Low-Level File I/O. 1. Open a file. 2. Read or write data into the file. 3. Close the file. Zheng-Liang Lu 1 / 14

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1 Low-Level File I/O Low-level file I/O functions allow the most control over reading or writing data to a file. However, these functions require that you specify more detailed information about your file than the easier-to-use high-level functions, such as importdata. If the high-level functions cannot import your data, you may consider to use low-level file I/O. The normal procedure looks like: 1. Open a file. 2. Read or write data into the file. 3. Close the file. Zheng-Liang Lu 1 / 14

2 Open Files fid = fopen(filename, permission) opens the file, filename, for binary read access, and returns an integer as fid equal to or greater than 3. fid refers to the file identifier. permission: file access type, specified as a string. MATLAB reserves fids 1 and 2 for standard output on the screen and standard error, respectively. (You will see later.) If fopen fails to open the file with r, then fid is 1. You can fopen a new file with permission w. Be aware that you can repeatedly open a new file with the same file name. Zheng-Liang Lu 2 / 14

3 fopen Permission Codes r is to open file for reading. r+ additionally for writing. w is to open or create new file for writing. 1 w+ additionally for reading. a is to open or create new file for writing. Also, it appends data to the end of the file. a+ additionally for reading. 1 Be aware that it will discard existing contents, if any. Zheng-Liang Lu 3 / 14

4 Common Commands in Low-Level File I/O feof(fid), which refers to end-of-file, returns 1 if a previous operation set the end-of-file indicator for the specified file. tline = fgetl(fid) returns the next line of the specified file, removing the newline characters. tline is a text string unless the line contains only the end-of-file marker. In this case, tline is the numeric value -1. Zheng-Liang Lu 4 / 14

5 Close Files fclose(fid) closes an open file. fclose( all ) closes all open files. You can also use fclose all. fclose returns a status of 0 when the close operation is successful. Otherwise, it returns 1. Zheng-Liang Lu 5 / 14

6 Example 1 clear; clc 2 % main 3 f = fopen('myfile.txt'); 4 str = fgetl(f); 5 while ischar(str) 6 disp(str) 7 str = fgetl(f); 8 end 9 fclose(fid); Zheng-Liang Lu 6 / 14

7 fprintf Recall that fprintf(format, A 1,..., A n ) formats data and displays the results on the screen 2. fprintf(fid, format, A 1,..., A n ) applies the format to all elements of arrays A 1,..., A n in column order, and writes the data to a text file. format: Format of the output fields, specified as a string. A1,..., A n : data arrays. 2 Recall disp. Zheng-Liang Lu 7 / 14

8 Recall that MATLAB reserves 1 and 2 for standard output on the screen, and standard error, respectively. Try fprintf(1, This is standard output!\n ); Try fprintf(2, This is standard error!\n ); sprintf(formatspec, A 1,..., A n ), which is similar to fprintf, but returns the results as a string. Zheng-Liang Lu 8 / 14

9 Example fprintf can print multiple numeric values and literal text to the screen. 1 A = [ ; ]; 2 % If you don't assign the destination, then the... default output is the screen. 3 fprintf('x is %4.2f meters or %8.3f mm.\n', A); 1 >> 2 3 X is 9.90 meters or mm. 4 X is 8.80 meters or mm. 5 X is 7.70 meters or mm. %4.2f Can you explain %8.3f? Zheng-Liang Lu 9 / 14

10 Escape Characters %%: Percent character \\: Backslash \b: Backspace \n: New line \t: Horizontal tab Zheng-Liang Lu 10 / 14

11 format Integer, signed: %d or %i Integer, unsigned %u: Base 10 %o: Base 8 %x: Base 16 Floating-point number 3 %f: Fixed-point notation %e: Exponential notation, such as e+00 Characters %c: Single character %s: String Note that the % can be followed by an optional field width to handle fixed width fields. 3 See IEEE 754. Zheng-Liang Lu 11 / 14

12 Example: grep in UNIX/Linux-like Systems findstr(s1, S2) returns the starting indices of any occurrences of the shorter of the two strings in the longer. 1 function grep(filename, pattern) 2 f = fopen(filename, 'r'); 3 line number = 1; 4 while feof(fid) == 0 5 str = fgetl(fid); 6 if ~isempty(findstr(str, pattern)) 7 fprintf('%d: %s \n', line number, line); 8 end 9 line number = line number + 1; 10 end 11 fclose(fid); Zheng-Liang Lu 12 / 14

13 Exercise Write a short table of the exponential function to a text file called exp.txt, which looks like: 1 x exp(x) Zheng-Liang Lu 13 / 14

14 1 clear; clc; 2 % main 3 x = 0:.1:1; 4 A = [x', (exp(x))']; 5 f = fopen('exp.txt', 'w'); 6 fprintf(f, '%6s%15s\n', 'x', 'exp(x)'); 7 fprintf(f, '%6.2f%15.8f\n', A); 8 fclose(fid); 9 dos('start exp.txt'); % Open exp.txt dos( command ), for Windows systems, calls upon the shell to execute the given command. Try dos( start ). Zheng-Liang Lu 14 / 14