State management (idle, moving up, moving down) in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding state transitions in a simple elevator system

Consider a system managing an elevator with three states: idle, moving up, and moving down. Which of the following best describes the correct state transition when the elevator receives a request to move to a higher floor?

A From moving up to idle, then moving down immediately.

B From moving down directly to moving up without going idle.

C From idle to moving down, then to moving up after reaching the floor.

D From idle to moving up, then to idle after reaching the floor.

💡 Hint

Think about the natural flow of elevator movement when going to a higher floor.

The elevator starts idle, then moves up when requested to a higher floor, and returns to idle once it reaches the destination.

❓ Architecture

intermediate

Designing a state machine for elevator movement

You need to design a state machine for an elevator with states: idle, moving up, moving down. Which component is essential to handle state transitions correctly?

A A controller that listens to floor requests and updates the state accordingly.

B A database to store all floor requests permanently.

C A UI component to display elevator status only.

D A logging system to record elevator movements.

💡 Hint

Focus on what controls the state changes in the system.

The controller is responsible for receiving requests and changing the elevator's state based on those requests.

❓ scaling

advanced

Scaling state management for multiple elevators

When managing multiple elevators in a building, what is the best approach to maintain consistent state management across all elevators?

A Let each elevator manage its own state independently without coordination.

B Use a centralized state manager that tracks all elevators and their states.

C Store elevator states only in local memory without synchronization.

D Use a random assignment of states to elevators to balance load.

💡 Hint

Consider how to avoid conflicting states and optimize elevator usage.

A centralized state manager ensures all elevators' states are tracked and coordinated to optimize performance and avoid conflicts.

❓ tradeoff

advanced

Tradeoffs in state polling vs event-driven updates

For an elevator system managing states (idle, moving up, moving down), which tradeoff is true when choosing between polling the elevator state frequently versus using event-driven updates?

A Polling and event-driven approaches have identical system load and complexity.

B Polling reduces system load and complexity; event-driven increases load and complexity.

C Polling increases system load but simplifies state detection; event-driven reduces load but requires more complex event handling.

D Event-driven updates always cause delays compared to polling.

💡 Hint

Think about how often the system checks for changes and how it reacts.

Polling checks state regularly, increasing load but is simple; event-driven updates only on changes, reducing load but needing event management.

❓ estimation

expert

Estimating capacity for state management in a high-rise elevator system

You are designing state management for 20 elevators in a 100-floor building. Each elevator changes state approximately every 10 seconds on average. Estimate the number of state transitions the system must handle per minute.

A 120 transitions per minute

B 60 transitions per minute

C 200 transitions per minute

D 100 transitions per minute

💡 Hint

Calculate transitions per elevator per minute, then multiply by number of elevators.

Each elevator changes state every 10 seconds → 6 times per minute (60/10=6). For 20 elevators: 6 × 20 = 120 transitions per minute.

Practice

1. What is the main purpose of state management in a system with states like idle, moving up, and moving down?

easy

A. To track and control what the system is doing at any moment

B. To store user data permanently

C. To speed up the system hardware

D. To create random outputs

Solution

1. Step 1: Understand the role of state management

   State management keeps track of the current condition or mode of the system, such as idle or moving.

2. Step 2: Identify the purpose in context

   It helps control system behavior by knowing what action to take based on the current state.

3. Final Answer:

   To track and control what the system is doing at any moment -> Option A

4. Quick Check:

   State management = track system state [OK]

Hint: State management controls system actions by tracking current state [OK]

Common Mistakes:

• Confusing state management with data storage
• Thinking it improves hardware speed
• Assuming it generates outputs randomly

2. Which of the following is the correct way to represent the state transitions for a system with states idle, moving up, and moving down?

easy

A. moving down -> moving up -> idle only

B. idle -> moving up -> moving down -> idle

C. moving up -> moving down -> idle only

D. idle -> moving up, idle -> moving down, moving up -> idle, moving down -> idle

Solution

1. Step 1: Identify valid transitions between states

   The system can start idle, then move up or down, and return to idle after movement.

