fprintf - Write data to text file - MATLAB (original) (raw)

Syntax

Description

fprintf([fileID](#btf8xsy-1-fileID),[formatSpec](#btf8xsy-1%5Fsep%5Fshared-formatSpec),[A1,...,An](#btf8xsy-1-A1An)) applies the formatSpec to all elements of arrays A1,...An in column order, and writes the data to a text file. fprintf uses the encoding scheme specified in the call to fopen.

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fprintf([formatSpec](#btf8xsy-1%5Fsep%5Fshared-formatSpec),[A1,...,An](#btf8xsy-1-A1An))formats data and displays the results on the screen.

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[nbytes](#btf8xsy-1-nbytes) = fprintf(___) returns the number of bytes that fprintf writes, using any of the input arguments in the preceding syntaxes.

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Examples

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Print multiple numeric values and literal text to the screen.

A1 = [9.9, 9900]; A2 = [8.8, 7.7 ; ... 8800, 7700]; formatSpec = 'X is %4.2f meters or %8.3f mm\n'; fprintf(formatSpec,A1,A2)

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

%4.2f in the formatSpec input specifies that the first value in each line of output is a floating-point number with a field width of four digits, including two digits after the decimal point. %8.3f in the formatSpec input specifies that the second value in each line of output is a floating-point number with a field width of eight digits, including three digits after the decimal point. \n is a control character that starts a new line.

Explicitly convert double-precision values with fractions to integer values.

a = [1.02 3.04 5.06]; fprintf('%d\n',round(a));

%d in the formatSpec input prints each value in the vector, round(a), as a signed integer. \n is a control character that starts a new line.

Write a short table of the exponential function to a text file called exp.txt.

x = 0:.1:1; A = [x; exp(x)];

fileID = fopen('exp.txt','w'); fprintf(fileID,'%6s %12s\n','x','exp(x)'); fprintf(fileID,'%6.2f %12.8f\n',A); fclose(fileID);

The first call to fprintf prints header text x and exp(x), and the second call prints the values from variable A.

If you plan to read the file with Microsoft® Notepad, use '\r\n' instead of '\n' to move to a new line. For example, replace the calls to fprintf with the following:

fprintf(fileID,'%6s %12s\r\n','x','exp(x)'); fprintf(fileID,'%6.2f %12.8f\r\n',A);

MATLAB® import functions, all UNIX® applications, and Microsoft Word and WordPad recognize '\n' as a newline indicator.

View the contents of the file with the type command.

 x       exp(x)

0.00 1.00000000 0.10 1.10517092 0.20 1.22140276 0.30 1.34985881 0.40 1.49182470 0.50 1.64872127 0.60 1.82211880 0.70 2.01375271 0.80 2.22554093 0.90 2.45960311 1.00 2.71828183

Write data to a file and return the number of bytes written.

Write an array of data, A, to a file and get the number of bytes that fprintf writes.

A = magic(4);

fileID = fopen('myfile.txt','w'); nbytes = fprintf(fileID,'%5d %5d %5d %5d\n',A)

The fprintf function wrote 96 bytes to the file.

Close the file.

View the contents of the file with the type command.

16 5 9 4 2 11 7 14 3 10 6 15 13 8 12 1

Display a hyperlink (The MathWorks Web Site) on the screen.

url = 'https://www.mathworks.com'; sitename = 'The MathWorks Web Site';

fprintf('%s\n',url,sitename)

%s in the formatSpec input indicates that the values of the variables url and sitename, should be printed as text.

Input Arguments

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File identifier, specified as one of the following:

• A file identifier obtained from fopen. fprintf does not support writing to internet URLs.
• 1 for standard output (the screen).
• 2 for standard error.

Data Types: double

Numeric or character arrays, specified as a scalar, vector, matrix, or multidimensional array.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical | char

Output Arguments

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Number of bytes that fprintf writes, returned as a scalar. When writing to a file, nbytes is determined by the character encoding. When printing data to the screen, nbytes is the number of characters displayed on the screen.

Tips

• Format specifiers for the reading functions sscanf and fscanf differ from the formats for the writing functions sprintf and fprintf. The reading functions do not support a precision field. The width field specifies a minimum for writing, but a maximum for reading.
• If you specify an invalid formatting operator or special character, then fprintf prints all text up to the invalid operator or character and discards the rest.
  Example: If formatSpec is 'value = %z', then fprintf prints 'value =' because %z is not a formatting operator.
  Example: If formatSpec is'character \x99999 = %s', then fprintf prints 'character' because \x99999 is not a valid special character.

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.

Extended Capabilities

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Usage notes and limitations:

• The formatSpec parameter must be constant.
• In formatSpec, hexadecimal numbers must be in the range [0 7F] and octal numbers must be in the range [0 177].
• If fileID has a constant value of 1 or2 and extrinsic calls are not possible, the code generator produces a C printf call. Extrinsic calls are not possible when extrinsic calls are disabled or when fprintf is called inside aparfor loop.
• The behavior of fprintf in the generated code matches the C compiler behavior instead of the MATLAB behavior in these cases:
  • The format specifier has a corresponding C format specifier, for example,%e or %E.
  • The fprintf call is inside aparfor loop.
  • Extrinsic calls are disabled.
• These options and capabilities are not supported:
  • The n$ position identifier for reordering input values
  • Printing arrays
  • Using subtypes to print a floating-point number as its octal, decimal, or hexadecimal value
• When you call fprintf with the format specifier%s, you cannot put a null character in the middle of the input character vector. To write a null character, use fprintf(fid, '%c', char(0)).
• Input argument types must match their format types. For example, ifn is a double, code generation does not allow the following code:
  For code generation, first cast n to a signed integer type such as int8.
  str = fprintf('%d',int8(n))
• When you call fprintf with an integer format specifier, the type of the integer argument must be a type that the target hardware can represent as a native C type. For example, if you call fprintf('%d', int64(n)), then the target hardware must have a native C type that supports a 64-bit integer.

The fprintf function supports GPU array input with these usage notes and limitations:

• This function accepts GPU arrays, but does not run on a GPU.

For more information, see Run MATLAB Functions on a GPU (Parallel Computing Toolbox).

Version History

Introduced before R2006a