Writing text files (writetable, fprintf) in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this fprintf code?

Consider the following MATLAB code that writes formatted text to a file. What will be the content of the file after running this code?

MATLAB

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

A

Value: %d

B 24 :eulaV

C

Value: 42

D Error: File not found

💡 Hint

Remember that fprintf with '%d' replaces the placeholder with the integer value and '\n' adds a new line.

The fprintf function writes 'Value: 42' followed by a newline character to the file. The fileread reads the file content including the newline, so the displayed content includes the line break.

❓ Predict Output

intermediate

What does writetable write to the file?

Given this MATLAB code, what will be the content of 'data.csv' after execution?

MATLAB

T = table([1;2;3], {'A';'B';'C'}, 'VariableNames', {'ID', 'Letter'});
writetable(T, 'data.csv');
content = fileread('data.csv');
disp(content);

A Error: VariableNames not supported

B

ID,Letter
1,A
2,B
3,C

C

ID Letter
1 A
2 B
3 C

D

1,A
2,B
3,C

💡 Hint

writetable writes variable names as headers by default and separates values with commas in CSV format.

The writetable function writes the table with headers separated by commas and each row on a new line, so the file content includes the header line 'ID,Letter' followed by the data rows.

🔧 Debug

advanced

Why does this fprintf code produce an error?

Examine the code below. Why does it cause an error when run?

MATLAB

fid = fopen('test.txt', 'w');
fprintf(fid, 'Number: %d %f\n', 5);
fclose(fid);

A fprintf expects two values for '%d' and '%f' but only one is provided, causing an error.

B fopen failed to open the file, so fprintf errors out.

C The format string is invalid because '%f' cannot be used with fprintf.

D fclose is called before fprintf, causing an error.

💡 Hint

Check how many values fprintf expects based on the format string placeholders.

The format string '%d %f\n' requires two values, but only one (5) is provided. This mismatch causes fprintf to error.

📝 Syntax

advanced

Which option correctly writes a table without row names?

You want to write a table to a CSV file without including row names. Which code does this correctly?

MATLAB

T = table([10;20], {'X';'Y'}, 'VariableNames', {'Num', 'Char'});

A writetable(T, 'file.csv', 'RowNames', false);

B writetable(T, 'file.csv', 'WriteRowNames', true);

C writetable(T, 'file.csv', 'IncludeRowNames', false);

D writetable(T, 'file.csv', 'WriteRowNames', false);

💡 Hint

Check the exact parameter name for controlling row names in writetable.

The correct parameter to exclude row names is 'WriteRowNames' set to false. Other options use incorrect parameter names or values.

🚀 Application

expert

How many lines are written by this fprintf loop?

This MATLAB code writes lines to a file. How many lines will the file contain after running?

MATLAB

fid = fopen('log.txt', 'w');
for i = 1:5
    fprintf(fid, 'Line %d\n', i);
end
fclose(fid);
content = fileread('log.txt');
lines = count(content, '\n');

A 5

B 6

C 4

D 1

💡 Hint

Each fprintf call writes one line ending with '\n'. Count how many times the loop runs.

The loop runs 5 times, each writing one line with a newline character, so the file has 5 lines.

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