Simple state machine with switch-case in Embedded C

Introduction

A state machine helps a program remember what it is doing and decide what to do next. Using switch-case makes it easy to check the current state and act accordingly.

Controlling a traffic light that changes colors in order.

Managing a simple game where the player moves between menus and playing states.

Handling a device that turns on, waits, and then turns off automatically.

Reading buttons where each press changes the mode of operation.

Syntax

Embedded C

switch (state) {
    case STATE1:
        // actions for STATE1
        break;
    case STATE2:
        // actions for STATE2
        break;
    // add more states as needed
    default:
        // default action
        break;
}

Each case matches a state value.

Use break; to stop checking other cases after one matches.

Examples

Simple two-state machine: IDLE and RUNNING.

Embedded C

switch (state) {
    case IDLE:
        // wait for event
        break;
    case RUNNING:
        // perform task
        break;
}

Traffic light cycle switching states inside each case.

Embedded C

switch (state) {
    case RED:
        // turn on red light
        state = GREEN;
        break;
    case GREEN:
        // turn on green light
        state = YELLOW;
        break;
    case YELLOW:
        // turn on yellow light
        state = RED;
        break;
}

Sample Program

This program cycles through three states: OFF, ON, and BLINK. It prints the current state and then moves to the next one. It repeats this 5 times.

Embedded C

#include <stdio.h>

// Define states
#define STATE_OFF 0
#define STATE_ON 1
#define STATE_BLINK 2

int main() {
    int state = STATE_OFF;
    int count = 0;

    while (count < 5) {
        switch (state) {
            case STATE_OFF:
                printf("State: OFF\n");
                state = STATE_ON;
                break;
            case STATE_ON:
                printf("State: ON\n");
                state = STATE_BLINK;
                break;
            case STATE_BLINK:
                printf("State: BLINK\n");
                state = STATE_OFF;
                break;
            default:
                printf("Unknown state\n");
                state = STATE_OFF;
                break;
        }
        count++;
    }
    return 0;
}

Important Notes

Always include a default case to catch unexpected states.

Remember to update the state inside each case to move the machine forward.

Summary

A state machine keeps track of what the program is doing.

Use switch-case to handle different states clearly.

Change the state inside cases to move between states.

Practice

1.

What is the main purpose of using a switch-case statement in a simple state machine?

easy

A. To create loops that repeat forever

B. To handle different states clearly and execute code based on the current state

C. To declare variables for each state

D. To stop the program immediately

Solution

1. Step 1: Understand the role of switch-case in state machines

   The switch-case statement lets the program choose what to do based on the current state value.

2. Step 2: Recognize how states are handled

   Each case corresponds to a state, so the program runs code specific to that state.

3. Final Answer:

   To handle different states clearly and execute code based on the current state -> Option B

4. Quick Check:

   switch-case manages states = To handle different states clearly and execute code based on the current state [OK]

Hint: Switch-case matches states to actions clearly [OK]

Common Mistakes:

• Thinking switch-case creates loops
• Confusing variable declaration with state handling
• Assuming switch-case stops the program

2.

Which of the following is the correct syntax for a switch statement in embedded C?

switch(state) {
  case 0:
    // code
    break;
  case 1:
    // code
    break;
  default:
    // code
}

easy

A. switch state { case 0: ... break; case 1: ... break; default: ... }

B. switch(state) { case 0: ... case 1: ... default: ... }

C. switch(state) case 0: ... break; case 1: ... break; default: ...

D. switch(state) { case 0: ... break; case 1: ... break; default: ... }

Solution

1. Step 1: Check the syntax structure of switch-case

   The switch statement requires parentheses around the variable and braces around the cases.

2. Step 2: Verify the use of colons and breaks

   Each case must have a colon and usually ends with a break to prevent fall-through.

3. Final Answer:

   switch(state) { case 0: ... break; case 1: ... break; default: ... } -> Option D

4. Quick Check:

   Correct switch-case syntax = switch(state) { case 0: ... break; case 1: ... break; default: ... } [OK]

Hint: Remember switch needs parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses after switch
• Missing colons after case labels
• Forgetting break statements

3.

What will be the output of this code snippet?

int state = 1;
switch(state) {
  case 0:
    printf("State is zero\n");
    break;
  case 1:
    printf("State is one\n");
    break;
  default:
    printf("Unknown state\n");
}

medium

A. Unknown state

B. State is zero

C. State is one

D. No output

Solution

1. Step 1: Identify the value of state

   The variable state is set to 1 before the switch statement.

2. Step 2: Match the case with state value

   The switch matches case 1, so it prints "State is one" and then breaks.

3. Final Answer:

   State is one -> Option C

4. Quick Check:

   state=1 prints "State is one" [OK]

Hint: Match state value to case label to find output [OK]

Common Mistakes:

• Confusing case 0 with case 1
• Forgetting break causes fall-through
• Assuming default runs when case matches

4.

Find the error in this state machine code snippet:

int state = 0;
switch(state) {
  case 0:
    printf("Start\n");
  case 1:
    printf("Running\n");
    break;
  default:
    printf("Unknown\n");
}

medium

A. Missing break after case 0 causes fall-through

B. Switch statement missing parentheses

C. Default case should be first

D. Variable state is not initialized

Solution

1. Step 1: Check each case for break statements

   Case 0 does not have a break, so after printing "Start", it continues to case 1.

2. Step 2: Understand fall-through effect

   This causes both "Start" and "Running" to print when state is 0, which may be unintended.

3. Final Answer:

   Missing break after case 0 causes fall-through -> Option A

4. Quick Check:

   Missing break causes fall-through = Missing break after case 0 causes fall-through [OK]

Hint: Always add break after each case to avoid fall-through [OK]

Common Mistakes:

• Ignoring missing break leads to unexpected output
• Thinking default must be first case
• Assuming uninitialized variable error

5.

You want to design a simple state machine with three states: INIT (0), PROCESS (1), and DONE (2). The machine should start at INIT, then move to PROCESS, and finally to DONE. Which code snippet correctly updates the state inside the switch-case?

int state = 0;
switch(state) {
  case 0:
    // Move to PROCESS
    state = 1;
    break;
  case 1:
    // Move to DONE
    state = 2;
    break;
  case 2:
    // Finished
    break;
}

hard

A. Correctly updates state from INIT to PROCESS to DONE

B. Does not update state, stays in INIT forever

C. Skips PROCESS and goes directly to DONE

D. Causes infinite loop without state change

Solution

1. Step 1: Check state transitions in each case

   Case 0 sets state to 1 (PROCESS), case 1 sets state to 2 (DONE), case 2 does nothing (finished).

2. Step 2: Confirm the order of states

   The code moves from INIT to PROCESS to DONE in order, updating the state variable correctly.

3. Final Answer:

   Correctly updates state from INIT to PROCESS to DONE -> Option A

4. Quick Check:

   State updates follow sequence = Correctly updates state from INIT to PROCESS to DONE [OK]

Hint: Update state variable inside each case to move forward [OK]

Common Mistakes:

• Not updating state variable inside cases
• Skipping intermediate states
• Forgetting break causing unexpected behavior