1. AN ENGINEER’S GUIDE TO MATLAB
3rd Edition
CHAPTER 3
DATA INPUT/OUTPUT
Copyright © Edward B. Magrab 2009

2. Chapter 3 – Objectives
Present the means of displaying annotated numerical results in the MATLAB command window and the means of storing and retrieving data from files.
Introduce cell arrays.
Copyright © Edward B. Magrab 2009

3. Topics Strings and Annotated Output Creating Strings
Converting Numerical Values to Strings and
Displaying Them
Entering Data With Input
Scalar, Vector, Matrix, and String
Input/Output Data Files
Cell Arrays
Input Microsoft Excel Files
Copyright © Edward B. Magrab 2009

4. Strings (Literals) -
Collections of any combination of letters, numbers, and special characters
Created, stored, and manipulated in arrays and defined similar to that for vectors and matrices
Differ from an array of numerical values in that –
Each character occupies one element in an array
Defined by enclosing all its characters between a pair of single quotes (' … ')
Typically used for –
Displaying information to the command window
Annotating data displayed to the command window
Annotating graphs
Copyright © Edward B. Magrab 2009

5. Examples – Let s be the string testing123. The MATLAB syntax is
s = 'testing123' or s = ['testing123']
To retrieve specific characters in the string s, we can use expressions like
s(7)  g
s(3:6)  stin
Strings can also be concatenated (added to form a longer string) as
sc = ['testing123', 'testing123']
which produces the (120) string
sc =
testing123testing123
Copyright © Edward B. Magrab 2009

6. sc = ['testing123'; 'testing123'] creates the (210) matrix scs =
But,
sc = ['testing123'; 'testing123']
creates the (210) matrix
scs =
testing123
Thus,
scs(1,:)  testing123
scs(2,:)  testing123
Copyright © Edward B. Magrab 2009

7. One can find the starting locations of strings within strings using
findstr(string1, string2)
which searches the longer of the two strings for the occurrences of the shorter of the two strings.
Let us find the occurrences of ‘123’ in the concatenated string shown in the script below
sc = ['testing123', 'testing123']
Loc=findstr(sc, '123')
Upon execution, we obtain
sc =
testing123testing123
Loc =
Copyright © Edward B. Magrab 2009

8. Thus, if we have the expression lab = ['first ';'last ';'middle'] then
If we place a string in each row of a matrix, we have a convenient way in which to access string expressions.
The requirement is that each row must contain the same number of characters.
This requirement can be met by employing blanks to pad the rest of the string when the individual string expressions are of unequal length.
Thus, if we have the expression
lab = ['first ';'last ';'middle']
then
lab(1,: )  firstb
lab(2,: )  lastbb
lab(3,: )  middle
and b indicates a blank space.
Copyright © Edward B. Magrab 2009

9. The padding is performed by char
Thus, the above expression can be replaced by the easier-to-use expression
lab = char('first','last','middle')
ord = size(lab)
which, when executed, displays
lab =
first
last
middle
ord =
The trailing blanks can be removed with
deblank
Copyright © Edward B. Magrab 2009

10. The leading and trailing blanks can be removed with strtrim(s)
where s is a string.
Two strings can be compared by using
L = strcmp(A, B)
where A and B are strings and
L = 1 (true) if A = B
L = 0 (false) if A  B
This function is intended to compare character data.
Copyright © Edward B. Magrab 2009

11. Example –
In the two strings defined in the script, A has two additional leading blanks and two additional trailing blanks than string B.
A = ' Yes and No ';
B = ' Yes and No ';
C1 = strcmp(A, B)
C2 = strcmp(strtrim(A), strtrim(B))
which, upon execution, gives
C1 =
C2 = 1
Copyright © Edward B. Magrab 2009

12. Converting Numerical Values to Strings and Displaying Them
To convert a numerical value to a string, we use
z = num2str(num) or z = num2str(num,N)
where
z is a string
num is either a number, an array of numbers, or an expression resulting in a number or an array of numbers
N is number of digits to be displayed (if omitted, N = 5)
Copyright © Edward B. Magrab 2009

