Elevator, Floor, Request classes in LLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main responsibility of the Elevator class in a building system?

The Elevator class manages the elevator's current state, including its current floor, direction, and the list of requests it needs to serve.

beginner

How does the Floor class typically interact with the Elevator system?

The Floor class represents a building floor and can generate requests when a user wants to go up or down, signaling the Elevator system to respond.

beginner

What information does a Request class usually contain in an elevator system?

A Request class contains details like the source floor, destination floor, and the direction of travel (up or down) to help the elevator decide its movement.

intermediate

Why is it important for the Elevator class to maintain a queue of Requests?

Maintaining a queue helps the Elevator class efficiently plan stops and serve multiple requests in an organized manner, improving system responsiveness.

intermediate

How can the Elevator system decide which request to serve next?

The system can use algorithms like 'nearest floor first' or 'direction-based scheduling' to pick the next request that minimizes travel time and wait time.

Which class is responsible for generating a request when a user presses a button on a floor?

A Floor

B Elevator

C Request

D Controller

The Floor class generates requests when users press buttons to go up or down.

What key data does a Request class NOT typically include?

A Destination floor

B User's name

C Source floor

D Direction

User's name is not usually part of the Request class; it focuses on floors and direction.

Why does the Elevator class keep track of its current floor?

A To know where to stop next

B To count passengers

C To open doors automatically

D To change the building's floor numbers

Knowing the current floor helps the elevator decide which requests to serve next.

Which scheduling strategy helps an elevator serve requests efficiently?

A Random floor selection

B Serving requests in reverse order

C Ignoring requests

D Nearest floor first

Nearest floor first reduces travel time and wait time.

What does the Floor class represent in the elevator system?

A The elevator's motor

B The elevator's control panel

C A building level where users wait

D A request queue

The Floor class models a building level where users can request elevators.

Explain the roles of Elevator, Floor, and Request classes in an elevator system.

Think about how a person calls and uses an elevator.

Describe how an elevator system decides which request to serve next and why this is important.

Consider how elevators avoid unnecessary travel.

Practice

1. What is the primary role of the Request class in an elevator system?

easy

A. To store the floor number and direction of a user's request

B. To move the elevator between floors

C. To open and close the elevator doors

D. To track the number of elevators in the building

Solution

1. Step 1: Understand the purpose of Request class

   The Request class holds information about where a user wants to go and in which direction.

2. Step 2: Compare roles of other classes

   Elevator moves and Floor represents building levels, but Request stores user input details.

3. Final Answer:

   To store the floor number and direction of a user's request -> Option A

4. Quick Check:

   Request = user floor and direction [OK]

Hint: Request class holds user floor and direction info [OK]

Common Mistakes:

• Confusing Request with Elevator movement
• Thinking Request controls doors
• Mixing Request with Floor class responsibilities

2. Which of the following is the correct way to define a Request class constructor in Python to store floor and direction?

easy

A. def Request(floor, direction): self.floor = floor; self.direction = direction

B. def __init__(self, floor, direction): self.floor = floor; self.direction = direction

C. def __init__(floor, direction): self.floor = floor; self.direction = direction

D. def __init__(self): floor = None; direction = None

Solution

1. Step 1: Recall Python constructor syntax

   Python constructors use def __init__(self, ...) and assign attributes with self.attribute = value.

2. Step 2: Check each option

   def __init__(self, floor, direction): self.floor = floor; self.direction = direction correctly uses self and assigns floor and direction. Others miss self or parameters.

3. Final Answer:

   def __init__(self, floor, direction): self.floor = floor; self.direction = direction -> Option B

4. Quick Check:

   Constructor with self and attributes = def __init__(self, floor, direction): self.floor = floor; self.direction = direction [OK]

Hint: Python constructors need self parameter and attribute assignment [OK]

Common Mistakes:

• Omitting self parameter
• Using class name as constructor
• Not assigning attributes to self

3. Given this Python snippet, what will be printed?

class Request:
    def __init__(self, floor, direction):
        self.floor = floor
        self.direction = direction

r = Request(5, 'up')
print(r.floor, r.direction)

medium

A. Error: missing self

B. floor direction

C. 5 up

D. None None

Solution

1. Step 1: Understand object creation and attribute assignment

   The Request object r is created with floor=5 and direction='up'. These are stored in attributes.

2. Step 2: Print attributes

   Printing r.floor and r.direction outputs 5 and 'up' respectively.

3. Final Answer:

   5 up -> Option C

4. Quick Check:

   Attributes print as assigned = 5 up [OK]

Hint: Print object attributes to see stored values [OK]

Common Mistakes:

• Expecting attribute names instead of values
• Confusing class variables with instance variables
• Assuming error without checking code carefully

4. Identify the error in this Elevator class snippet:

class Elevator:
    def __init__(self, current_floor):
        self.current_floor = current_floor

    def move_to(self, floor):
        current_floor = floor

medium

A. Indentation error in move_to method

B. Missing return statement in move_to method

C. Constructor should not have parameters

D. The move_to method updates a local variable, not the elevator's floor

Solution

1. Step 1: Analyze move_to method

   The method assigns current_floor = floor without self., so it changes a local variable only.

2. Step 2: Understand instance variable update

   To update the elevator's floor, it should be self.current_floor = floor.

3. Final Answer:

   The move_to method updates a local variable, not the elevator's floor -> Option D

4. Quick Check:

   Missing self. means local variable used [OK]

Hint: Use self. to update instance variables inside methods [OK]

Common Mistakes:

• Forgetting self. prefix
• Thinking return is needed to update state
• Assuming indentation is wrong without checking

5. In designing an elevator system with Elevator, Floor, and Request classes, which approach best handles multiple simultaneous requests efficiently?

hard

A. Use a priority queue in Elevator to process requests by nearest floor and direction

B. Process requests in the order they arrive without sorting

C. Assign each request to a random elevator immediately

D. Ignore direction and always move elevators to the highest requested floor first

Solution

1. Step 1: Understand multiple request handling

   Efficient elevator systems prioritize requests to minimize travel and wait time.

2. Step 2: Evaluate options for request processing

   Using a priority queue to pick nearest floors and matching direction optimizes movement. Others cause inefficiency or randomness.

3. Final Answer:

   Use a priority queue in Elevator to process requests by nearest floor and direction -> Option A

4. Quick Check:

   Priority queue for nearest requests = Use a priority queue in Elevator to process requests by nearest floor and direction [OK]

Hint: Prioritize nearest requests with direction for efficient elevator movement [OK]

Common Mistakes:

• Ignoring direction leads to inefficient routes
• Random assignment causes delays
• Processing requests strictly by arrival order wastes time