Writing text files (writetable, fprintf) in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the writetable function do in MATLAB?

The writetable function saves a table variable to a text file, such as a CSV or TXT file, making it easy to export structured data.

beginner

How do you use fprintf to write formatted text to a file in MATLAB?

You first open a file with fopen, then use fprintf with a file identifier and a format string to write text, and finally close the file with fclose.

beginner

What is the purpose of the format specifier in fprintf?

The format specifier tells MATLAB how to display the data (like %d for integers, %f for floating-point numbers, %s for strings), controlling the output style.

beginner

Why should you always use fclose after writing to a file?

Closing the file with fclose ensures all data is saved properly and frees system resources, preventing file corruption or access issues.

intermediate

Can writetable write files other than CSV? Give an example.

Yes, writetable can write text files like tab-delimited TXT files by specifying the delimiter option, e.g., writetable(T, 'file.txt', 'Delimiter', '\t').

Which MATLAB function is best for writing a table directly to a CSV file?

A writetable

B fprintf

C fopen

D load

writetable is designed to save tables directly to files like CSV.

What must you do before using fprintf to write to a file?

A Use <code>load</code> first

B Close the file with <code>fclose</code>

C Convert data to string

D Open the file with <code>fopen</code>

You must open the file with fopen to get a file ID for fprintf.

What does the format specifier %f mean in fprintf?

A Floating-point number

B Integer

C String

D Character

%f formats numbers as floating-point values.

What happens if you forget to call fclose after writing a file?

A Nothing happens

B File deletes automatically

C Data might not be saved properly

D MATLAB crashes

Not closing the file can cause data loss or file corruption.

Which option allows writetable to write tab-delimited files?

A 'Format', '%s'

B 'Delimiter', '\t'

C 'Mode', 'append'

D 'Encoding', 'UTF-8'

Setting 'Delimiter', '\t' writes tab-separated values.

Explain the steps to write a matrix of numbers to a text file using fprintf in MATLAB.

Think about opening, writing with formatting, and closing.

Describe how writetable simplifies saving data compared to fprintf.

Focus on ease and automation.

Practice

1. Which MATLAB function is best suited to save a table as a CSV file quickly?

easy

A. writetable

B. fprintf

C. fopen

D. load

Solution

1. Step 1: Understand the purpose of writetable

   writetable is designed to save MATLAB tables directly to files like CSV with minimal code.

2. Step 2: Compare with other functions

   fprintf is for custom text formatting, fopen opens files but doesn't save tables, and load is for loading data, not saving.

3. Final Answer:

   writetable -> Option A

4. Quick Check:

   Save table fast = writetable [OK]

Hint: Use writetable to save tables directly as CSV files [OK]

Common Mistakes:

• Confusing fprintf with writetable for saving tables
• Using fopen alone to save data
• Thinking load saves files

2. Which of the following is the correct syntax to open a file named 'data.txt' for writing in MATLAB?

easy

A. fid = fopen('data.txt', 'w');

B. fid = fopen('data.txt', 'r');

C. fid = fopen('data.txt', 'a');

D. fid = fopen('data.txt', 'x');

Solution

1. Step 1: Understand fopen modes

   'r' is read-only, 'w' is write (creates or overwrites), 'a' appends, 'x' creates new file but errors if exists.

2. Step 2: Choose mode for writing

   To write fresh data, use 'w' mode to open file for writing.

3. Final Answer:

   fid = fopen('data.txt', 'w'); -> Option A

4. Quick Check:

   Open file for writing = 'w' mode [OK]

Hint: 'w' mode in fopen opens file for writing (overwrite) [OK]

Common Mistakes:

• Using 'r' mode which is read-only
• Using 'a' when overwrite is needed
• Confusing 'x' mode with 'w'

3. What will be the output file content after running this MATLAB code?

fid = fopen('output.txt', 'w');
fprintf(fid, 'Value: %d\n', 42);
fclose(fid);

medium

A. 42

B. Value: 42

C. Value: %d

D. Error: File not closed

Solution

1. Step 1: Analyze fprintf formatting

   fprintf writes formatted text. '%d' is replaced by integer 42, '\n' adds a newline.

2. Step 2: Check file closing

   fclose(fid) properly closes the file, so no error occurs.

3. Final Answer:

   Value: 42 -> Option B

4. Quick Check:

   Formatted integer output = Value: 42 [OK]

Hint: fprintf replaces %d with number, remember to fclose [OK]

Common Mistakes:

• Forgetting to close the file
• Expecting %d to appear literally
• Confusing output with just the number

4. Identify the error in this MATLAB code snippet that writes numbers to a file:

fid = fopen('numbers.txt', 'w');
fprintf(fid, '%d\n', 1:5);
% Missing fclose(fid)

medium

A. Missing file open mode

B. Using range 1:5 is invalid

C. Incorrect format specifier in fprintf

D. Missing fclose to close the file

Solution

1. Step 1: Check file opening

   fopen uses 'w' mode correctly, so file opens for writing.

2. Step 2: Check file closing

   The code does not call fclose(fid), which is necessary to save and release the file.

3. Final Answer:

   Missing fclose to close the file -> Option D

4. Quick Check:

   Always fclose after writing [OK]

Hint: Always fclose after writing to save file properly [OK]

Common Mistakes:

• Forgetting fclose causes incomplete writes
• Thinking fprintf needs different format for vectors
• Assuming fopen mode is missing

5. You want to save a table T with columns 'Name' and 'Score' to a CSV file, but only include rows where Score > 50. Which MATLAB code correctly does this?

hard

A. fid = fopen('filtered.csv', 'w'); fprintf(fid, '%s, %d\n', T.Name, T.Score); fclose(fid);

B. fprintf('filtered.csv', '%s, %d\n', T.Name, T.Score);

C. writetable(T(T.Score > 50, :), 'filtered.csv');

D. writetable(T, 'filtered.csv');

Solution

1. Step 1: Filter rows where Score > 50

   Use logical indexing: T.Score > 50 selects rows with Score greater than 50.

2. Step 2: Use writetable to save filtered table

   Pass filtered rows T(T.Score > 50, :) to writetable to save only those rows.

3. Final Answer:

   writetable(T(T.Score > 50, :), 'filtered.csv'); -> Option C

4. Quick Check:

   Filter rows then writetable = writetable(T(T.Score > 50, :), 'filtered.csv'); [OK]

Hint: Filter table rows before writetable to save subset [OK]

Common Mistakes:

• Using fprintf without opening file
• Not filtering rows before saving
• Saving entire table without filtering