Stack Programs for the Board: Conditional Push and Pop All (Class 12 Computer Science)

Understand Stack Programs: Conditional Push and Pop All

def push_big(stack, data):
    for x in data:
        if x > 50:
            stack.append(x)

def pop_all(stack):
    if stack == []:
        print("Stack Empty")
    else:
        while stack != []:
            print(stack.pop())

Stack Programs: Conditional Push and Pop All: key facts

Conditional push

• •for x in data:
• •if condition:
• •stack.append(x)

Push order follows the order of the data.

Pop all

• •while stack != []:
• •print(stack.pop())
• •After the loop the stack is empty

Print Stack Empty when it is empty at the start.

Dictionary data

• •for k in d: use d[k] for the value
• •or for k, v in d.items()
• •append k, v or (k, v) as the question asks

State what each stack item is.

Output order

Pop order is the reverse of push order | The last pushed item prints first

Check by writing the stack after the pushes.

How to answer Stack Programs: Conditional Push and Pop All questions

1. 1Read what goes on the stack: the whole item, only the key, or a pair.
2. 2Write the condition from the question word by word (greater than, even, starts with a vowel, and so on).
3. 3Write push as a loop with an if and append; do not return anything.
4. 4Write pop-all as a while loop that pops until the list is empty; add the Stack Empty message for the starting-empty case.
5. 5Trace with the sample data: list the pushed items in order, then reverse them to get the output.
6. 6State the output clearly, line by line, after the code.

Stack Programs: Conditional Push and Pop All 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 Stack Programs: Conditional Push and Pop All

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

• Filter, then flip

  Push only passers; the output is the passers in reverse.

  Use it when: Predicting the output order.

• One loop to push, one loop to pop

  Each function has a single loop and one job.

  Use it when: Structuring the 3-mark answer.

• While not empty

  while stack != []: keeps popping until nothing remains.

  Use it when: Pop-all loop.

• Dictionary: keys for names, values for tests

  d[k] is the mark when k is the name.

  Use it when: Dictionary stack questions.

Stack Programs: Conditional Push and Pop All: solved examples

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

Where students lose marks in Stack Programs: Conditional Push and Pop All

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

• ✗Mistake: Printing in the push function instead of pushing

  The stack stays empty, so nothing can be popped later.

  Wrong working

  for x in data:
  if x > 50:
  print(x)

  Right working

  for x in data:
  if x > 50:
  stack.append(x)

  ✓Do this: Use append in the push function; print only in the pop function.

  Usually costs: 1 to 1.5 marks

• ✗Mistake: Wrong output order

  Pop gives the last pushed first; many students write the push order.

  Wrong working

  Pushed 70, 66, 90 and printed 70, 66, 90

  Right working

  Printed 90, 66, 70

  ✓Do this: Write the stack, then read it from the top.

  Usually costs: 1 mark

• ✗Mistake: Looping with for over the stack while popping

  Changing a list while iterating over it skips items.

  Wrong working

  for x in stack:
  print(stack.pop())

  Right working

  while stack != []:
  print(stack.pop())

  ✓Do this: Use a while loop to empty a stack.

  Usually costs: 1 mark

• ✗Mistake: Missing the empty message

  The question may say print Stack Empty when no item remains; without the test it silently prints nothing.

  Wrong working

  while stack != []: print(stack.pop())

  Right working

  if stack == []: print("Stack Empty") with else: loop

  ✓Do this: Add the test if the question mentions it.

  Usually costs: 0.5 mark

Practice questions on Stack Programs: Conditional Push and Pop All

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 · Very short answerBoard level

   In a stack program, the items are printed by popping until the stack is empty. In what order do they appear compared with the order they were pushed?

   Hint

   Think LIFO.

   Answer

   In the reverse order.

2. Q2 · 1 mark · Very short answerBoard level

   Write a statement that pushes the value x onto a stack st only if x is divisible by 5.

   Hint

   One if and one append.

   Answer

   if x % 5 == 0: st.append(x)

3. Q3 · 3 marks · Short answerHarder board level

   Write push_odd(stack, data) to push the odd numbers in data and show the pop-all output for data = [4, 7, 9, 10, 3].

   Hint

   Odd means remainder 1 when divided by 2.

   Answer

   def push_odd(stack, data):
       for x in data:
           if x % 2 == 1:
               stack.append(x)

   Odds in order: 7, 9, 3. Popping prints 3, 9, 7.

4. Q4 · 3 marks · Short answerHarder board level

   A dictionary price = {"pen": 10, "book": 120, "bag": 450, "ink": 60} is given. Write code to push the names of items costing more than 50 into a stack s and then pop and print them, and state the output.

   Hint

   Dictionary order follows insertion.

   Answer

   s = []
   for item in price:
       if price[item] > 50:
           s.append(item)
   while s != []:
       print(s.pop())

   Pushed: book, bag, ink. Output: ink, bag, book.

5. Q5 · 3 marks · Short answerHOTS / competency

   A stack st = [3, 8] is used with this code. What is the output?

   st = [3, 8]
   while len(st) < 5:
       if st[-1] % 2 == 0:
           st.append(st[-1] // 2)
       else:
           st.append(st[-1] * 3 + 1)
   print(st)

   Hint

   Check the top each time; stop when len is 5.

   Answer

   Top 8 is even: append 4 gives [3, 8, 4]. Top 4 is even: append 2 gives [3, 8, 4, 2]. Top 2 is even: append 1 gives [3, 8, 4, 2, 1]. len is 5 so stop. Output: [3, 8, 4, 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.In what order do popped items appear compared with the pushed order?Show answer

   Reverse.

2. 2.Which loop empties a stack safely?Show answer

   while stack != []: with pop inside.

3. 3.How is a dictionary value read when the key is name?Show answer

   d[name].

4. 4.What does stack.append((k, v)) push?Show answer

   A pair (tuple) of key and value as one item.

5. 5.Which function should print, push or pop-all?Show answer

   Pop-all; the push function should only append.

6. 6.What should pop-all show if the stack starts empty?Show answer

   Stack Empty (or the message the question names).

Stack Programs: Conditional Push and Pop All: frequently asked questions