Chapter 11 Input/Output (I/O) Functions

Transcription

1 EGR115 Introduction to Computing for Engineers Input/Output (I/O) Functions from: S.J. Chapman, MATLAB Programming for Engineers, 5 th Ed Cengage Learning Topics Introduction: MATLAB I/O 11.1 The textread Function 11.2 More about the load & save Commands 11.3 An Introduction to MATLAB File Processing 11.4 File Opening & Closing 11.5 Binary I/O Functions 11.6 Formatted I/O Functions 11.7 Comparing Formatted & Binary I/O Functions

2 Slide 1 of 17 Introduction: MATLAB I/O MATLAB I/O In chapter 2, we learned how to load (input) and save MATLAB data using the load and save commands, and how to write out (display output at command window) using fprinf command. In this chapter, we will learn more about MATLAB s Input/Output (I/O) capabilities: Reading text data from a file: textread and textscan commands More about load and save commands Other MATLAB I/O options File Input/Output in MATLAB In addition to the text I/O (between the disk and MATLAB workspace), MATLAB can open the file, read from or write to the file, and close the file. Open the file: fid = fopen(filename, r ); fid = fopen(filename, w ); (permission = r or w tells MATLAB that we want to read from or write to the file) Read from the file; a = fscanf(fid,format,size); Write to the file: fprintf(fid,format,variables); Close the file: fclose(fid);

3 Slide 2 of /11.2 The textread Function More about the load & save Commands MATLAB textread Function (see EXAMPLE 5-6) In MATLAB, the textread function reads text files that are formatted into columns of data, where each column can be of different type, and stores the contents of each column in a separate output array. [a,b,c,...] = textread(filename,format,n) MATLAB save/load Command In MATLAB, the save command saves workplace data to disk, and the load command loads data from disk into workspace. save filename [content] [options] load filename [options] [content] load Command Options The textread command reads text files that are formatted into columns of data (Example 5-6). Editor [first,last,blood,gpa,age,answer] =... textread('test_input.dat','%s %s %s %f %d %s') >> example_5_6.m first = James Sally last = Jones Smith blood = O+ A+ gpa = age = answer = Yes No Command Window The textread command can also skip selected columns by adding an asterisk to the corresponding format descriptor (for example, %*s). Editor [first,last,blood,gpa,age,answer] =... textread('test_input.dat','%s %s %*s %f %*d %*s') >> example_5_6.m first = James Sally last = Jones Smith gpa = Command Window

4 Slide 3 of /11.2 The textread Function More about the load & save Commands load Command Contents load Command Options The save command saves MATLAB workspace data to disk, and the load command loads data from disk into the workspace. save filename [content] [options] load filename [options] [content] The MAT and ASCII Files -mat MAT files are MATLAB default file format files. MAT files preserve all of the information about each variable in the workspace, including its class, name, and whether or not it is global. MAT files are unique to MATLAB and cannot be used to share data with other programs. If the file name and the names of the variables to be loaded or saved are in strings, then it is necessary to require file name input from the user: filename = input( Enter save file name:, s ); save(filename, -mat ); -ascii This is American Standard Code for Information Exchange (ASCII) standard and can be used to share data between MATLAB and other programs. The save -ascii command will not save cell or structure array data, and it converts string data to numbers before saving it. The load -ascii command will only load space or tab separated data with an equal number of elements on each row, and it will place all of the data into a single variable with the same name as the input file.

5 11.3 An Introduction to MATLAB File Processing Slide 4 of 17 MATLAB Input/Output Functions The fopen Function fid = fopen(filename, permission, format) filename: is a string specifying the name of the file to open permission: is a character string specifying the mode in which the file is opened format: is an optional string specifying the numeric format of the data in the file Examples of fopen Functions (Case 1) Opening a Text File for Data Reading The command below opens a text file named example.dat for data reading. fid = fopen( example.dat, r ) The permission string is r, indicating that the file is to be opened for reading only. (Case 2) Opening a Text File for Data Writing The command below open a binary file named outdat for data writing. fid = fopen( outdat, wt ) or fid = fopen( outdat, at ) The wt permissions string specifies that the file is a new text file; if it already exists, then the old file will be deleted and a new empty file will be opened for writing. The at permissions string specifies that we want to append to an existing text file.

