) 1, 2, 3 on separate lines (bottom first, not pop order). (ii) for i in range(len(stack) - 1, -1, -1): print(stack[i]) prints 3, 2, 1.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Which list method is push?Show answer

   append().

2. 2.Which list method is pop?Show answer

   pop() with no argument.

3. 3.What does stack[-1] give?Show answer

   The top item, without removing it.

4. 4.What does pop() do on an empty list?Show answer

   Raises IndexError.

5. 5.What is the stack state called when pop is attempted on an empty stack?Show answer

   Underflow.

6. 6.Which end of the list is the top?Show answer

   The last (highest index) end.

Push and Pop Operations on a Stack Using a List: frequently asked questions