MATLAB Function Reference

fprintf - Write formatted data to file

Syntax

count = fprintf(fid, format, A, ...)

Description

count = fprintf(fid, format, A, ...) formats the data in the real part of matrix A (and in any additional matrix arguments) under control of the specified format string, and writes it to the file associated with file identifier fid. fprintf returns a count of the number of bytes written.

Argument fid is an integer file identifier obtained from fopen. (It can also be 1 for standard output (the screen) or 2 for standard error. See fopen for more information.) Omitting fid causes output to appear on the screen.

For more detailed information on using string formatting commands, see Formatting Strings in the MATLAB^® Programming Fundamentals documentation.

Format String

The format argument is a string containing ordinary characters and/or C language conversion specifications. A conversion specification controls the notation, alignment, significant digits, field width, and other aspects of output format. The format string can contain escape characters to represent nonprinting characters such as newline characters and tabs.

Conversion specifications begin with the % character and contain these optional and required elements:

• Flags (optional)

• Width and precision fields (optional)

• A subtype specifier (optional)

• Conversion character (required)

You specify these elements in the following order:

Flags

You can control the alignment of the output using any of these optional flags.

Field Width and Precision Specifications

You can control the width and precision of the output by including these options in the format string.

Conversion Characters

Conversion characters specify the notation of the output.

Conversion characters %o, %u, %x, and %X support subtype specifiers. See Remarks for more information.

Escape Characters

This table lists the escape character sequences you use to specify nonprinting characters in a format specification.

Remarks

When writing text to a file on a Windows^® system, The MathWorks recommends that you open the file in write-text mode (e.g., fopen(file_id, 'wt')). This ensures that lines in the file are terminated in such a way as to be compatible with all applications that might use the file.

MATLAB writes characters using the encoding scheme associated with the file. See fopen for more information.

The fprintf function behaves like its ANSI^® C language namesake with these exceptions and extensions:

• If you use fprintf to convert a MATLAB double into an integer, and the double contains a value that cannot be represented as an integer (for example, it contains a fraction), MATLAB ignores the specified conversion and outputs the value in exponential format. To successfully perform this conversion, use the fix, floor, ceil, or round function to change the value in the double into a value that can be represented as an integer before passing it to sprintf.

• The following nonstandard subtype specifiers are supported for the conversion characters %o, %u, %x, and %X.

  b	The underlying C data type is a double rather than an unsigned integer. For example, to print a double-precision value in hexadecimal, use a format like '%bx'.
  t	The underlying C data type is a float rather than an unsigned integer.

  For example, to print a double value in hexadecimal, use the format '%bx'.

• The fprintf function is vectorized for nonscalar arguments. The function recycles the format string through the elements of A (columnwise) until all the elements are used up. The function then continues in a similar manner through any additional matrix arguments.

Note   fprintf displays negative zero (-0) differently on some platforms, as shown in the following table.

Examples

Example 1

Create a text file called exp.txt containing a short table of the exponential function. (On Windows platforms, it is recommended that you use fopen with the mode set to 'wt' to open a text file for writing.)

x = 0:.1:1;
y = [x; exp(x)];
fid = fopen('exp.txt', 'wt');
fprintf(fid, '%6.2f %12.8f\n', y);
fclose(fid)

Now examine the contents of exp.txt:

type exp.txt
   0.00    1.00000000
   0.10    1.10517092
       ...
   1.00    2.71828183

Example 2

The command:

fprintf( ...
  'A unit circle has circumference %g radians.\n', 2*pi)

displays a line on the screen:

A unit circle has circumference 6.283186 radians.

Example 3

To insert a single quotation mark in a string, use two single quotation marks together. For example:

fprintf(1,'It''s Friday.\n')

displays on the screen:

It's Friday.

Example 4

Use fprintf to display a hyperlink on the screen. For example:

site = '"http://www.mathworks.com"';
title = 'The MathWorks Web Site';
fprintf(['<a href = ' site '>' title '</a>'])

creates the hyperlink:

The Mathworks Web Site

in the Command Window. Click this link to display The MathWorks home page in a MATLAB Web browser.

Example 5

The commands

B = [8.8  7.7; 8800  7700]
fprintf(1, 'X is %6.2f meters or %8.3f mm\n', 9.9, 9900, B)

display the lines

X is 9.90 meters or 9900.000 mm
X is 8.80 meters or 8800.000 mm
X is 7.70 meters or 7700.000 mm

Example 6

Explicitly convert MATLAB double-precision variables to integer values for use with an integer conversion specifier. For instance, to convert signed 32-bit data to hexadecimal format,

a = [6 10 14 44];
fprintf('%9X\n', a + (a<0)*2^32)
        6
        A
        E
       2C

See Also

disp, fclose, ferror, fopen, fread, fscanf, fseek, ftell, fwrite

References

[1] Kernighan, B.W., and D.M. Ritchie, The C Programming Language, Second Edition, Prentice-Hall, Inc., 1988.

[2] ANSI specification X3.159-1989: "Programming Language C," ANSI, 1430 Broadway, New York, NY 10018.

fplot

fprintf (serial)