6 Slide 5 of File Opening & Closing fopen File Permissions fopen Numeric Format Strings Examples of fopen Functions (continued) (Case 3) Opening an Existing Binary File for Read/Write Access The command below opens a binary file named junk for data reading/writing. fid = fopen( junk, r+ ) fid = fopen( junk, w+ ) The permission strings r+ and w+ indicate that the existing files are opened and available for reading and writing accesses. Read/Write to the Open File (Text Files) Read from the file; a = fscanf(fid,format,size); Write to the file: fprintf(fid,format,variables); The fclose Function status = fclose(fid) status = fclose( all ) fid: is the file ID status: is the result of the operation (if the operation is successful, status will be 0, and if it is unsuccessful, status will be 1)

7 Slide 6 of Binary I/O Functions MATLAB Precision Strings The fwrite Function The fwrite function writes binary data in a user-specified format to a file. count = fwrite(fid,array,precision) array: is the array of values to write out count: the number of values written to the file MATLAB writes out data in column order, which means that the entire first column is written out, followed by the entire second column, and so forth. 1 2 array = [ 3 4] will be written out in the order of: 1, 3, 5, 2, 4, The fread Function The fread function reads binary data in a user-specified format from a file, and returns the data in a (possibly different) user-specified format. [array,count] = fread(fid,size,precision) size: is the number of values to read precision: specifies both the format of the data on the disk and the format of the data array to be returned to the calling program. The general form of the precision string is: disk_precision=>array_precision single single=>single *single double=>real*4 Reads data in single precision format from disk and returns it in a double array Reads data in single precision format from disk and returns it in a single array Reads data in single precision format from disk and returns it in a single array (a shorthand version) Reads data in double precision format from disk and returns it in a single array

8 Slide 7 of Binary I/O Functions EXAMPLE 11-1 Develop a MATLAB script that creates an array containing 10,000 random values, opens a user-specified file for writing only, writes the array to disk in 64-bit floating-point format, and closes the file. Then, make the MATLAB script open the file for reading and read the data back into a array. >> binary_io Enter file name: testfile values written values read... binary_io.m % Script file: binary_io.m Editor % Prompt for file name filename = input('enter file name: ','s'); % Generate the data array out_array = randn(1,10000); % Open the output file for writing. [fid,msg] = fopen(filename,'w'); % Was the open successful? if fid > 0 % Write the output data. count = fwrite(fid,out_array,'float64'); % Tell user disp([int2str(count) ' values written...']); % Close the file status = fclose(fid); else % Output file open failed. Display message. disp(msg); end % Now try to recover the data. Open the file for reading. [fid,msg] = fopen(filename,'r'); % Was the open successful? if fid > 0 % Read the input data. [in_array, count] = fread(fid,[ ],'float64'); % Tell user disp([int2str(count) ' values read...']); % Close the file status = fclose(fid); else % Input file open failed. Display message. disp(msg); end (NOTE) You cannot open binary files in notepad (need a special software, such as: Binary Viewer)

9 Slide 8 of Binary I/O Functions Do-It-Yourself (DIY) EXERCISE 11-1 (a) Answer the following questions: 1. Why is the textread function especially useful for reading data created by programs written in other languages? 2. What are the advantages and disadvantages of saving data in a MAT file? 3. What MATLAB functions are used to open and close files? What is the difference between opening a binary file and opening a text file? (b) Examine the following MATLAB statements. Are they correct? If they are in error, specify what is wrong with them. 1. fid = fopen( file1, rt ); array = fread(fid,inf); fclose(fid); 2. fid = fopen( file1, w ); x = 1:10; count = fwrite(fid,x); fclose(fid); fid = fopen( file1, r ); array = fread(fid,[2 Inf]); fclose(fid); (a) 1. The textread function reads text files that are formatted into columns of data, where each column can be of a different type, and stores the contents of each column in a separate output array. This function is very useful for importing tables of data printed out (written in ASCII format) by other software applications. 2. MAT files are MATLAB default and efficient, as long as you use MATLAB as your programming language platform. MAT files preserve all of the information about each variable in the workspace, including its class, name, and whether or not it is global. MAT files can be compressed to save disk data space. MAT files preserve full precision of all variables. MAT files are, however, unique to MATLAB and cannot be used to share data with other programs. 3. MATLAB functions fopen and fclose can be used for opening and closing the file. Text files need to have file extension.dat while binary files do not have any file extension. For text files, fscanf and fprintf functions will be used; while binary files use fwrite and fread functions. (b) 1. Incorrect: opening a file as a text file, but reading the data in binary format (use fscanf). 2. Correct: creating a 10-element array x, opening a binary output file file1, writing the array to the file, and closing file. Next, opening the file again for reading, and reading the data into array array in a [2 Inf] format. The result is: array = [ ]

