Importing and Exporting Data

Transcription

1 Class14

2 Importing and Exporting Data MATLAB is often used for analyzing data that was recorded in experiments or generated by other computer programs. Likewise, data that is produced by MATLAB sometimes needs to be transferred to other computer applications. Various type of data can be imported and exported from MATLAB (numerical, text, audio, graphics, and images). Text is probably the most common.

3 Function csvread M = csvread('filename') Reads a comma separated value formatted file FILENAME. The result is returned in M. The file can only contain numeric values. FILENAME must be in the current folder, or you must specify the path.

4 Function csvwrite csvwrite( file.csv, M) Writes matrix M into the file file.csv as commaseparated values. If file.csv does not exist, it will be created If file.csv does exist, it will be over-written

5 Other Text I/O functions csvread and csvwrite are very useful but somewhat limited. For more options and flexibility see dlmread, dlmwrite readtable, writetable xlsread, xlswrite (these have a history of not working well on Macs)

6 Output Commands auto display as we ve seen, suppress using a semicolon. disp( text as string ) or disp(variable_name) only one variable can be displayed in a disp command.

7 Output as Table %Example yr=[1984:2:1996]; pop=[ ]; tableyp(:,1)=yr'; tableyp(:,2)=pop'; disp(' YEAR Population') disp(' disp(' ') disp(tableyp) (Millions)') Population data entered in two row vectors yr is entered as the 1 st column in the array tableyp pop is entered as the 2 nd column in the array Display heading 1 st line Display heading 2 nd line Display empty line Display the array tableyp

8 Output as Table fprintf Command fprintf command can be used to display output (text and data) on the screen or to save it to a file. Output can also be formatted (unlike with disp command) Text and numerical values of variables can be intermixed and displayed on same line. Format of numbers can be controlled. fprintf command can be long and complicated we will approach gradually.

9 fprintf to display text General form: fprintf( text typed in as a string ) Special character \n for new line: fprintf( The problem has no solution. \nplease check the input data.\n ) The backslash is an escape character. It indicates special meaning along with the character which follows. Other escape characters: \b Backspace \t Tab

10 Can also use \n in middle of text to make MATLAB display remainder of text on next line >> fprintf('a man\na plan\npanama\n') A man A plan Panama >>

11 Include variable data using a format specifier The % sign marks the spot where the number is inserted within text General form: fprintf( text as string %-5.2f additional text, variable_name) The name of the variable whose value is displayed Formatting elements (define the format of the number)

12 Formatting Elements Example Character used for flag %-8.2f flag (optional) Description - (minus sign) Left justifies the number within the field + (plus sign) Prints a sign character (+ or -) in front of the number Try: 0 (zero) Adds zeros if the number is shorter than the field %-8.2f %-5.2f %5.2f -%-5.2f %--5.2f

13 Formatting Elements Example %-5.2f Field width and precision (optional) The 1 st number (5 in example) is the field width (5 spots) that specifies the minimum number of digits in the display. If the number to be displayed is shorter than the field width, spaces or zeros are added in front of the number. The precision is the second number (2 in example), specifies the number of digits to be displayed to the right of the decimal point.

14 Formatting Elements Example %-5.2f Conversion character (required) The last element in the formatting elements, which is required, is the conversion character which specifies the notation in which the number is displayed. Common notations: e Exponential notation using lower case e ( e+001) E Exponential notation using upper case E ( E+001) f Fixed point notation ( ) g The shorter of e or f notations. G The shorter of E or f notations i Integer d Decimal notation does not include trailing zeros if value displayed is an integer.

15 Examples Format specifier PI displayed Notes %f Default width will accommodate the size of the number; Default precision=6 %4.1f 3.1 Leading space to make width=4 %10.8f %10.8e e+00 %10.2e 3.14e+00 2 leading spaces %10.8g Compact form, will not print insignificant 0s The format commands ( format short, format long ) do not affect fprintf.

16 Use escape characters to display characters used in conversion specifiers To display a percent sign, use %% in the text To display a single quote, use ' ' sequential single quotes) in the text (two To display a backslash, use \\ in the text (two sequential backslashes)

17 Make the following strings Mom's apple 3.14 Mom's apple Mom's apple 3.1e+000 >> fprintf( 'Mom''s apple %.2f\n', pi ) Mom's apple 3.14 >> fprintf( 'Mom''s apple %.7f\n', pi ) Mom's apple >> fprintf( 'Mom''s apple %.1e\n', pi ) Mom's apple 3.1e+000

18 Format strings are often long. Can break a string by 1. Put an open square bracket ( [ ) in front of first single quote 2. Put a second single quote where you want to stop the line 3. Follow that quote with an ellipsis (three periods) 4. Press ENTER, which moves cursor to next line 5. Type in remaining text in single quotes 6. Put a close square bracket ( ] ) 7. Put the rest of the fprintf command

19 Example >> weight = 178.3; >> age = 17; >> fprintf( ['Tim weighs %.1f lbs'... ' and is %d years old'], weight, age ) Tim weighs lbs and is 17 years old Multi-line fprintf statements can be difficult to read. I prefer to avoid them. In many cases you can use multiple fprintf statements instead.

20 Inserting more than 1 variable within text General format: fprintf(..text %g %g %f, variable1, variable2, variable3) If your variable is a matrix, for example, x=1:5; MATLAB will repeat the string in the fprintf command until it uses all the values in the matrix. fprintf( %8.2f \n, x) returns , etc.

21 Write a table fprintf ('x\tsqrt(x)\n'); fprintf ('--\t \n'); fprintf ('%g\t%g\n', T); x sqrt(x)

22 Write to a File 3 Steps to write to a file: 1. Create/Open the file (fopen command) 2.Write to the file (fprintf command) 3.Close the file (fclose command)

23 1. Open a File fid=fopen('file_name','permission') fid file identifier, will be used by fprintf to know what file to write its output in file_name whatever you want the file name to be permission tells how file will be used, e.g., for reading, writing, both

24 Some common permissions r - open file for reading w - open file for writing. If file exists, content deleted. If file doesn't exist, new file created a - same as w except if file exists the written data is appended to the end of the file If no permission code specified, fopen uses r See Help on fopen for all permission codes

25 2 Write to the File Write to file with fprintf. Use it exactly as before but insert the variable fid before the format string: fprintf(fid,'fmt string',variables) The fid is how fprintf knows to write to the file instead of display on the screen

26 3 Close the File When you're done writing to the file, close it with the command fclose(fid) Once you close it, you can't use that fid anymore until you get a new one by calling fopen Make sure to close every file you open. Too many open files makes problems for MATLAB

27 Miscellaneous fid is a variable called the file identifier and needs to be unique for each file your write to. If the file name you give to fopen has no path, MATLAB writes it to the current directory, also called the working directory You can have multiple files open simultaneously and use fprintf to write to all of them just by passing it different fids You can read the files you make with fprintf in any text editor, e.g., MATLAB's Editor/Debugger window or Notepad

28 Example Using fprintf command to write output to two different files. X = 1:10; Y = 11:20; Table1 = [X]; Table2 = [Y]; fid1=fopen( x_first.txt, w ); fid2=fopen( y_firsttxt, w ); fprintf(fid1, %5i\n,Table1); fprintf(fid2, %5i\n,Table2); fclose(fid1); fclose(fid2);