Reading data from SD card in Arduino - Time & Space Complexity

Time Complexity: Reading data from SD card

O(n)

Understanding Time Complexity

When reading data from an SD card, it is important to understand how the time taken grows as the amount of data increases.

We want to know how the program's running time changes when reading more data.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#include <SD.h>
File dataFile;

void setup() {
  SD.begin(10);
  dataFile = SD.open("data.txt");
}

void loop() {
  while (dataFile.available()) {
    char c = dataFile.read();
  }
  dataFile.close();
  while(true) {}
}
    

This code opens a file on the SD card and reads it character by character until the end.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Reading each character from the file inside the while loop.
• How many times: Once for every character in the file until no more data is available.

How Execution Grows With Input

As the file size grows, the number of read operations grows directly with it.

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

Pattern observation: The time grows in a straight line as the file size increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to read the file grows directly with the number of characters in the file.

Common Mistake

[X] Wrong: "Reading from the SD card takes the same time no matter how big the file is."

[OK] Correct: Each character must be read one by one, so more data means more time spent reading.

Interview Connect

Understanding how reading data scales helps you write efficient code for devices with limited speed and memory.

Self-Check

"What if we read the file in larger chunks instead of one character at a time? How would the time complexity change?"

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