Why Writing text files (writetable, fprintf) in MATLAB? - Purpose & Use Cases

The Big Idea

What if you could save your data perfectly every time with just one command?

The Scenario

Imagine you have a list of student scores in a class, and you want to save them to a file so you can share or review later. Doing this by hand means opening a text editor, typing each score carefully, and formatting it so others can understand.

The Problem

Manually typing data is slow and easy to mess up. You might forget a score, make typos, or format the file inconsistently. If the data changes, you have to redo everything from scratch, which wastes time and causes frustration.

The Solution

Using writetable or fprintf in MATLAB lets you save data automatically and neatly. These commands write your data directly to a file with the right format, so you don't have to type or worry about mistakes.

Before vs After

✗ Before

Open text editor
Type: "John, 85"
Type: "Mary, 90"
Save file

✓ After

T = table({'John';'Mary'}, [85;90], 'VariableNames', {'Name', 'Score'});
writetable(T, 'scores.txt')

What It Enables

You can quickly save and share large amounts of data without errors, making your work faster and more reliable.

Real Life Example

A teacher collects test scores from hundreds of students and uses writetable to save all results into a file that can be opened in Excel or shared with colleagues instantly.

Key Takeaways

Manual data saving is slow and error-prone.

writetable and fprintf automate writing data to files.

This saves time and reduces mistakes when handling data.

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