State diagrams in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the initial state in a state diagram.

LLD

stateDiagram-v2
    [*] --> [1]

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

AIdle

BStart

CRunning

DStopped

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a state that is not the starting point.

The initial state is usually represented by an arrow pointing to the Start state.

2fill in blank

medium

Complete the code to add a transition triggered by an event.

LLD

stateDiagram-v2
    Idle --> Running : [1]

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

Astop

Breset

Cpause

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using an event that stops or pauses instead of starting.

The transition from Idle to Running is triggered by the start event.

3fill in blank

hard

Fix the error in the transition label to correctly represent the event.

LLD

stateDiagram-v2
    Running --> Paused : [1]

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

Apause

Bstart

Cstop

Dresume

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' or 'stop' instead of 'pause'.

The transition from Running to Paused should be triggered by the pause event.

4fill in blank

hard

Fill both blanks to define a transition with an action on event.

LLD

stateDiagram-v2
    Paused --> Running : [1] / [2]

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

Aresume

Bstart

CnotifyUser

Dstop

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up events and actions in the transition label.

The transition from Paused to Running is triggered by the resume event and performs the notifyUser action.

5fill in blank

hard

Fill all three blanks to define a transition with event, guard condition, and action.

LLD

stateDiagram-v2
    Running --> Stopped : [1] [[2]] / [3]

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

Astop

BisEmergency

ClogStop

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing event with action or guard condition.

The transition from Running to Stopped is triggered by the stop event, guarded by the isEmergency condition, and performs the logStop action.

Practice

1. What is the main purpose of a state diagram in system design?

easy

A. To display the database schema and relationships

B. To list all the classes and their methods in a system

C. To show different states of a system and how it moves between them

D. To outline the network topology of system components

Solution

1. Step 1: Understand what a state diagram represents

   A state diagram shows the various states a system or component can be in and the transitions triggered by events.

2. Step 2: Compare with other diagrams

   Class diagrams show classes and methods, database diagrams show data structure, and network diagrams show connections, not states.

3. Final Answer:

   To show different states of a system and how it moves between them -> Option C

4. Quick Check:

   State diagram = system states and transitions [OK]

Hint: State diagrams focus on states and transitions only [OK]

Common Mistakes:

• Confusing state diagrams with class diagrams
• Thinking state diagrams show data structure
• Assuming state diagrams show network layout

2. Which of the following is the correct syntax to represent a transition from state A to state B on event start in a state diagram?

easy

A. A => start => B

B. A -> B : start

C. A --start--> B

D. A -start-> B

Solution

1. Step 1: Recall standard state diagram notation

   Transitions are usually shown as arrows with the event label after a colon, like A -> B : event.

2. Step 2: Match options with notation

   A -> B : start matches this format exactly. Others use incorrect symbols or order.

3. Final Answer:

   A -> B : start -> Option B

4. Quick Check:

   Transition syntax = A -> B : event [OK]

Hint: Use arrow then colon for event label in transitions [OK]

Common Mistakes:

• Using double dashes instead of single arrow
• Placing event before arrow
• Omitting colon before event label

3. Given the following state diagram code snippet:

Idle -> Processing : start
Processing -> Idle : stop
Processing -> Error : fail

What is the next state if the system is in Processing and receives the fail event?

medium

A. Error

B. Idle

C. Processing

D. Start

Solution

1. Step 1: Identify current state and event

   The system is in Processing state and receives fail event.

2. Step 2: Find matching transition

   From Processing, on fail, the transition goes to Error.

3. Final Answer:

   Error -> Option A

4. Quick Check:

   Processing + fail = Error [OK]

Hint: Match current state and event to transition target [OK]

Common Mistakes:

• Choosing Idle instead of Error
• Confusing event names
• Ignoring event and state pairing

4. Identify the error in this state diagram transition:

Start -> Running : begin
Running -> Paused : pause
Paused -> Running pause

What is wrong with the last transition?

medium

A. Missing colon before event name

B. Wrong arrow direction

C. State name misspelled

D. Extra event name

Solution

1. Step 1: Review transition syntax

   Transitions should have a colon before the event name, e.g., State1 -> State2 : event.

2. Step 2: Check last transition

   The last line is Paused -> Running pause missing the colon before pause.

3. Final Answer:

   Missing colon before event name -> Option A

4. Quick Check:

   Colon before event is required [OK]

Hint: Check for colon before event label in transitions [OK]

Common Mistakes:

• Ignoring missing colon
• Assuming arrow direction is wrong
• Thinking state name is incorrect

5. You are designing a state diagram for a vending machine with states: Idle, Selecting, Dispensing, and OutOfStock. Which transition should be added to handle the event stockEmpty when the machine is in Idle?

hard

A. Dispensing -> OutOfStock : stockEmpty

B. Idle -> Dispensing : stockEmpty

C. OutOfStock -> Idle : stockEmpty

D. Idle -> OutOfStock : stockEmpty

Solution

1. Step 1: Understand event context

   The event stockEmpty means no items are left to dispense.

2. Step 2: Determine correct transition from Idle

   If the machine is Idle and stock is empty, it should move to OutOfStock state.

3. Final Answer:

   Idle -> OutOfStock : stockEmpty -> Option D

4. Quick Check:

   stockEmpty from Idle leads to OutOfStock [OK]

Hint: Event triggers transition to correct state logically [OK]

Common Mistakes:

• Transitioning to Dispensing when stock is empty
• Reversing transition direction
• Ignoring event meaning