13. Example -
Let a = 1000 = Then the various values of N will display the digits shown below.
num2str(a,1)  3e+003
num2str(a,3)  3.14e+003
num2str(a,4)  3142
num2str(a,5) 
num2str(a,8) 
Notice that the decimal point (.) does not count as a digit.
Copyright © Edward B. Magrab 2009

14. Example -
Let a =  /1000 = Then the various values of N will display the digits shown below
num2str(a,1)  0.003
num2str(a,3) 
num2str(a,4) 
num2str(a,5) 
num2str(a,8) 
Copyright © Edward B. Magrab 2009

15. To convert an integer to a string, we use z = int2str(num)
where num is an integer. If num is not an integer, then it is rounded to one.
Copyright © Edward B. Magrab 2009

16. disp(['Product weight = ' num2str(num) ' kg'])
A typical construct is to use num2str to concatenate the converted numerical value with some identifying text.
Thus, if num is, say, the weight in kilograms, and it is to be identified as such, then to display it to the command window we use disp as follows:
num = ;
disp(['Product weight = ' num2str(num) ' kg'])
Upon execution, we obtain
Product weight = kg
Note: At least one blank space on each side of num2str is required.
Copyright © Edward B. Magrab 2009

17. Let num be a vector of the lengths of the object.
Then, the script that displays a vector of values is
num = [12.567, 3.458, 9.111];
disp(['Object length = ' num2str(num) ' m'])
Upon execution, we obtain
Object length = m
Copyright © Edward B. Magrab 2009

18. n = length(num); % Let MATLAB do the counting
To create annotation that accompanies each value of num, we use repmat as follows.
num = [12.567, 3.458, 9.111];
n = length(num); % Let MATLAB do the counting
disp([repmat('Object length = ', n, 1)
num2str(num') repmat(' m', n, 1)])
which, upon execution, displays
Object length = m
Object length = m
Object length = m
Copyright © Edward B. Magrab 2009

19. fprintf(1,'%…', variables) where
An alternative way to display formatted data to the MATLAB command window is with
fprintf(1,'%…', variables)
where
‘1’ indicates that the output goes to the command window
'%….' is the format specification pertaining to variables
% precedes each specific format specification, which is of the form
x.yq
q specifies the format
x specifies the minimum number of digits to be displayed
y is the number of digits to the right of the decimal point
Copyright © Edward B. Magrab 2009

20. Example –
We select q = f, which is a fixed point format, and use it to display the vector
num = [12, -14, , ];
several different ways.
To display this vector on one line, we have
fprintf(1,'%5.2f ', num)
which results in
Copyright © Edward B. Magrab 2009

21. which, when executed, displays 12.00 -14.00 3098.46 0.11
To display these values as a column of four numbers, we use the delimiter \n as follows:
num = [ ];
fprintf(1,'%5.2f\n', num)
which, when executed, displays
12.00
-14.00
0.11
Copyright © Edward B. Magrab 2009

22. The execution of this script results in weight = 12 kg
To display each number with the appropriate number of digits, annotate each value, and print it in column form, we use
num = [12, -14, , ];
fprintf(1,'weight = %2.0f kg\npressure = %2.0f kPa\ntime = %5.3f s\nlength = %5.5f m', num)
The execution of this script results in
weight = 12 kg
pressure = -14 kPa
time = s
length = m
Copyright © Edward B. Magrab 2009

23. If this number were x = 0.00045, then the script is x = 0.00045;
num2str can also employ the format specifications of fprintf by replacing the second argument N in num2str with a string format specification.
Suppose that we wanted to display a very small number as 0 instead of in exponent form.
If this number were x = , then the script is
x = ;
disp(['x = ' num2str(x,'%2.1f')])
displays
x = 0.0
Copyright © Edward B. Magrab 2009

24. Entering Data with input
Entering a Scalar –
To input a single numerical quantity, we use
InputData=input('Enter temperature in degrees C: ');
Upon execution, we have displayed in the command window
Enter temperature in degrees C:
where the number was entered by the user.
The semicolon at the end of the expression in the script suppresses the echoing of the value entered.
The variable InputData has the value of after Enter is depressed.
Copyright © Edward B. Magrab 2009

25. One can also modify user-entered values in the same expression.
Consider the conversion of degrees to radians. Here
InputData = input('Enter the starting angle in
degrees: ')*pi/180;
When executed, we have in the command window
Enter the starting angle in degrees: 45
where the value 45 was entered by the user.
However, the value of InputData is (= 45/180).
Copyright © Edward B. Magrab 2009

