Writing text files (writetable, fprintf) in MATLAB - Step-by-Step Execution

Concept Flow - Writing text files (writetable, fprintf)

Prepare data to write

↓

Choose method: writetable or fprintf

↓

Use writetable

↓

Write table to file

↓

File saved

↓

Close file with fclose

↓

File saved

First, prepare your data. Then pick writetable for tables or fprintf for formatted text. writetable saves directly, fprintf needs fopen and fclose.

Execution Sample

MATLAB

T = table([1;2;3],[4;5;6],'VariableNames',{'A','B'});
writetable(T,'data.txt');

fid = fopen('output.txt','w');
fprintf(fid,'Row %d: A=%d, B=%d\n', [1 2 3; 4 5 6; 1 2 3]);
fclose(fid);

This code saves a table to 'data.txt' using writetable, then writes formatted lines to 'output.txt' using fprintf.

Execution Table

Step	Action	Function/Operation	Result/Output
1	Create table T with columns A and B	table()	T = [1 4; 2 5; 3 6]
2	Write table T to 'data.txt'	writetable(T,'data.txt')	'data.txt' file created with table data
3	Open 'output.txt' for writing	fopen('output.txt','w')	fid = file identifier (e.g. 3)
4	Write formatted text to 'output.txt'	fprintf(fid,'Row %d: A=%d, B=%d\n', [1 2 3; 4 5 6; 1 2 3])	Lines written: Row 1: A=1, B=4 Row 2: A=2, B=5 Row 3: A=3, B=6
5	Close 'output.txt'	fclose(fid)	File closed, data saved
6	End of script	N/A	All files saved successfully

💡 All writing operations completed and files closed properly

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
T	undefined	[1 4; 2 5; 3 6]	[1 4; 2 5; 3 6]	[1 4; 2 5; 3 6]	[1 4; 2 5; 3 6]	[1 4; 2 5; 3 6]	[1 4; 2 5; 3 6]
fid	undefined	undefined	undefined	3 (example)	3 (example)	closed	closed

Key Moments - 2 Insights

Why do we need to use fopen and fclose with fprintf but not with writetable?

writetable handles file opening and closing internally, so you just give it the filename. fprintf writes formatted text to an open file, so you must open the file with fopen and close it with fclose yourself (see steps 3 and 5 in execution_table).

How does fprintf know how many lines to write when given a matrix?

fprintf reads the matrix column-wise and repeats the format string for each column. Here, the matrix [1 2 3; 4 5 6; 1 2 3] has 3 columns, so fprintf writes 3 lines (step 4 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What text is written to 'output.txt'?

A Row 1: A=4, B=1 Row 2: A=5, B=2 Row 3: A=6, B=3

B Row 1: A=1, B=2 Row 2: A=3, B=4 Row 3: A=5, B=6

C Row 1: A=1, B=4 Row 2: A=2, B=5 Row 3: A=3, B=6

D Row 1: A=3, B=6 Row 2: A=2, B=5 Row 3: A=1, B=4

💡 Hint

Check the 'Result/Output' column at step 4 in execution_table.

At which step is the file identifier 'fid' assigned a value?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Function/Operation' column for fopen in execution_table.

If you forget to call fclose after fprintf, what happens?

A The file is saved correctly anyway.

B The file remains open and data may not be fully saved.

C writetable will close the file automatically.

D MATLAB will throw an error immediately.

💡 Hint

Refer to step 5 in execution_table where fclose is called to close the file.

Concept Snapshot

Writing text files in MATLAB:
- Use writetable(T,'file.txt') to save tables easily.
- Use fprintf(fid,format,data) for formatted text.
- fopen opens a file, fclose closes it.
- Always close files after writing with fclose.
- writetable handles file open/close internally.

Full Transcript

This visual execution shows how to write text files in MATLAB using writetable and fprintf. First, a table T is created with two columns A and B. Then writetable saves this table directly to 'data.txt'. Next, fopen opens 'output.txt' for writing and returns a file identifier fid. fprintf writes formatted lines to this file using the data matrix. Finally, fclose closes the file to save changes. Variables T and fid are tracked through each step. Key points include that writetable manages file opening and closing automatically, while fprintf requires manual fopen and fclose. The quiz questions check understanding of file writing steps and outputs.

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