Reading data from SD card in Arduino - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to include the SD library.

Arduino

#include [1]

Drag options to blanks, or click blank then click option'

A<SD.h>

B<SPI.h>

C<Wire.h>

D<EEPROM.h>

Attempts:

3 left

💡 Hint

Common Mistakes

Including SPI.h instead of SD.h

Forgetting to include any library

The SD library is included with #include to work with SD cards.

2fill in blank

medium

Complete the code to initialize the SD card in setup().

Arduino

if (!SD.[1](4)) {
  Serial.println("Initialization failed!");
  return;
}

Drag options to blanks, or click blank then click option'

Ainit

Bopen

Cstart

Dbegin

Attempts:

3 left

💡 Hint

Common Mistakes

Using SD.init() instead of SD.begin()

Using wrong pin number

SD.begin(4) initializes the SD card on pin 4.

3fill in blank

hard

Fix the error in opening the file for reading.

Arduino

File dataFile = SD.[1]("test.txt", FILE_READ);

Drag options to blanks, or click blank then click option'

AopenFile

Bread

Copen

DreadFile

Attempts:

3 left

💡 Hint

Common Mistakes

Using SD.read() which reads data, not opens file

Using non-existent functions like openFile

Use SD.open() to open a file for reading or writing.

4fill in blank

hard

Fill both blanks to read and print each character from the file.

Arduino

while (dataFile.[1]()) {
  char c = dataFile.[2]();
  Serial.print(c);
}

Drag options to blanks, or click blank then click option'

Aavailable

Bread

Copen

Dclose

Attempts:

3 left

💡 Hint

Common Mistakes

Using open() or close() instead of available() or read()

Not checking if data is available before reading

Use available() to check if data remains and read() to get each byte.

5fill in blank

hard

Fill all three blanks to close the file and print a message.

Arduino

dataFile.[1]();
Serial.[2]("File closed.");
while (!Serial.[3]()) {}

Drag options to blanks, or click blank then click option'

Aclose

Bprintln

Cavailable

Dprint

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to close the file

Using print() instead of println() for message

Not waiting for serial data before continuing

Close the file with close(), print message with println(), and wait for serial data with available().

Practice

1. What is the correct function to open a file for reading from an SD card in Arduino?

easy

A. SD.open(filename, FILE_READ)

B. SD.read(filename)

C. SD.begin(filename)

D. SD.load(filename)

Solution

1. Step 1: Identify the function to open files on SD card

   The Arduino SD library uses SD.open() to open files.

2. Step 2: Specify the mode for reading

   To read a file, the mode FILE_READ must be passed as the second argument.

3. Final Answer:

   SD.open(filename, FILE_READ) -> Option A

4. Quick Check:

   Open file for reading = SD.open(filename, FILE_READ) [OK]

Hint: Use SD.open() with FILE_READ to open files for reading [OK]

Common Mistakes:

• Using SD.read() to open files
• Forgetting FILE_READ mode
• Using SD.begin() to open files
• Using SD.load() which doesn't exist

2. Which of the following is the correct syntax to check if a file opened successfully before reading?

easy

A. if (file != NULL) { /* read file */ }

B. if (file.isOpen()) { /* read file */ }

C. if (file) { /* read file */ }

D. if (file.exists()) { /* read file */ }

Solution

1. Step 1: Understand file object truthiness in Arduino

   In Arduino, File objects can be checked directly in if statements to see if they opened successfully.

2. Step 2: Recognize correct syntax

   Using if (file) is the standard way to check if the file opened.

3. Final Answer:

   if (file) { /* read file */ } -> Option C

4. Quick Check:

   Check file open with if (file) [OK]

Hint: Use if (file) to check if file opened successfully [OK]

Common Mistakes:

• Comparing file to NULL (not Arduino style)
• Using file.isOpen() which doesn't exist
• Using file.exists() which checks SD, not file open

3. What will be the output of this Arduino code snippet reading from an SD card file named "data.txt" containing the text "Hello"?

File file = SD.open("data.txt", FILE_READ);
if (file) {
  char c = file.read();
  Serial.println(c);
  file.close();
} else {
  Serial.println("Error opening file");
}

medium

A. H

B. Hello

C. Error opening file

D. 104

Solution

1. Step 1: Understand file.read() returns a byte

   The read() method returns the ASCII value of the first character, but when assigned to a char and printed with Serial.println(char), it prints the character itself.

2. Step 2: Check code carefully

   Since c is a char and Serial.println(c) prints the character, the output is 'H'.

3. Final Answer:

   H -> Option A

4. Quick Check:

   file.read() returns char 'H' printed as H [OK]

Hint: file.read() returns one char; Serial.println prints char [OK]

Common Mistakes:

• Thinking read() returns whole string
• Expecting ASCII number printed
• Confusing file.read() with readStringUntil()

4. Identify the error in this Arduino code snippet that reads a line from an SD card file:

File file = SD.open("log.txt", FILE_READ);
if (file) {
  String line = file.readStringUntil('\n');
  Serial.println(line);
}
file.close();

medium

A. readStringUntil() is not a valid File method

B. file.close() is called outside the if block

C. Missing SD.begin() before opening file

D. Using single quotes instead of double quotes in readStringUntil

Solution

1. Step 1: Check file.close() placement

   file.close() should be called only if file opened successfully, so it must be inside the if block.

2. Step 2: Understand consequences

   If file failed to open, calling file.close() causes error because file is invalid.

3. Final Answer:

   file.close() is called outside the if block -> Option B

4. Quick Check:

   Close file only if opened [OK]

Hint: Call file.close() only inside if (file) block [OK]

Common Mistakes:

• Calling file.close() unconditionally
• Assuming readStringUntil() is invalid
• Forgetting SD.begin() but not shown here
• Confusing single vs double quotes in char argument

5. You want to read all lines from a file "config.txt" on an SD card and print each line to Serial. Which code snippet correctly does this?

hard

A. File file = SD.open("config.txt", FILE_READ);
while (file) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

B. File file = SD.open("config.txt", FILE_READ);
while (!file.available()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

C. File file = SD.open("config.txt", FILE_READ);
while (file.read()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

D. File file = SD.open("config.txt", FILE_READ);
while (file.available()) {
Serial.println(file.readStringUntil('\n'));
}
file.close();

Solution

1. Step 1: Use file.available() to check for data

   file.available() returns true if there is data left to read, so loop while available() is true.

2. Step 2: Read each line with readStringUntil('\n')

   This method reads until newline, perfect for line-by-line reading.

3. Step 3: Close file after reading

   Call file.close() after finishing reading to free resources.

4. Final Answer:

   Use while (file.available()) with readStringUntil('\n') -> Option D

5. Quick Check:

   Loop with file.available() to read lines [OK]

Hint: Loop while file.available() to read all lines [OK]

Common Mistakes:

• Using while (!file.available()) which loops incorrectly
• Using while (file.read()) which reads bytes, not lines
• Using while (file) which doesn't check data availability