MATLAB: Strings and File IO

Transcription

1 MATLAB: Strings and File IO Kipp Martin University of Chicago Booth School of Business February 9, 2012

2 The M-files The following files are used in this lecture. filein.txt fileio.m

3 Outline Strings File Input-Output

4 Strings In MATLAB everything is an array We can have an array of numbers (doubles) We can have an array of strings In this module we first look at strings.

5 Strings In MATLAB you can also work with strings. A string is a sequence of characters. The sequence can be digits, letters, symbols, and spaces enclosed by single quotation marks. Examples of string variables are x = abcd123 y = ae _ bx%$#( In MATLAB everything is an array so what have we created? >> whos Name Size Bytes Class x 1x7 14 char y 1x12 24 char Each character takes two bytes.

6 Strings Since a string is an array, you can access parts of the string just like you would any other array. For example >> y(5:7) ans = _ b >> x(1, 3) ans = c >> x(1,:) ans = abcd123

7 Strings It is also possible to define matrices where each row is a string. >> student_record =char( Name:, Joe Schmoe, Quiz 1 Score, 96 ) student_record = Name: Joe Schmoe Quiz 1 Score 96 >> whos student_record Name Size Bytes Class student_record 4x12 96 char Why 4 by 12? Where does the 12 come from?

8 Strings 8 We can address student record like any other matrix of numbers. >> student_record(2, [1:2, 10]) ans = Joe What! How did we get Joe? What will student_record(2, [1, 2, 5]) give?

9 Strings 9 Now what about something like: x = [ Hello ; Goodbye ] What will happen? As an alternative to the char function you can use the strvact function (it will automatically pad). >> x = strvcat( Hello, Goodbye ) x = Hello Goodbye >> whos Name Size Bytes Class x 2x7 28 char

10 Strings Here are some useful string functions in MATLAB. The strcmp function. This function will compare two strings and return a true if they are equal. False if not. Very useful in if statements. >> x =strcmp( hello, goodbye ) x = 0 >> y = strcmp( abc, abc ) y = 1

11 Strings The strcat function. This function is the equivalent of the & operator in VBA for strings. This function allows us to concatenate strings. >> x = William ; >> y = strcat(x, Barret, Travis ) y = William Barret Travis

12 Strings 12 We may wish to convert string data to numbers. Use str2num. >> x = ; >> y = str2num(x) y = >> whos Name Size Bytes Class x 1x5 10 char y 1x1 8 double

13 Strings 13 We may wish doubles to strings. Use num2str. >> x = 77.11; >> y = num2str( x) y = >> whos Name Size Bytes Class x 1x1 8 double y 1x5 10 char

14 File Input-Output It is pretty easy to read from and write to text files using MATLAB. See Chapter 4 of Gilat. Let s start with reading a file. Consider the file filein.txt Hi 1.77 Goodbye 34 a b, c, d HiAgain Read the above file with fileio.m

15 File Input-Output First open the file. In VBA we had testfile = "C:\temp\data.txt" Open testfile For Input As #1 In MATLAB we have: fid = fopen( filein.txt ); I am assuming filein.txt is in the MATLAB path.

16 Read the first line: File Input-Output Hi 1.77 Goodbye 34 Use the code: v1 = fscanf(fid, %s, 1) v2 = fscanf(fid, %f, 1) v3 = fscanf(fid, %s, 1) v4 = fscanf(fid, %f, 1) Use the %s to read a string We read the first string until we hit the white space delimiter. The 1 is telling to read one string Use the %f to read a floating point number

17 File Input-Output Read the second line using the comma as a delimiter: a b, c, d Use the code v5 = fscanf(fid, %[^,], 1) fscanf(fid, %[,], 1) ; %trash the comma v6 = fscanf(fid, %[^,], 1) fscanf(fid, %[,], 1) ; %trash the comma v7 = fscanf(fid, %s, 1) I want to preserve the white space between a and b Use %[^,] to read all characters until a comma is found. Burn off the commas with %[,]

18 File Input-Output 18 Read the third line: HiAgain Use the code fgetl(fid); v8 = fscanf(fid, %s, 1) I use the fgetl(fid); to make sure I have read any control characters before going to the next line

19 File Input-Output Now read the fourth line with the code x = fscanf(fid, %f %f %f %f, 4) What is x?

20 File Input-Output 20 Now read the all of the remaining numbers using the code: k = 1; while ~feof(fid); A(k ) = fscanf(fid, %f, 1); k = k + 1; end;

21 File Input-Output Create a Structure From a File: Consider the following data set (studentdata.txt) Tom,Jones,57 Bill,Uehling,100 Mary,Honda,95 Kathy,Murigami,67 Bill,Jones,99 Jill,Doe,83 Mark,Anderson,56 Jody,Ruebush,99 Alice,Tyx,89 Mary,Chin,100 Richard,Valens,77 Tom,Choi,80 Dave,Sweeney,45 Download this file at the data link.

22 File Input-Output In-class Exercise: Read the file (studentdata.txt) and create a structure, studentstruct, that has the following properties: FirstName LastName Quiz1 Question: Instead of writing code to read a file, why not A = load( studentdata.txt ) and let MATLAB do the work?

23 File Input-Output Now let s write the input file we worked with. First open a file with write permission. fid = fopen( fileout.txt, w ); If you replace the w with a data will be appended to the end of the file. Write the first line. fprintf(fid, Hi %-5.2f Goodbye %i\n, 1.77, 34) We use \n to move the file pointer to a new line The %-5.2f and %i get replaced by 1.77 and 34, respectively. (the minus inf -5.2f causes left justification

24 File Input-Output Write the next two lines of text. fprintf(fid, a b, c, d\n ) fprintf(fid, HiAgain\n ) Next print the line of integers. fprintf(fid, %i %i %i %i\n, 1, 54, 3, 4)

25 File Input-Output Write the line of seven real numbers and then a new line. for i=1:7 fprintf(fid, %-5.1f, A( i)) end fprintf(fid, \n ) and then the last line fprintf(fid, %-5.1f %-5.1f \n, 27.2, 11.9)

26 File Input-Output Note that when we printed, we specified the file id (in this case fid) fprintf(fid, %-5.1f %-5.1f \n, 27.2, 11.9) You can also print directly to the screen fprintf( Print Directly to the screen\n ) Note that the file id is missing. If you do not care about formatting you can also print to the screen with the disp() function.