fprintf MATLAB function: Beginner’s Guide

In this article, you’ll learn to generate text files from MATLAB using the MATLAB command fprintf. You’ll find out about:

• The MATLAB fprintf Syntax
• How to print a matrix in a text file
  
• How to print complex numbers in a text file

MATLAB fprintf Syntax

How to use fprintf in MATLAB

In MATLAB, you can print text into a file by using the fprintf MATLAB command. To do this, go through the following 3 steps:

1. Open a file using fopen.
2. Write content using fprintf.
3. Close the file using fclose.

For example, let’s write the word “Text” into a file named “textFileName.txt”:

fid = fopen('textFileName.txt', 'at');
fprintf(fid, 'Text');
fclose(fid);

• fid: output of the fopen command, required to reference the file on which to write on (and to write on different files at once).
• fopen(‘textFileName.txt’, ‘at’): open or create (if it doesn’t exist) a file named “textFileName.txt’
  • “a” is for appending data to the end of the file
  • “t” is for opening the file in text mode
• fprintf(fid, ‘Text’): print the word ‘Text’ into the file referenced by the “fid” variable.
• fclose(fid): close the file referenced by the “fid” variable.

If you run this piece of code in MATLAB, a file named ‘textFileName.txt’ will be created in your current folder:

Delete Existing Content When Writing

There are 2 ways to delete the content before writing when the file already exists:

1. Either use the argument ‘w’ instead of ‘a’ in the fopen MATLAB command:

   fid = fopen('textFileName.txt', 'wt');
   fprintf(fid, 'Text');
   fclose(fid);

2. Or, use the delete MATLAB command to delete the existing file before using fopen to create a new file with the same name:

   delete('textFileName.txt');
   fid = fopen('textFileName.txt', 'at');
   fprintf(fid, 'Text');
   fclose(fid);

If ‘textFileName.txt’ doesn’t exist, MATLAB will generate a warning; otherwise, it will delete the file before creating a new one with fopen.

Print Numbers in a Text File

You can format as follows:

delete('textFileName.txt');
fid = fopen('textFileName.txt', 'at');
fprintf(fid, ['%d + %d = %d'], 2, 3, 5);
fclose(fid);

Or, you can just use the num2str MATLAB command to convert your number into a text:

delete('textFileName.txt');
fid = fopen('textFileName.txt', 'at');
fprintf(fid, [num2str(2) '+' num2str(3) '=' num2str(5)]);
fclose(fid);

Warning: File not Found or Permission Denied

When you encounter a warning saying that the file cannot be overwritten, it could be because there have been some issues with closing the file the last time it was opened:

In this case, you can close it by using:

fclose all

After running this command you should be able to delete/overwrite the file.

MATLAB fprintf: Matrix

Let’s say we have a matrix:

\(m = \begin{pmatrix}1 & 2 & 3\\4 & 5 & 6\end{pmatrix}\)

It can be defined in MATLAB as follows:

m = [1 2 3 ; 4 5 6];

The intuitive way to write a matrix would be to use “%d” for each column of the matrix such as:

fid = fopen('textFileName.txt', 'at');
fprintf(fid, '%d %d %d\n', m);
fclose(fid);

However, because fprintf doesn’t store matrices in an intuitive way, in MATLAB you’d get:

Instead, you need to use the transpose of that matrix:

fid = fopen('textFileName.txt', 'at');
fprintf(fid, '%d %d %d\n', m'); % notice the single quotation mark at the right of m
fclose(fid);

Now, you get:

MATLAB fprintf: Complex Number

If you want to print complex numbers, there is no specific format, you need to separate the real from the imaginary part:

x = 1+3i;
fid = fopen('textFileName.txt', 'at');
fprintf(fid, '%d+%dj\n', real(x), imag(x));
fclose(fid);

Here’s what the text file will look like:

Key takeaways:

1. The basic structure to write text into a file is the following:

   fid = fopen(fileName, 'at'); % a is for append
   fprintf(fid, text);
   fclose(fid);

2. To delete the content in the file before writing using the syntax:
   1. Use “w” as an argument of fopen:

      fid = fopen(fileName, 'wt'); % w is for write
      fprintf(fid, text);
      fclose(fid);

   2. Use the delete MATLAB command:

      delete(fileName);
      fid = fopen(fileName, 'at');
      fprintf(fid, text);
      fclose(fid);

3. To print a matrix in MATLAB, you need to transpose the matrix:

   fid = fopen('textFileName.txt', 'at'); 
   fprintf(fid, '%d %d %d\n', m'); % notice the single quotation mark at the right of m 
   fclose(fid);

4. To print a complex number in MATLAB, separate the real from the imaginary part:

   x = 1+3i; 
   fid = fopen('textFileName.txt', 'at'); 
   fprintf(fid, '%d+%dj\n', real(x), imag(x)) 
   fclose(fid);

To learn more about fprintf, check out the following links:

• A real-life example of writing model information in a file: https://realtechnologytools.com/fprintf-matlab/
• MathWorks documentation: https://www.mathworks.com/help/matlab/ref/fprintf.html

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