Hierarchical state machine concept in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Embedded C

typedef enum { STATE_IDLE, STATE_PROCESSING, STATE_ERROR } State;

State currentState = [1];

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

ASTATE_IDLE

BSTATE_UNKNOWN

CSTATE_ERROR

DSTATE_PROCESSING

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a state that represents an error or processing as the initial state.

The initial state is usually set to STATE_IDLE to indicate the machine is waiting to start.

2fill in blank

medium

Complete the code to declare a nested (sub) state inside the PROCESSING state.

Embedded C

typedef enum { SUBSTATE_READ, SUBSTATE_WRITE } ProcessingSubState;

ProcessingSubState currentSubState = [1];

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ASUBSTATE_WRITE

BSUBSTATE_READ

CSUBSTATE_IDLE

DSUBSTATE_ERROR

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a substate that does not exist or is unrelated.

The substate SUBSTATE_READ is often the first step in processing data.

3fill in blank

hard

Fix the error in the transition function to correctly change the state.

Embedded C

void transitionToProcessing(State *state) {
    *state = [1];
}

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ASTATE_PROCESSING

BSTATE_ERROR

CSTATE_IDLE

DPROCESSING_STATE

Attempts:

3 left

💡 Hint

Common Mistakes

Using an undefined state name or a wrong state.

The function should set the state to STATE_PROCESSING to indicate the machine is now processing.

4fill in blank

hard

Fill both blanks to complete the hierarchical state check.

Embedded C

if (currentState == [1]) {
    if (currentSubState == [2]) {
        // Handle read substate
    }
}

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ASTATE_PROCESSING

BSTATE_IDLE

CSUBSTATE_READ

DSUBSTATE_WRITE

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up main states and substates in the condition.

Check if the main state is STATE_PROCESSING and the substate is SUBSTATE_READ to handle reading.

5fill in blank

hard

Fill all three blanks to complete the hierarchical state machine transition.

Embedded C

switch (currentState) {
    case [1]:
        switch (currentSubState) {
            case [2]:
                currentSubState = [3];
                break;
        }
        break;
}

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

ASTATE_PROCESSING

BSUBSTATE_READ

CSUBSTATE_WRITE

DSTATE_IDLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong state or substate names in the switch cases.

This code switches from SUBSTATE_READ to SUBSTATE_WRITE inside the STATE_PROCESSING main state.

Practice

1. What is the main benefit of using a hierarchical state machine in embedded C programming?

easy

A. It removes the need for any state transitions.

B. It eliminates the need for events to change states.

C. It allows states to run in parallel without interaction.

D. It organizes states into groups, making complex behavior easier to manage.

Solution

1. Step 1: Understand hierarchical state machine purpose

   Hierarchical state machines group related states inside bigger states to simplify complex logic.

2. Step 2: Identify the benefit

   This grouping helps share common actions and manage transitions more clearly.

3. Final Answer:

   It organizes states into groups, making complex behavior easier to manage. -> Option D

4. Quick Check:

   Grouping states = easier management [OK]

Hint: Think of states like folders inside folders for clarity [OK]

Common Mistakes:

• Thinking events are not needed in hierarchical state machines
• Believing states run independently without interaction
• Assuming no transitions are required

2. Which of the following is the correct way to define a sub-state inside a parent state in embedded C code for a hierarchical state machine?

easy

A. typedef enum { STATE_PARENT, STATE_CHILD } State;

B. typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD as a separate enum

C. typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD nested inside STATE_PARENT logically

D. typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD declared inside STATE_PARENT block

Solution

1. Step 1: Understand enum usage for states

   In embedded C, states are often defined as enums. Logical nesting is done by naming and design, not by code nesting.

2. Step 2: Identify correct sub-state definition

   typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD nested inside STATE_PARENT logically correctly states that STATE_CHILD is logically nested inside STATE_PARENT, even though enums are flat.

3. Final Answer:

   typedef enum { STATE_PARENT, STATE_CHILD } State; // with STATE_CHILD nested inside STATE_PARENT logically -> Option C

4. Quick Check:

   Logical nesting, not code nesting [OK]

Hint: Enums are flat; nesting is logical, not syntax [OK]

Common Mistakes:

• Trying to nest enums inside enums (not allowed in C)
• Assuming code blocks define sub-states
• Declaring sub-states as separate enums without relation

3. Given the following code snippet for a hierarchical state machine, what will be the output when event EVENT_START occurs in STATE_IDLE?

