Programming

Writing Formatted Text Files

The fprintf function converts data to character strings and outputs them to the screen or a file. A format control string containing conversion specifiers and any optional text specify the output format. The conversion specifiers control the output of array elements; fprintf copies text directly.

Common conversion specifiers include

Conversion Specifier	Description
%e	Exponential notation
%f	Fixed-point notation
%g	Automatically select the shorter of %e and %f

Optional fields in the format specifier control the minimum field width and precision. For example, this code creates a text file containing a short table of the exponential function:

• x = 0:0.1:1;
  y = [x; exp(x)];
  

The code below writes x and y into a newly created file named exptable.txt:

• fid = fopen('exptable.txt','w');
  fprintf(fid,'Exponential Function\n\n');
  fprintf(fid,'%6.2f  %12.8f\n',y);
  status = fclose(fid);
  

The first call to fprintf outputs a title, followed by two carriage returns. The second call to fprintf outputs the table of numbers. The format control string specifies the format for each line of the table:

• A fixed-point value of six characters with two decimal places
• Two spaces
• A fixed-point value of twelve characters with eight decimal places

fprintf converts the elements of array y in column order. The function uses the format string repeatedly until it converts all the array elements.

Now use fscanf to read the exponential data file:

• fid = fopen('exptable.txt','r');
  title = fgetl(fid);
  [table,count] = fscanf(fid,'%f %f',[2 11]);
  table = table';
  status = fclose(fid);
  

The second line reads the file title. The third line reads the table of values, two floating-point values on each line, until it reaches end of file. count returns the number of values matched.

A function related to fprintf, sprintf, outputs its results to a string instead of a file or the screen. For example,

• root2 = sprintf('The square root of %f is %10.8e.\n',2,sqrt(2));
  

Reading Formatted ASCII Data

Closing a File

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