Receiving commands from computer in Arduino - Time & Space Complexity

Time Complexity: Receiving commands from computer

O(n)

Understanding Time Complexity

When an Arduino receives commands from a computer, it often reads data repeatedly. Understanding how the time it takes grows as more data comes in helps us write better programs.

We want to know how the time to process commands changes as the number of commands increases.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

void loop() {
  while (Serial.available() > 0) {
    char command = Serial.read();
    // process command
  }
}
    

This code reads all available characters sent from the computer and processes each one.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The while loop that reads each character from the serial buffer.
• How many times: Once for each character available to read.

How Execution Grows With Input

Each new character sent means one more loop iteration to read and process it.

Input Size (n)	Approx. Operations
10	About 10 reads and processes
100	About 100 reads and processes
1000	About 1000 reads and processes

Pattern observation: The time grows directly with the number of commands received.

Final Time Complexity

Time Complexity: O(n)

This means the time to read and process commands grows in a straight line with the number of commands.

Common Mistake

[X] Wrong: "Reading commands takes the same time no matter how many commands arrive."

[OK] Correct: Each command must be read and processed, so more commands mean more time spent.

Interview Connect

Understanding how input size affects processing time helps you write efficient code that handles data smoothly, a skill valuable in many programming tasks.

Self-Check

What if we changed the code to process commands only when a full line is received? How would the time complexity change?

Practice

1. What is the purpose of Serial.begin(9600); in an Arduino sketch?

easy

A. It reads data from the serial port.

B. It starts serial communication at 9600 bits per second.

C. It sends data to the computer automatically.

D. It stops the serial communication.

Solution

1. Step 1: Understand Serial.begin()

   Serial.begin(9600); initializes the serial communication at a speed of 9600 bits per second.

2. Step 2: Identify its role in communication

   This function sets up the Arduino to send and receive data through the serial port at the specified speed.

3. Final Answer:

   It starts serial communication at 9600 bits per second. -> Option B

4. Quick Check:

   Serial.begin() = start communication [OK]

Hint: Serial.begin() always starts communication at given speed [OK]

Common Mistakes:

• Thinking Serial.begin() reads or sends data
• Confusing Serial.begin() with Serial.read()
• Assuming Serial.begin() stops communication

2. Which of the following is the correct way to check if data is available to read from the serial port?

easy

A. if (Serial.available()) { }

B. if (Serial.write()) { }

C. if (Serial.begin()) { }

D. if (Serial.read() > 0) { }

Solution

1. Step 1: Identify function to check data availability

   Serial.available() returns the number of bytes available to read from the serial buffer.

2. Step 2: Understand usage in condition

   Using if (Serial.available()) checks if there is any data to read (non-zero means data is available).

3. Final Answer:

   if (Serial.available()) { } -> Option A

4. Quick Check:

   Serial.available() checks data presence [OK]

Hint: Use Serial.available() to check before reading [OK]

Common Mistakes:

• Using Serial.read() to check availability
• Calling Serial.begin() inside loop
• Using Serial.write() to check data

3. What will be the output on the Serial Monitor if the following code receives the input string "HELLO"?

void setup() {
  Serial.begin(9600);
}

void loop() {
  if (Serial.available()) {
    String command = Serial.readStringUntil('\n');
    Serial.println(command);
  }
}

medium

A. HELLO\n

B. H

C. HELLO

D. No output

Solution

1. Step 1: Understand Serial.readStringUntil()

   This function reads characters from the serial buffer until it finds the newline character '\n'. It returns the string without the '\n'.

2. Step 2: Analyze the code output

   The input "HELLO" followed by Enter sends "HELLO\n". The code reads "HELLO" and prints it exactly.

3. Final Answer:

   HELLO -> Option C

4. Quick Check:

   readStringUntil('\n') returns string without newline [OK]

Hint: readStringUntil('\n') excludes newline from output [OK]

Common Mistakes:

• Expecting newline character printed
• Thinking only one character is read
• Assuming no output without delay

4. Identify the error in this Arduino code snippet that tries to read a command from the serial port:

void loop() {
  if (Serial.available > 0) {
    char c = Serial.read();
    Serial.print(c);
  }
}

medium

A. Serial.available is used without parentheses

B. Serial.read() is missing a parameter

C. Serial.print() cannot print char variables

D. The if condition should check for Serial.read() instead

Solution

1. Step 1: Check Serial.available usage

   Serial.available is a function and must be called with parentheses: Serial.available().

2. Step 2: Verify other function calls

   Serial.read() correctly reads one byte without parameters; Serial.print() can print chars.

3. Final Answer:

   Serial.available is used without parentheses -> Option A

4. Quick Check:

   Functions need parentheses to call [OK]

Hint: Always use parentheses when calling functions like Serial.available() [OK]

Common Mistakes:

• Forgetting parentheses on function calls
• Thinking Serial.read() needs parameters
• Assuming Serial.print() can't print chars

5. You want to receive a command string from the computer and turn on an LED if the command is "ON" and turn it off if "OFF". Which code snippet correctly implements this?

void setup() {
  Serial.begin(9600);
  pinMode(13, OUTPUT);
}

void loop() {
  if (Serial.available()) {
    String cmd = Serial.readStringUntil('\n');
    // What goes here?
  }
}

hard

A. if (cmd === "ON") digitalWrite(13, HIGH); else if (cmd === "OFF") digitalWrite(13, LOW);

B. if (cmd = "ON") digitalWrite(13, HIGH); else if (cmd = "OFF") digitalWrite(13, LOW);

C. if (cmd.equal("ON")) digitalWrite(13, HIGH); else if (cmd.equal("OFF")) digitalWrite(13, LOW);

D. if (cmd == "ON") digitalWrite(13, HIGH); else if (cmd == "OFF") digitalWrite(13, LOW);

Solution

1. Step 1: Understand String comparison in Arduino

   Arduino String objects overload the == operator to compare contents with string literals like "ON".

2. Step 2: Check each option's correctness

   if (cmd == "ON") correctly compares string contents. if (cmd = "ON") performs assignment, not comparison. cmd.equal("ON") fails--no such method (it's equals()). === is invalid C++ syntax.

3. Final Answer:

   if (cmd == "ON") digitalWrite(13, HIGH); else if (cmd == "OFF") digitalWrite(13, LOW); -> Option D

4. Quick Check:

   Arduino String == compares content [OK]

Hint: Use cmd == "ON" to compare Arduino Strings [OK]

Common Mistakes:

• Using = instead of == for comparison
• Calling non-existent cmd.equal()
• Using JavaScript === operator in Arduino