Writing text files (writetable, fprintf) in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to write a table to a text file using writetable.

MATLAB

T = table([1; 2; 3], {'A'; 'B'; 'C'}, 'VariableNames', {'ID', 'Letter'});
writetable(T, '[1]');

Drag options to blanks, or click blank then click option'

A'output.txt'

Boutput.txt

C'output'

Doutput

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the filename.

Omitting the file extension.

The filename must be a string enclosed in single quotes, like 'output.txt'.

2fill in blank

medium

Complete the code to open a file for writing using fopen.

MATLAB

fid = fopen('data.txt', '[1]');

Drag options to blanks, or click blank then click option'

A'rw'

B'w'

C'a'

D'r'

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'r' which is read-only mode.

Using 'a' which appends instead of overwriting.

Use 'w' to open a file for writing, which creates or overwrites the file.

3fill in blank

hard

Fix the error in the fprintf statement to write a number and a string to the file.

MATLAB

fid = fopen('log.txt', 'w');
num = 10;
str = 'test';
fprintf(fid, '[1]', num, str);
fclose(fid);

Drag options to blanks, or click blank then click option'

A'%d %d\n'

B'%s %d\n'

C'%f %s\n'

D'%d %s\n'

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of %d and %s.

Using %f for integers.

Use '%d' for integer and '%s' for string in the format specifier.

4fill in blank

hard

Fill both blanks to write a table to a CSV file without row names.

MATLAB

T = table([4; 5], {'X'; 'Y'}, 'VariableNames', {'Num', 'Char'});
writetable(T, '[1]', 'WriteRowNames', [2]);

Drag options to blanks, or click blank then click option'

A'data.csv'

Btrue

Cfalse

D'data.txt'

Attempts:

3 left

💡 Hint

Common Mistakes

Using .txt extension instead of .csv.

Setting 'WriteRowNames' to true by mistake.

Use 'data.csv' as filename and false to avoid writing row names.

5fill in blank

hard

Fill all three blanks to write formatted data to a file using fprintf.

MATLAB

fid = fopen('results.txt', 'w');
values = [1.5, 2.5, 3.5];
for i = 1:length(values)
    fprintf(fid, '[1]', values(i));
end
fclose(fid);

Drag options to blanks, or click blank then click option'

AValue: %.1f\n

BValue: %d\n

CValue: %f\n

DValue: %s\n

Attempts:

3 left

💡 Hint

Common Mistakes

Using %d which is for integers.

Using %s which is for strings.

Use '%.1f' to format the number with one decimal place.

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