26. Entering a String –
To input a single string quantity, we append an 's' at the end of the input function
InputData = input('Enter file name, including its
extension: ','s');
which displays command to the window.
Enter file name, including its extension: DataSet3.txt
where the string DataSet3.txt was entered by the user.
Notice that no single quotation marks are required.
The value of InputData is the string DataSet3.txt, which is a vector of length 12.
Copyright © Edward B. Magrab 2009

27. The square brackets are required.
Entering a Vector –
To input a vector of numerical values, we use
InputData = input('Enter four temperatures in
degrees C: ');
which, upon execution, displays
Enter four temperatures in degrees C: [120, 141,169, 201]
where the vector of numbers [120, 141, 169, 201] was entered by the user.
The square brackets are required.
If a column vector was required, then the user's response would be either
[120, 141, 169, 201]' or [120; 141; 169; 201]
Copyright © Edward B. Magrab 2009

28. Entering a Matrix - To input a matrix of numerical values, we use
InputData = input('Enter three temperatures in
degrees C\nfor levels 1 and 2: ');
which displays
Enter the three temperatures in degrees C
for levels 1 and 2: [67, 35, 91; 44, 51, 103]
where the array [67, 35, 91; 44, 51, 103] was entered by the user.
The variable InputData is a (23) array.
Copyright © Edward B. Magrab 2009

29. Input/Output Data Files – load/save
Reads data on a row-by-row basis.
Each row separated by an Enter, where Enter is used instead of the semicolon.
Each data value separated by either one or more blanks or by a comma.
The number of columns of data in each row must be the same.
For a row vector, data entered without using Enter.
For a column vector, each data value followed by an
Enter.
Copyright © Edward B. Magrab 2009

30. Example –
Assume that data reside in an ASCII text file DataSection33.txt in the form
The load function is
load('DataSection33.txt')
Copyright © Edward B. Magrab 2009

31. load('DataSection33.txt') y = DataSection33.^2
Let us square each element of the matrix in DataSection33.txt. The script is
load('DataSection33.txt')
y = DataSection33.^2
which, upon execution, results in
y =
Copyright © Edward B. Magrab 2009

32. FileName1 = input('Enter file name containing data
If one wanted to operate on data in different files, each having a different file name, then we have to employ a different technique.
Here, the user will enter the file name when requested to do so and, as before, the script will square the data residing in the file whose name is specified when the script is executed. The script is
FileName1 = input('Enter file name containing data
(including suffix): ','s');
load(FileName1);
m = findstr(FileName1,'.');
data1 = eval(FileName1(1:m-1));
y = data1.^2
Copyright © Edward B. Magrab 2009

33. Since FileName1 is still unknown to the remaining expressions in the script, it must be converted to a numerical quantity.
This is done by the eval function, which evaluates the string quantity appearing in its argument.
Copyright © Edward B. Magrab 2009

34. save
If one wants to save numerical values resulting from the execution of a script or function to a file, then we use
save('File name', 'Variable 1', 'Variable 2', …, '-ascii')
where
'File name' is a string containing the name of the file to be saved and its directory, if other that the current directory.
'Variable n' are strings containing the names of the n variables that are to be saved in the order that they appear.
'-ascii' is a string indicating that the data will be saved in ascii format.
Copyright © Edward B. Magrab 2009

35. Example –
Save the square of each value in a file named DataSection33.txt as ASCII text. The script is
load('DataSection33.txt')
y = DataSection33.^2;
save('SavedDataSection33.txt', 'y', '-ascii')
Upon execution, the script creates a text file whose contents are
e e e+002
e e e+002
e e e+003
e e e+003
Copyright © Edward B. Magrab 2009

36. A Note About Path Name –
When just the file name is given, MATLAB places the file in the current directory.
In order to place the file in a specific directory, the entire path name must be given.
For example,
load('DataSection33.txt') % File in current directory
y = DataSection33.^2;
z = sqrt(Datasection33);
save('c:\Matlab mfiles\Matlab
results\SavedDataSection33.txt', 'y', ‘z', '-ascii')
Copyright © Edward B. Magrab 2009

