Why elevator design tests state machines in LLD - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the initial state of the elevator.

LLD

elevator_state = '[1]'

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

ADOOR_OPEN

BMOVING

CIDLE

DMAINTENANCE

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing MOVING as initial state, which is incorrect because elevator starts idle.

The elevator starts in the IDLE state when it is not moving or opening doors.

2fill in blank

medium

Complete the code to transition the elevator state to moving when a request is received.

LLD

if request_received:
    elevator_state = '[1]'

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

AMOVING

BIDLE

CDOOR_OPEN

DSTOPPED

Attempts:

3 left

💡 Hint

Common Mistakes

Setting state to IDLE after request, which means no movement.

When a request is received, the elevator state changes to MOVING to reach the requested floor.

3fill in blank

hard

Fix the error in the state transition when the elevator reaches the floor.

LLD

if elevator_state == 'MOVING' and at_floor:
    elevator_state = '[1]'

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

AMOVING

BIDLE

CMAINTENANCE

DDOOR_OPEN

Attempts:

3 left

💡 Hint

Common Mistakes

Setting state to IDLE immediately, skipping door opening.

When the elevator reaches the floor, it should open the door, so the state changes to DOOR_OPEN.

4fill in blank

hard

Fill both blanks to complete the state machine transition for closing doors and going idle.

LLD

if elevator_state == 'DOOR_OPEN' and door_closed:
    elevator_state = '[1]'
if elevator_state == '[2]' and no_requests:
    elevator_state = 'IDLE'

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

AMOVING

BDOOR_OPEN

CIDLE

DSTOPPED

Attempts:

3 left

💡 Hint

Common Mistakes

Setting state to IDLE immediately after door closes without checking requests.

After doors close, elevator moves if there are requests; if no requests, it goes idle.

5fill in blank

hard

Fill all three blanks to complete the dictionary for elevator states and their descriptions.

LLD

state_descriptions = {
    '[1]': 'Elevator is idle and waiting',
    '[2]': 'Elevator is moving between floors',
    '[3]': 'Elevator doors are open'
}

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

AIDLE

BMOVING

CDOOR_OPEN

DMAINTENANCE

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up state names and descriptions.

The dictionary maps elevator states to their human-readable descriptions.

Practice

1. Why is elevator design often used to test state machines in system design?

easy

A. Because elevators have clear states and transitions that model real-world behavior

B. Because elevators require complex database management

C. Because elevators use machine learning algorithms

D. Because elevators operate without any state changes

Solution

1. Step 1: Understand elevator operation basics

   Elevators move between floors and have states like moving up, moving down, idle, door open, and door closed.

2. Step 2: Connect elevator states to state machine concept

   State machines model systems with defined states and transitions, matching elevator behavior perfectly.

3. Final Answer:

   Because elevators have clear states and transitions that model real-world behavior -> Option A

4. Quick Check:

   Elevator states = clear states and transitions [OK]

Hint: Elevators have clear states and transitions [OK]

Common Mistakes:

• Thinking elevators don't have states
• Confusing state machines with databases
• Assuming elevators use AI for basic movement

2. Which of the following correctly represents a state transition in an elevator state machine?

easy

A. Idle -> Door Open -> Moving Up

B. Idle -> Moving Up -> Door Open

C. Door Open -> Moving Down -> Door Closed

D. Moving Up -> Door Closed -> Idle

Solution

1. Step 1: Identify valid elevator state order

   An elevator usually goes from Idle to Moving Up, then Door Open when it reaches the floor.

2. Step 2: Check each option's sequence

   Idle -> Moving Up -> Door Open correctly shows Idle -> Moving Up -> Door Open, a valid transition sequence.

3. Final Answer:

   <code>Idle -> Moving Up -> Door Open</code> -> Option B

4. Quick Check:

   Idle to Moving Up to Door Open = valid transition [OK]

Hint: Elevator moves before doors open [OK]

Common Mistakes:

• Opening doors before moving
• Closing doors while idle
• Skipping moving state

3. Given this simplified elevator state machine code snippet, what is the final state after these events: callElevator(), arriveFloor(), openDoor()?

states = ['Idle', 'Moving', 'DoorOpen']
current_state = 'Idle'

def callElevator():
    global current_state
    if current_state == 'Idle':
        current_state = 'Moving'

def arriveFloor():
    global current_state
    if current_state == 'Moving':
        current_state = 'DoorOpen'

def openDoor():
    global current_state
    if current_state == 'DoorOpen':
        current_state = 'Idle'

medium

A. Idle

B. Moving

C. DoorOpen

D. Error

Solution

1. Step 1: Trace callElevator()

   Starting at 'Idle', callElevator() changes state to 'Moving'.

2. Step 2: Trace arriveFloor() and openDoor()

   arriveFloor() changes 'Moving' to 'DoorOpen', then openDoor() changes 'DoorOpen' back to 'Idle'.

3. Final Answer:

   Idle -> Option A

4. Quick Check:

   Idle after all events = Idle [OK]

Hint: Follow state changes step-by-step [OK]

Common Mistakes:

• Skipping openDoor() effect
• Assuming state stays at DoorOpen
• Confusing event order

4. In this elevator state machine code, what is the bug causing the elevator to never open doors?

current_state = 'Idle'

def callElevator():
    global current_state
    if current_state == 'Idle':
        current_state = 'Moving'

def arriveFloor():
    global current_state
    if current_state == 'Moving':
        current_state = 'Idle'  # Bug here

def openDoor():
    global current_state
    if current_state == 'DoorOpen':
        current_state = 'Idle'

medium

A. current_state is not initialized

B. callElevator() does not change state

C. openDoor() changes state incorrectly

D. arriveFloor() sets state to 'Idle' instead of 'DoorOpen'

Solution

1. Step 1: Analyze arriveFloor() function

   arriveFloor() changes 'Moving' state directly to 'Idle', skipping 'DoorOpen'.

2. Step 2: Understand effect on door opening

   Since state never becomes 'DoorOpen', openDoor() condition never triggers, so doors never open.

3. Final Answer:

   arriveFloor() sets state to 'Idle' instead of 'DoorOpen' -> Option D

4. Quick Check:

   arriveFloor() wrong state change = no door open [OK]

Hint: Check if all states are reachable in transitions [OK]

Common Mistakes:

• Ignoring wrong state assignment
• Assuming callElevator() is faulty
• Overlooking openDoor() condition

5. You are designing an elevator system with multiple elevators and floors. Why is modeling the system as a state machine important for safety and scalability?

hard

A. It eliminates the need for sensors and hardware checks

B. It allows elevators to learn user preferences automatically

C. It ensures predictable behavior and clear transitions, preventing unsafe states

D. It reduces the number of elevators needed by half

Solution

1. Step 1: Understand state machine benefits in complex systems

   State machines define clear states and transitions, making system behavior predictable and easier to manage.

2. Step 2: Connect predictability to safety and scalability

   Predictable transitions prevent unsafe states like doors opening while moving, and help scale by managing multiple elevators consistently.

3. Final Answer:

   It ensures predictable behavior and clear transitions, preventing unsafe states -> Option C

4. Quick Check:

   Predictable states = safety and scalability [OK]

Hint: Predictable states prevent unsafe elevator behavior [OK]

Common Mistakes:

• Confusing state machines with AI
• Ignoring safety in design
• Assuming hardware replaces software logic