10 Slide 9 of Formatted I/O Functions fprintf Format Conversion Specifier The MATLAB fprintf Command The MATLAB fprintf command writes (or displays) formatted data in a user-specified format to a file (or on Command Window). The syntax is: count = fprintf(fid,format,val1,val2,...) fprintf(format,val1,val2,...) format: is the format string controlling the appearance of the data. count: the number of values written to the file If fid is not specified, the data will be written to the standard output device (default is the display on Command Window; this is what you have been doing so far!).

11 Slide 10 of Formatted I/O Functions fprintf Format Flags fprintf Escape Characters Understanding fprintf Format Marker (the very first % ): marks the beginning of the format. If the ordinary % character is to be printed out, then it must appear in the format string as %%. Modifier (minus sign <= NOTE: does NOT mean minus sign added but LEFT-JUSTIFY, plus sign <= added redundant plus sign for positive value, or padded zero <= added redundant leading zero): will be optional modifier (format flag). Field Width: will specify the field width of data. If not specified, MATLAB will set the default field width. Precision: will specify the minimum number of significant digit to display. If not specified, MATLAB will set the default field width. Format Descriptor (c, d, e,... x): will set the description of the data format type (Format Conversion Specifier). This (together with the Marker, the very first % ) is required as a minimum for fprintf command. Escape Characters: in addition to ordinary characters and formats, certain special characters can be used in a format string (adding new line, tab, carriage return, etc.) in fprintf.

12 Slide 11 of Formatted I/O Functions fprintf Examples fprintf( %e\n,123.4) e+002 Case 1: Displaying Decimal Data fprintf( %d\n,123) 123 fprintf( %6d\n,123) 123 fprintf( %6.4d\n,123) 0123 fprintf( %-6.4d\n,123) 0123 fprintf( %+6.4d\n,123) fprintf Examples Case 1: Displaying Decimal Data Display default decimal notation (%d) of a value (123) Display decimal notation with field width 6 (%6d) of a value (123) Display decimal notation with field width 6 and precision 4 (%6.4d) of a value (123) Display decimal notation, left justified, with field width 6 and precision 4 (%-6.4d) of a value (123) Display decimal notation, added leading plus sign, with field width 6 and precision 4 (%+6.4d) of a value (123)

13 Slide 12 of Formatted I/O Functions Case 2: Displaying Floating-Point Data fprintf( %f\n,123.4) fprintf( %8.2f\n,123.4) fprintf( %6.2f\n,123.4) fprintf( %10.2e\n,123.4) fprintf( %10.2E\n,123.4) e E+002 fprintf Examples (continued) Case 2: Displaying Floating-Point Data Display default floating-point notation (%f) of a value (123.4) Display floating-point notation with field width 8 and precision 2 (%8.2f) of a value (123.4) Display floating-point notation with field width 4 and precision 3 (%4.2f) of a value (123.4) Display exponential notation with field width 10 and precision 2 (%10.2e) of a value (123.4) using lower case e Display exponential notation with field width 10 and precision 2 (%10.2E) of a value (123.4) using upper case E

14 Slide 13 of Formatted I/O Functions Case 3: Displaying Character Data fprintf( %c\n, s ) s fprintf( %s\n, string ) string fprintf( %8s\n, string ) string fprintf( %-8s\n, string ) string fprintf Examples (continued) Case 3: Displaying Character Data Display default single character (%c) of a character ( s ) Display default string of characters (%s) of a character string ( string ) Display string of characters with field width 8 (%8s) of a character string ( string ) Display string of characters, left justified, with field width 8 (%-8s) of a character string ( string )