37. Execution of this script creates the file SavedDataSection331
Execution of this script creates the file SavedDataSection331.txt with the contents
e e e+002
e e e+002
e e e+003
e e e+003
e e e+000
e e e+000
e e e+000
e e e+000 y z
Copyright © Edward B. Magrab 2009

38. Cell Arrays –
Cell arrays are a special class of arrays whose elements consist of cells that themselves contain arrays.
They provide a hierarchical way of storing dissimilar kinds of data.
Any cell in a cell array can be accessed through matrix indexing as is done with standard vectors and matrices.
Cell notation differs from standard matrix notation in that open braces ‘{’ and the closed braces ‘}’ are used instead of open ‘[’ and closed ‘]’ brackets.
Copyright © Edward B. Magrab 2009

39. Example –
Let us create four different arrays of data as shown in the following script
A = ones(3,2)
B = magic(3)
C = char('Pressure', 'Temperature', 'Displacement')
D = [6+7j, 15]
Upon executing this script, we obtain
Copyright © Edward B. Magrab 2009

40. A = 1 1 B = 8 1 6 3 5 7 4 9 2 C = Pressure Temperature Displacement
B =
C =
Pressure
Temperature
Displacement
D = i
Copyright © Edward B. Magrab 2009

41. C = char('Pressure', 'Temperature', 'Displacement'); D = [6+7j, 15];
We now add to this script the cell assignment statement to create a (22) cell array, which is analogous to that used for standard arrays, except that we use braces as delimiters. Thus,
A = ones(3,2);
B = magic(3);
C = char('Pressure', 'Temperature', 'Displacement');
D = [6+7j, 15];
Cel = {A, B; C, D}
After executing this script, we obtain
Cel =
[3x2 double] [3x3 double]
[3x12 char ] [1x2 double]
Copyright © Edward B. Magrab 2009

42. To display the contents of Cel to the command window, we use
Notice that we do not get what is specifically in each cell, only what the size of the data arrays in each cell are and their type.
To display the contents of Cel to the command window, we use
celldisp(x)
Then, the program becomes
A = ones(3,2);
B = magic(3);
C = char('Pressure', 'Temperature', 'Displacement');
D = [6+7j, 15];
Cel = {A, B; C, D};
celldisp(Cel)
Copyright © Edward B. Magrab 2009

43. which, upon execution, gives Cel{1,1} = 1 1
Cel{2,1} =
Pressure
Temperature
Displacement
Cel{1,2} =
Cel{2,2} = i
Copyright © Edward B. Magrab 2009

44. C = char('Pressure', 'Temperature', 'Displacement'); D = [6+7j,15];
To access each cell individually, we use the index notation used for standard array variables, except that we use braces instead of parentheses. Thus,
A = ones(3,2);
B = magic(3);
C = char('Pressure', 'Temperature', 'Displacement');
D = [6+7j,15];
Cel = {A, B; C, D};
Cell_1_2 = Cel{1,2}
which, upon execution, gives
Cell_1_2 =
Copyright © Edward B. Magrab 2009

45. Words = {'application', 'apple', 'friend', 'apply', 'fiend'};
With cell arrays, the use of sort can be extended to sort words in dictionary order.
Consider the following script, where we will create a cell array of five words and then sort them.
Words = {'application', 'apple', 'friend', 'apply', 'fiend'};
WordSort = sort(Words)'
The execution of this script gives
'apple'
'application'
'apply'
'fiend'
'friend'
Copyright © Edward B. Magrab 2009

46. Input Microsoft Excel Files
Data files created in Microsoft Excel can be read into MATLAB with the function
[X, Y] = xlsread('Filename')
where X will be an array containing the columns and rows of data and Y will be a cell array containing any text headers that accompany the data.
The file name must contain the suffix ‘.xls’.
Copyright © Edward B. Magrab 2009

47. Example –
Consider the data generated in Excel shown below and saved in ForceDispData.xls.
The script to read this file is
[X, Y] = xlsread('ForceDispData.xls')
X =
Y =
[1x20 char] []
'Force' 'Displacement'
'(kPa)' '(mm)'
Note: Y is a (3×2) cell array: Y{1,1}=Transducer Linearity
Copyright © Edward B. Magrab 2009