Writing text files (writetable, fprintf) in MATLAB - Mini Project: Build & Apply

Writing Text Files with writetable and fprintf in MATLAB

📖 Scenario: You work in a small shop and want to save your sales data to a text file so you can share it with your manager.

🎯 Goal: Learn how to write data to text files in MATLAB using writetable and fprintf.

📋 What You'll Learn

Create a table with sales data

Set a filename variable for the output file

Write the table to a text file using writetable

Write formatted sales data lines using fprintf

Display confirmation message after writing files

💡 Why This Matters

🌍 Real World

Saving sales or inventory data to text files is common in shops and businesses to share reports or keep records.

💼 Career

Knowing how to write and format text files is useful for data reporting, exporting results, and communicating between software tools.

1

Create the sales data table

Create a table called salesData with these columns and values:
Product with entries 'Apples', 'Bananas', 'Cherries'
Quantity with values 10, 15, 7
Price with values 1.2, 0.5, 2.0

MATLAB

% Create the salesData table with Product, Quantity, and Price columns
% Your code here

Hint

Use the table function with cell arrays for text and numeric arrays for numbers.

2

Set the filename for the output file

Create a variable called filename and set it to the string 'sales.txt'

MATLAB

salesData = table({'Apples'; 'Bananas'; 'Cherries'}, [10; 15; 7], [1.2; 0.5; 2.0], 'VariableNames', {'Product', 'Quantity', 'Price'});
% Set the filename variable to 'sales.txt'
% Your code here

Hint

Use single quotes for the string in MATLAB.

3

Write the table to a text file using writetable

Use writetable to write the salesData table to the file named by filename with the option 'Delimiter' set to ','

MATLAB

salesData = table({'Apples'; 'Bananas'; 'Cherries'}, [10; 15; 7], [1.2; 0.5; 2.0], 'VariableNames', {'Product', 'Quantity', 'Price'});
filename = 'sales.txt';
% Write the salesData table to the file using writetable with comma delimiter
% Your code here

Hint

Use writetable with the 'Delimiter' option set to ',' to create a CSV style file.

4

Write formatted sales lines using fprintf and display confirmation

Open the file named by filename for writing using fopen, then use a for loop with index i from 1 to height of salesData to write each product line with fprintf in the format: Product: Quantity units at $Price each. Close the file with fclose. Finally, print 'Sales data saved to sales.txt' using fprintf.

MATLAB

salesData = table({'Apples'; 'Bananas'; 'Cherries'}, [10; 15; 7], [1.2; 0.5; 2.0], 'VariableNames', {'Product', 'Quantity', 'Price'});
filename = 'sales.txt';
writetable(salesData, filename, 'Delimiter', ',');
% Open the file, write formatted lines with fprintf in a for loop, close the file, and print confirmation
% Your code here

Hint

Use fopen with 'w' to open for writing, fprintf inside a for loop to write each line, then fclose. Use fprintf without fileID to print confirmation.

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