15 Slide 14 of Formatted I/O Functions EXAMPLE 11-2 Develop a MATLAB script that generates and prints a table of data (the square roots, squares, and cubes of all integers between 1 to 10). >> table Table of Square Roots, Squares, and Cubes Number Square Root Square Cube ====== =========== ====== ==== create_table.m % Script file: create_table.m Editor % Print the title of the table. fprintf(' Table of Square Roots, Squares, and Cubes\n\n'); % Print column headings fprintf(' Number Square Root Square Cube\n'); fprintf(' ====== =========== ====== ====\n'); % Generate the required data ii = 1:10; square_root = sqrt(ii); square = ii.^2; cube = ii.^3; % Create the output array out = [ii' square_root' square' cube']; % Print the data for ii = 1:10 fprintf (' %2d %11.4f %6d %8d\n',out(ii,:)); end

16 The MATLAB sprintf Command The MATLAB sprintf command writes formatted data to a character string (instead of a file). The syntax is: string = sprintf(fid, format,val1,val2,...) format: is the format string controlling the appearance of the data. fid: is the ID of a file to which the data will be written. The MATLAB fscanf Command The MATLAB fscanf command reads formatted data in a user-specified format from a file. The syntax is: array = fscanf(fid,format) [array,count] = fscanf(fid,format,size) format: is the format string controlling how the data is read. fid: is the ID of a file to which the data will be read. array: is the array that receives the data. count: returns the number of values read from the file. The optional argument size specifies the amount of data to be read from the file: n Read exactly n values. After this statement, array will be a column vector containing n values read from the file. inf Read until the end of the file. After this statement, array will be a column vector containing all of the data until the end of the file. [n,m] Read exactly n by m values, and format the data as an n by m array.

17 Formatted Files (Text Files) The MATLAB formatted I/O operations produce formatted files. A formatted file contains recognizable characters, numbers, and so forth that are stored as ordinary text. These files are easy to distinguish, because we can see the characters and numbers in the file when we display them on the screen or print them out. It can usually be moved between different types of computers with different internal data representations (Windows, Linux, Mac O/S, etc.) However, to use data in a formatted file, MATLAB must translate the characters in the file into the internal data format used by the computer. Unformatted Files (Binary Files) Unformatted files simply copy the information from the computer s memory directly to the disk file with no conversions at all. Since no conversions occur, no computer time is wasted formatting the data. However, data cannot be examined and interpreted directly by humans. In addition, it is usually cannot be moved between different types of computers with different internal data representations (Windows, Linux, Mac O/S, etc.)

18 Slide 17 of Comparing Formatted & Binary I/O Functions Do-It-Yourself (DIY) EXERCISE 11-2 (a) Answer the following questions: 1. What is the difference between unformatted (binary) and formatted I/O operations? 2. When should formatted I/O be used? When should unformatted I/O be used? (b) Examine the following MATLAB statements. Are they correct? If they are in error, specify what is wrong with them. 1. a = 2*pi; b = 6; c = hello ; fprintf(fid, %s %d %g\n,a,b,c); 2. data1 = 1:20; data2 = 1:20; fid = fopen( xxx, w+ ); fwrite(fid,data1); fprintf(fid, %g\n,data2); (a) 1. Unformatted (binary) data is simply a copy of the information from the computer s memory directly to the disk file with no conversions at all. Since no conversions occur, no computer time is wasted formatting the data. Formatted (text) data, on contrary, is a data file containing recognizable characters, numbers, and so forth that are stored as ordinary text. These files are easy to distinguish, because we can see the characters and numbers in the file when we display them on the screen or print them out. 2. Formatted I/O should be used to create formatted (text) files, in order to freely move (transport) data files between different types of computers with different internal data representations (Windows, Linux, Mac O/S, etc.). Unformatted I/O should be used to create unformatted (binary) files, to be used within a single computer that will not require file transfer with external computers (files only used within a computer with a single consistent internal data representations). (b) 1. Incorrect. The %s descriptor must correspond to a character string. The %g descriptor must not correspond to a character string. 2. Technically correct, but undesirable. It is possible to mix unformatted (binary) and formatted (text) data in a single file, but the file is then very hard to use for any purpose. Binary and text data should be written to separate files.