#define EVENT_START 1
typedef enum { STATE_IDLE, STATE_ACTIVE, STATE_ACTIVE_SUB } State;
State current_state = STATE_IDLE;

void handle_event(int event) {
  switch (current_state) {
    case STATE_IDLE:
      if (event == EVENT_START) {
        current_state = STATE_ACTIVE;
        printf("Entered ACTIVE state\n");
      }
      break;
    case STATE_ACTIVE:
      if (event == EVENT_START) {
        current_state = STATE_ACTIVE_SUB;
        printf("Entered ACTIVE_SUB state\n");
      }
      break;
    default:
      printf("No action\n");
  }
}

handle_event(EVENT_START);

medium

A. No action

B. Entered ACTIVE state

C. Entered ACTIVE_SUB state

D. Compilation error

Solution

1. Step 1: Check initial state and event

   The current_state is STATE_IDLE. The event is EVENT_START.

2. Step 2: Follow switch-case logic for STATE_IDLE

   On EVENT_START in STATE_IDLE, current_state changes to STATE_ACTIVE and prints "Entered ACTIVE state".

3. Final Answer:

   Entered ACTIVE state -> Option B

4. Quick Check:

   STATE_IDLE + EVENT_START = Entered ACTIVE state [OK]

Hint: Trace current state before event handling [OK]

Common Mistakes:

• Assuming it jumps directly to sub-state
• Ignoring initial state value
• Confusing event handling in different states

4. Identify the error in this hierarchical state machine event handler code snippet:

#define EVENT_GO 1
typedef enum { STATE_TOP, STATE_SUB } State;
State current_state = STATE_TOP;

void handle_event(int event) {
  switch (current_state) {
    case STATE_TOP:
      if (event = EVENT_GO) {
        current_state = STATE_SUB;
        printf("Moved to SUB state\n");
      }
      break;
    case STATE_SUB:
      printf("In SUB state\n");
      break;
  }
}

medium

A. Using assignment (=) instead of comparison (==) in if condition

B. Missing default case in switch statement

C. Incorrect enum values for states

D. No return statement in function

Solution

1. Step 1: Examine if condition in STATE_TOP case

   The condition uses single '=' which assigns EVENT_GO to event instead of comparing.

2. Step 2: Understand impact of assignment in if

   This causes the condition to always be true if EVENT_GO is non-zero, leading to unintended behavior.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in if condition -> Option A

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Check if conditions for == vs = errors [OK]

Common Mistakes:

• Confusing assignment and comparison operators
• Ignoring missing default case (not critical here)
• Expecting return in void function

5. You want to design a hierarchical state machine in embedded C where STATE_RUNNING has two sub-states: STATE_RUNNING_FAST and STATE_RUNNING_SLOW. Which approach correctly models this hierarchy and allows shared entry actions for STATE_RUNNING and specific actions for its sub-states?

hard

A. Define separate enums for all states and use nested switch-case to handle parent and sub-states with shared and specific actions.

B. Use a single enum with flat states and duplicate code for shared actions in each sub-state case.

C. Use multiple variables to track parent and sub-state separately without nesting logic.

D. Implement sub-states as independent states without any relation to the parent state.

Solution

1. Step 1: Understand hierarchical state machine design

   Parent states group sub-states to share common behavior and allow specific sub-state actions.

2. Step 2: Identify correct coding approach

   Defining separate enums for parent and sub-states and using nested switch-case allows shared entry actions for parent and specific actions for sub-states efficiently.

3. Final Answer:

   Define separate enums for all states and use nested switch-case to handle parent and sub-states with shared and specific actions. -> Option A

4. Quick Check:

   Nested switch-case models hierarchy best [OK]

Hint: Use nested switch-case for parent and sub-state actions [OK]

Common Mistakes:

• Duplicating shared actions in sub-states
• Tracking states separately without hierarchy
• Ignoring parent-child relationship in code