String parsing from serial input in Arduino

Introduction

We use string parsing from serial input to read and understand text sent to the Arduino. This helps the Arduino react to commands or data from a computer or sensor.

When you want to control an Arduino using commands typed on a computer.

When reading sensor data sent as text from another device.

When you need to split a sentence or command into parts to understand it.

When you want to check if a specific word or number was sent to the Arduino.

When you want to convert text input into numbers for calculations.

Syntax

Arduino

if (Serial.available() > 0) {
  String input = Serial.readStringUntil('\n');
  // parse input here
}

Serial.available() checks if data is ready to read.

Serial.readStringUntil('\n') reads characters until a newline is found.

Examples

Reads a command and checks if it is "ON".

Arduino

if (Serial.available()) {
  String command = Serial.readStringUntil('\n');
  if (command == "ON") {
    // turn something on
  }
}

Reads a number sent as text and converts it to an integer.

Arduino

if (Serial.available()) {
  String data = Serial.readStringUntil('\n');
  int value = data.toInt();
  // use value as a number
}

Splits input text into two parts separated by a comma.

Arduino

if (Serial.available()) {
  String input = Serial.readStringUntil('\n');
  int commaIndex = input.indexOf(',');
  String part1 = input.substring(0, commaIndex);
  String part2 = input.substring(commaIndex + 1);
  // use part1 and part2
}

Sample Program

This program waits for you to type a command in the Serial Monitor. If you type "ON", it replies "LED turned ON". If you type "OFF", it replies "LED turned OFF". For anything else, it says "Unknown command".

Arduino

#include <Arduino.h>

void setup() {
  Serial.begin(9600);
  while (!Serial) { ; }
  Serial.println("Send a command like ON or OFF:");
}

void loop() {
  if (Serial.available() > 0) {
    String input = Serial.readStringUntil('\n');
    input.trim();
    if (input == "ON") {
      Serial.println("LED turned ON");
    } else if (input == "OFF") {
      Serial.println("LED turned OFF");
    } else {
      Serial.println("Unknown command");
    }
  }
}

Important Notes

Always call Serial.begin() in setup() to start serial communication.

Use input.trim() to remove extra spaces or newline characters from the input.

Parsing strings carefully helps avoid errors when commands are typed incorrectly.

Summary

String parsing from serial input lets Arduino read and understand text sent from a computer or device.

Use Serial.readStringUntil('\n') to read a full line of input.

Split or convert the input string to use commands or numbers in your program.

Practice

1. What does Serial.readStringUntil('\n') do in Arduino programming?

easy

A. Reads characters from serial input until a newline character is found

B. Sends a newline character over serial

C. Clears the serial buffer

D. Reads only one character from serial input

Solution

1. Step 1: Understand the function purpose

   Serial.readStringUntil('\n') reads characters from the serial buffer until it finds the newline character '\n'.

2. Step 2: Compare with other options

   It does not send data, clear buffer, or read only one character; it reads a full line until newline.

3. Final Answer:

   Reads characters from serial input until a newline character is found -> Option A

4. Quick Check:

   Read until '\n' means read full line [OK]

Hint: Remember '\n' means newline, so it reads until line ends [OK]

Common Mistakes:

• Thinking it reads only one character
• Confusing reading with sending data
• Assuming it clears the buffer

2. Which of the following is the correct way to read a full line from serial input in Arduino?

easy

A. Serial.read()

B. Serial.available()

C. Serial.readStringUntil('\n')

D. Serial.write()

Solution

1. Step 1: Identify function to read full line

   Serial.readStringUntil('\n') reads characters until newline, capturing a full line.

2. Step 2: Understand other functions

   Serial.read() reads one byte, Serial.write() sends data, and Serial.available() checks bytes available.

3. Final Answer:

   Serial.readStringUntil('\n') -> Option C

4. Quick Check:

   Read full line = readStringUntil('\n') [OK]

Hint: Use readStringUntil('\n') to get whole line input [OK]

Common Mistakes:

• Using Serial.read() to get full line
• Confusing read and write functions
• Using Serial.available() to read data

3. What will be the output of this Arduino code if the serial input is "TEMP:25\n"?

String input = Serial.readStringUntil('\n');
int value = input.substring(5).toInt();
Serial.println(value);

medium

A. 25

B. TEMP:25

C. 0

D. Error

Solution

1. Step 1: Read the input string

   The input string is "TEMP:25" (newline removed by readStringUntil).

2. Step 2: Extract substring and convert to integer

   input.substring(5) takes characters from index 5 onward, which is "25". Then toInt() converts "25" to integer 25.

3. Final Answer:

   25 -> Option A

4. Quick Check:

   Substring from 5 = "25", toInt() = 25 [OK]

Hint: Use substring index to isolate number, then toInt() converts [OK]

Common Mistakes:

• Forgetting substring index starts at 0
• Expecting full string printed
• Not converting substring to int

4. Identify the error in this Arduino code snippet for parsing serial input:

String data = Serial.readStringUntil('\n');
int num = data.toInt();
if(num = 10) {
  Serial.println("Number is 10");
}

medium

A. toInt() cannot convert string to int

B. Missing semicolon after Serial.println

C. Serial.readStringUntil('\n') does not read input

D. Using assignment '=' instead of comparison '==' in if condition

Solution

1. Step 1: Check if condition syntax

   The condition if(num = 10) uses assignment '=' instead of comparison '=='. This causes a bug.

2. Step 2: Verify other parts

   Semicolon is present, toInt() works correctly, and readStringUntil('\n') reads input properly.

3. Final Answer:

   Using assignment '=' instead of comparison '==' in if condition -> Option D

4. Quick Check:

   Use '==' to compare values in if [OK]

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

Common Mistakes:

• Using '=' instead of '==' in conditions
• Assuming toInt() fails on valid numbers
• Thinking readStringUntil doesn't read input

5. You receive serial input in the format "CMD:VALUE\n", for example "LED:1\n". How can you parse the command and value separately in Arduino?

hard

A. Use String input = Serial.readStringUntil('\n'); int val = input.substring(0,3).toInt(); String cmd = input.substring(4);

B. Use String input = Serial.readStringUntil('\n'); String cmd = input.substring(0,3); int val = input.substring(4).toInt();

C. Use String input = Serial.readString(); String cmd = input.split(':')[0]; int val = input.split(':')[1].toInt();

D. Use String input = Serial.read(); String cmd = input.substring(0,3); int val = input.substring(4).toInt();

Solution

1. Step 1: Read full line input

   Serial.readStringUntil('\n') reads the entire line including command and value.

2. Step 2: Extract command and value

   input.substring(0,3) extracts the first 3 characters as command (e.g., "LED"), and input.substring(4).toInt() converts the value part after ':' to integer.

3. Final Answer:

   Use String input = Serial.readStringUntil('\n'); String cmd = input.substring(0,3); int val = input.substring(4).toInt(); -> Option B

4. Quick Check:

   Substring command and value parsing works [OK]

Hint: Use substring with indexes to split command and value [OK]

Common Mistakes:

• Using read() instead of readStringUntil
• Trying to split string with split() which is not available
• Mixing up substring indexes for command and value