MATLAB: Strings and File IO

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

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

Outline Strings File Input-Output

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.

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.

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

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?

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?

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

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

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

Strings 12 We may wish to convert string data to numbers. Use str2num. >> x = 17.45 ; >> y = str2num(x) y = 17.4500 >> whos Name Size Bytes Class x 1x5 10 char y 1x1 8 double

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

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 1 54 3 4 17 16.6 77.7 7 9 10 11.4 27.2 11.9 Read the above file with fileio.m

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.

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

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 %[,]

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

File Input-Output Now read the fourth line 1 54 3 4 with the code x = fscanf(fid, %f %f %f %f, 4) What is x?

File Input-Output 20 Now read the all of the remaining numbers 17 16.6 77.7 7 9 10 11.4 27.2 11.9 using the code: k = 1; while ~feof(fid); A(k ) = fscanf(fid, %f, 1); k = k + 1; end;

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.

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?

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

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)

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)

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.