String parsing from serial input in Arduino - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Arduino serial parsing code?

Consider this Arduino code snippet that reads a serial input string and extracts a number:

String input = "TEMP:25;";
int value = 0;

int index = input.indexOf(':');
if (index != -1) {
  String numberPart = input.substring(index + 1, input.indexOf(';'));
  value = numberPart.toInt();
}
Serial.println(value);

What will be printed on the Serial Monitor?

Arduino

String input = "TEMP:25;";
int value = 0;

int index = input.indexOf(':');
if (index != -1) {
  String numberPart = input.substring(index + 1, input.indexOf(';'));
  value = numberPart.toInt();
}
Serial.println(value);

A 25

B 0

C TEMP

D Syntax error

💡 Hint

Look at how substring and toInt() work on the extracted part.

The code finds the colon, extracts the substring between ':' and ';' which is "25", then converts it to integer 25 and prints it.

🧠 Conceptual

intermediate

Which function reads incoming serial data as a string until a newline?

In Arduino, which function reads characters from the serial buffer until it encounters a newline character and returns the result as a String?

A Serial.available()

B Serial.read()

C Serial.readStringUntil('\n')

D Serial.parseInt()

💡 Hint

Think about reading until a specific character.

Serial.readStringUntil('\n') reads characters until it finds a newline, returning the collected string.

🔧 Debug

advanced

What error does this Arduino code cause when parsing serial input?

Analyze this Arduino code snippet:

String data = Serial.readStringUntil(';');
int val = data.toInt();
Serial.println(val);

If the serial input is "123abc;", what happens?

Arduino

String data = Serial.readStringUntil(';');
int val = data.toInt();
Serial.println(val);

A Prints 123

B Prints 0

C Causes a runtime error

D Prints 123abc

💡 Hint

Check how toInt() handles strings with trailing letters.

toInt() converts the initial numeric part and stops at the first non-digit, so it returns 123.

📝 Syntax

advanced

Which option causes a syntax error in Arduino string parsing?

Which of these Arduino code snippets will cause a syntax error?

1) String s = Serial.readStringUntil(';');
2) int n = s.toInt();
3) String part = s.substring(2, 5);
4) int x = s.substring(1, 3).toInt()

A Option 3 (substring parameters invalid)

B Option 1 (wrong function name)

C Option 2 (toInt() not valid)

D Option 4 (missing semicolon)

💡 Hint

Look carefully for missing punctuation.

Option 4 misses a semicolon at the end, causing a syntax error.

🚀 Application

expert

How many items are in the parsed array after splitting serial input?

Given this Arduino code:

String input = "CMD,100,ON,45";
int count = 0;
int start = 0;
int commaIndex = -1;

while ((commaIndex = input.indexOf(',', start)) != -1) {
  String part = input.substring(start, commaIndex);
  count++;
  start = commaIndex + 1;
}
count++;
Serial.println(count);

What number will be printed?

Arduino

String input = "CMD,100,ON,45";
int count = 0;
int start = 0;
int commaIndex = -1;

while ((commaIndex = input.indexOf(',', start)) != -1) {
  String part = input.substring(start, commaIndex);
  count++;
  start = commaIndex + 1;
}
count++;
Serial.println(count);

A 3

B 4

C 5

D 2

💡 Hint

Count how many commas split the string and add one.

The string has 3 commas, so 4 parts. The code counts each comma found and adds one at the end.

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