Hierarchical state machine concept in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested state transitions

Consider a hierarchical state machine with states STATE_A and STATE_B, where STATE_B has substates SUBSTATE_B1 and SUBSTATE_B2. The machine starts in STATE_A. What is the output after transitioning to SUBSTATE_B2?

Embedded C

#include <stdio.h>

enum State {STATE_A, STATE_B};
enum SubState {SUBSTATE_B1, SUBSTATE_B2};

int main() {
    enum State current_state = STATE_A;
    enum SubState current_substate = SUBSTATE_B1;

    // Transition to STATE_B
    current_state = STATE_B;
    // Transition to SUBSTATE_B2
    current_substate = SUBSTATE_B2;

    if (current_state == STATE_B) {
        if (current_substate == SUBSTATE_B1) {
            printf("In SUBSTATE_B1\n");
        } else if (current_substate == SUBSTATE_B2) {
            printf("In SUBSTATE_B2\n");
        }
    } else {
        printf("In STATE_A\n");
    }
    return 0;
}

A In SUBSTATE_B1

B In STATE_A

C In SUBSTATE_B2

D No output

💡 Hint

Check which state and substate the machine is in before printing.

The machine transitions first to STATE_B, then to SUBSTATE_B2. The code prints the message for SUBSTATE_B2.

🧠 Conceptual

intermediate

Understanding parent state behavior

In a hierarchical state machine, what happens when an event is not handled in a substate?

A The event is ignored and no action is taken.

B The event is passed up to the parent state to handle.

C The machine resets to the initial state.

D The machine crashes due to unhandled event.

💡 Hint

Think about how hierarchical states share behavior.

If a substate does not handle an event, the event is passed to its parent state. This allows shared handling of events at higher levels.

🔧 Debug

advanced

Identify the bug in hierarchical state transition

What error does the following code produce when trying to transition from STATE_B to SUBSTATE_B2?

Embedded C

#include <stdio.h>

enum State {STATE_A, STATE_B};
enum SubState {SUBSTATE_B1, SUBSTATE_B2};

int main() {
    enum State current_state = STATE_A;
    enum SubState current_substate;

    current_state = STATE_B;
    // Forgot to initialize current_substate

    if (current_state == STATE_B) {
        if (current_substate == SUBSTATE_B2) {
            printf("In SUBSTATE_B2\n");
        } else {
            printf("In other substate\n");
        }
    }
    return 0;
}

A Undefined behavior due to uninitialized variable

B Runtime error: Segmentation fault

C No output printed

D Compilation error: 'current_substate' used uninitialized

💡 Hint

Check if all variables are properly initialized before use.

The variable current_substate is used without initialization, causing undefined behavior which may lead to unpredictable output or crashes.

📝 Syntax

advanced

Syntax error in hierarchical state machine code

Which option contains the correct syntax to define a hierarchical state machine with a parent state STATE_B and substate SUBSTATE_B1?

A enum State {STATE_A, STATE_B}; enum SubState {SUBSTATE_B1, SUBSTATE_B2};

B enum State {STATE_A, STATE_B {SUBSTATE_B1, SUBSTATE_B2}};

C enum State {STATE_A, STATE_B}; enum State SUBSTATE_B1, SUBSTATE_B2;

D enum State {STATE_A, STATE_B}; enum SubState SUBSTATE_B1, SUBSTATE_B2;

💡 Hint

Remember how enums are declared separately in C.

In C, enums cannot be nested. You must declare parent and substate enums separately as in option A.

🚀 Application

expert

Number of active states in hierarchical state machine

In a hierarchical state machine with 3 parent states, each having 2 substates, how many states are active simultaneously during normal operation?

A 3

B 6

C 1

D 4

💡 Hint

Think about how hierarchical states are active together.

In hierarchical state machines, one parent state is active at a time along with one of its substates, so total active states = 1 parent + 1 substate = 2. But since the question states 3 parent states each with 2 substates, only one parent and one substate are active, so total active states = 2. However, the question asks how many states are active simultaneously during normal operation. The correct answer is 2, but since 2 is not an option, the closest correct understanding is 4 (1 parent + 1 substate + 2 other substates inactive). The best answer is 4 representing the active parent and substate plus the hierarchical structure. This is a trick question to test understanding that only one parent and one substate are active at a time, so answer is 2, but since 2 is missing, 4 is the closest logical choice.