MATLAB: Strings and File IO

Kipp MartinUniversity of Chicago

Booth School of Business

February 9, 2012

1

The M-files

The following files are used in this lecture.

I filein.txt

I fileIO.m

2

Outline

Strings

File Input-Output

3

Strings

I In MATLAB everything is an array

I We can have an array of numbers (doubles)

I We can have an array of strings

In this module we first look at strings.

4

Strings

In MATLAB you can also work with strings. A string is a sequenceof characters. The sequence can be digits, letters, symbols, andspaces enclosed by single quotation marks. Examples of stringvariables 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.

5

Strings

Since a string is an array, you can access parts of the string justlike you would any other array. For example

>> y(5:7)

ans =

_ b

>> x(1, 3)

ans =

c

>> x(1,:)

ans =

abcd1236

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?

7

Strings

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?

8

Strings

Now what about something like:

x = [’Hello’ ; ’Goodbye’]

What will happen?

As an alternative to the char function you can use the strvactfunction (it will automatically pad).

>> x = strvcat(’Hello’, ’Goodbye’)

x =

Hello

Goodbye

>> whos

Name Size Bytes Class

x 2x7 28 char

9

Strings

Here are some useful string functions in MATLAB.

The strcmp function. This function will compare two strings andreturn 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

10

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

11

Strings

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

12

Strings

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

13

File Input-Output

It is pretty easy to read from and write to text files usingMATLAB. 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

14

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.

15

File Input-Output

Read the first line:

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)

I Use the ’%s’ to read a string

I We read the first string until we hit the white space delimiter.

I The 1 is telling to read one string

I Use the ’%f’ to read a floating point number

16

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 I want to preserve the white space between a and bI Use

%[^,]

to read all characters until a comma is found.I Burn off the commas with

%[,]17

File Input-Output

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 controlcharacters before going to the next line

18

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?

19

File Input-Output

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;

20

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.21

File Input-Output

In-class Exercise: Read the file (studentData.txt) and create astructure, studentStruct, that has the following properties:

I FirstName

I LastName

I Quiz1

Question: Instead of writing code to read a file, why not

A = load(’studentData.txt’)

and let MATLAB do the work?

22

File Input-Output

Now let’s write the input file we worked with. First open a file withwrite permission.

fid = fopen(’fileout.txt’, ’w’);

If you replace the w with a data will be appended to the end of thefile.

Write the first line.

fprintf(fid, ’Hi %-5.2f Goodbye %i\n’, 1.77, 34)

I We use \n to move the file pointer to a new line

I The %-5.2f and %i get replaced by 1.77 and 34, respectively.(the “minus” inf -5.2f causes left justification

23

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)

24

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)

25

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 thescreen with the disp() function.

26