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

Concept Flow - Writing text files (writetable, fprintf)

Prepare data to write

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Choose method: writetable or fprintf

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Use writetable

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Write table to file

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File saved

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Close file with fclose

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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.