2. Step 2: Check which option lists all valid transitions

   idle -> moving up, idle -> moving down, moving up -> idle, moving down -> idle correctly lists transitions from idle to moving states and back to idle.

3. Final Answer:

   idle -> moving up, idle -> moving down, moving up -> idle, moving down -> idle -> Option D

4. Quick Check:

   Valid transitions include idle to moving and back [OK]

Hint: State transitions must include all valid moves between states [OK]

Common Mistakes:

• Assuming linear transitions only
• Missing transitions back to idle
• Ignoring that moving up and down are separate states

3. Given the following pseudo-code for state transitions, what will be the final state after these events: start at idle, move up, move down, idle?

state = 'idle'
if event == 'move up' and state == 'idle':
    state = 'moving up'
elif event == 'move down' and state == 'idle':
    state = 'moving down'
elif event == 'stop' and state in ['moving up', 'moving down']:
    state = 'idle'

medium

A. error

B. moving down

C. moving up

D. idle

Solution

1. Step 1: Trace the events and state changes

   Start: state = 'idle'
   Event 'move up': matches first if, state = 'moving up'
   Event 'move down': does not match any condition (state != 'idle', event != 'stop'), no change
   Event 'idle': does not match any condition, no change. Final state = 'moving up'

2. Step 2: Determine final state

   After all events, the state is 'moving up'.

3. Final Answer:

   moving up -> Option C

4. Quick Check:

   Trace confirms final state 'moving up' [OK]

Hint: Follow events step-by-step to track state changes [OK]

Common Mistakes:

• Assuming move down changes state from moving up
• Thinking event 'idle' triggers return to idle
• Confusing event names with states

4. Identify the error in this state transition logic for a system with states idle, moving up, and moving down:

if state == 'idle' and event == 'move up':
    state = 'moving up'
elif state == 'moving up' and event == 'move down':
    state = 'moving down'
elif state == 'moving down' and event == 'stop':
    state = 'idle'

medium

A. Missing transition from idle to moving down

B. Missing transition from idle to moving up

C. Incorrect event name for stopping

D. State variable is not updated

Solution

1. Step 1: Review all possible transitions

   The code allows idle to moving up, moving up to moving down, and moving down to idle, but no direct transition from idle to moving down.

2. Step 2: Identify missing transitions

   Since the system should allow moving down from idle, this transition is missing.

3. Final Answer:

   Missing transition from idle to moving down -> Option A

4. Quick Check:

   Check all valid transitions included [OK]

Hint: Check if all state-event pairs have transitions [OK]

Common Mistakes:

• Assuming moving up can switch directly to moving down
• Ignoring missing transitions from idle
• Confusing event names with states

5. You are designing a state machine for an elevator with states idle, moving up, and moving down. Which design choice best ensures scalability and clear control flow when adding more states like door open or maintenance?

hard

A. Hardcode state changes inside each function without a state map

B. Use a centralized state manager with explicit allowed transitions and event handlers

C. Use global variables and if-else checks scattered across code

D. Ignore state management and rely on random delays

Solution

1. Step 1: Consider scalability and clarity

   A centralized state manager clearly defines states and allowed transitions, making it easier to add new states and maintain control flow.

2. Step 2: Evaluate other options

   Using global variables or hardcoding state changes leads to messy, error-prone code. Ignoring state management causes unpredictable behavior.

3. Final Answer:

   Use a centralized state manager with explicit allowed transitions and event handlers -> Option B

4. Quick Check:

   Centralized state management = scalable and clear [OK]

Hint: Centralize state logic for easier scaling and maintenance [OK]

Common Mistakes:

• Scattering state logic causing bugs
• Hardcoding states making changes hard
• Ignoring state management leads to chaos