Why stacks follow LIFO principle in Data Structures Theory - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the sentence to explain the LIFO principle of stacks.

Data Structures Theory

A stack follows the [1] principle, meaning the last item added is the first one to be removed.

Drag options to blanks, or click blank then click option'

AFIFO

BRandom

CLIFO

DPriority

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing LIFO with FIFO, which is used in queues.

Thinking stacks remove items in the order they were added.

The LIFO principle means "Last In, First Out". In a stack, the last element you put in is the first one you take out.

2fill in blank

medium

Complete the sentence to describe a real-life example of LIFO.

Data Structures Theory

When you stack books on a table, the book you put [1] is the first one you pick up.

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Afirst

Blast

Cin the middle

Dat the bottom

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'first' instead of 'last' for the book picked up.

Not relating the example to the LIFO principle.

The last book placed on the stack is on top, so it is the first one you pick up, illustrating LIFO.

3fill in blank

hard

Fix the error in the explanation about stack behavior.

Data Structures Theory

In a stack, the element removed is always the [1] element that was added.

Drag options to blanks, or click blank then click option'

Alargest

Bfirst

Crandom

Dlast

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing 'first' with 'last' element removal.

Thinking stacks remove elements randomly.

The correct behavior is that the last element added is the one removed first, following LIFO.

4fill in blank

hard

Fill both blanks to complete the explanation of stack operations.

Data Structures Theory

To add an item, we use [1]; to remove the last added item, we use [2].

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Apush

Bpop

Cenqueue

Ddequeue

Attempts:

3 left

💡 Hint

Common Mistakes

Using queue operation names like enqueue or dequeue.

Mixing up push and pop operations.

Stacks use push to add items and pop to remove the last added item, following LIFO.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that models stack behavior.

Data Structures Theory

stack = [1]: [2] for [3] in range(1, 4)}

Drag options to blanks, or click blank then click option'

Ai

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to fill this task as a real stack comprehension.

This task is a placeholder and does not represent a valid stack comprehension. Please ignore.

Practice

1. Why do stacks follow the Last In, First Out (LIFO) principle?

easy

A. Because the last item added is the first one removed

B. Because items are removed in the order they were added

C. Because stacks allow random access to any item

D. Because the first item added is the first one removed

Solution

1. Step 1: Understand the LIFO principle

   LIFO means the last item put in is the first one taken out.

2. Step 2: Relate to stack operations

   Stacks add (push) and remove (pop) items only from the top, so the newest item is removed first.

3. Final Answer:

   Because the last item added is the first one removed -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember: last added, first removed [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking stacks allow random access
• Assuming first added is removed first

2. Which of the following is the correct way to remove an item from a stack?

easy

A. pop() removes the last item added

B. push() removes the first item added

C. peek() removes the bottom item

D. insert() removes a random item

Solution

1. Step 1: Identify stack operations

   pop() removes the top item, which is the last added in a stack.

2. Step 2: Check other options

   push() adds items, peek() only views the top item, insert() is not a stack operation.

3. Final Answer:

   pop() removes the last item added -> Option A

4. Quick Check:

   pop() = remove last added [OK]

Hint: pop() always removes the top item [OK]

Common Mistakes:

• Mixing push and pop functions
• Thinking peek() removes items
• Assuming insert() is a stack method

3. Consider this sequence of stack operations:
push(1); push(2); push(3); pop(); push(4); pop(); pop();
What is the value returned by the last pop()?

medium

A. 1

B. 3

C. 2

D. 4

Solution

1. Step 1: Track stack after each operation

   push(1) -> [1], push(2) -> [1,2], push(3) -> [1,2,3], pop() removes 3 -> [1,2], push(4) -> [1,2,4], pop() removes 4 -> [1,2], pop() removes 2 -> [1]

2. Step 2: Identify last pop() value

   The last pop() removes 2, which is the correct answer.

3. Final Answer:

   2 -> Option C

4. Quick Check:

   Last pop() = 2 [OK]

Hint: Track stack top after each push/pop [OK]

Common Mistakes:

• Forgetting to update stack after pop
• Mixing order of operations
• Assuming pop removes bottom item

4. A programmer wrote this code to simulate a stack but it removes the first item added instead of the last:
stack = [] stack.append(1) stack.append(2) stack.append(3) removed = stack.pop(0)
What is the error causing this behavior?

medium

A. pop() without index removes the first item

B. Using append() adds items to the bottom

C. Stack should use insert() to remove items

D. Using pop(0) removes the first item, not the last

Solution

1. Step 1: Understand pop() with index

   pop(0) removes the item at index 0, which is the first item added.

2. Step 2: Correct stack removal method

   To follow LIFO, pop() without index should be used to remove the last item.

3. Final Answer:

   Using pop(0) removes the first item, not the last -> Option D

4. Quick Check:

   pop(0) removes first item [OK]

Hint: pop() without index removes last item [OK]

Common Mistakes:

• Using pop(0) instead of pop()
• Thinking append() adds to bottom
• Confusing insert() with removal

5. In a text editor, the undo feature uses a stack to reverse actions. If a user performs actions A, B, C in order, which sequence of undo operations correctly follows the LIFO principle?

hard

A. Undo B, then C, then A

B. Undo C, then B, then A

C. Undo A, then B, then C

D. Undo all actions simultaneously

Solution

1. Step 1: Understand stack use in undo

   Actions are pushed onto a stack as they occur: A, then B, then C.

2. Step 2: Apply LIFO to undo

   Undo removes the last action first, so undo order is C, then B, then A.

3. Final Answer:

   Undo C, then B, then A -> Option B

4. Quick Check:

   Undo order = last done, first undone [OK]

Hint: Undo reverses last action first [OK]

Common Mistakes:

• Undoing actions in original order
• Undoing multiple actions at once
• Ignoring stack order in undo