Transcript Introduction to MATLAB - Georgia Institute of Technology

Cell Arrays and Structures
Learning
Objectives
Topics
Data Types
Character Strings
Cell Arrays
Structures
Summary
Learn about
characters, cell
arrays &
structures
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Storing More Than Numbers
• MATLAB matrices store only numeric results
– What about words, names, strings?
– What about arrays of arrays?
• MATLAB provides three more containers to store data
– Character arrays
– Cell arrays
– Structures
• Examples
– Gradebooks
– Hierarchical geographic data
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What are Character Arrays?
• Examples:
» C = 'Hello'; %C is a 1x5 character array.
» D = 'Hello there’; %D is a 1x11 character array.
» A = 43; %A is a 1x1 double array.
» T = 'How about this character string?’
» size(T)
ans =
1
» whos
32
% What do you observe?
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Learn?
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How are Characters Stored?
• Character arrays are similar to vectors, except:
– Each cell contains a single digit
• Example
» u = double(T)
» char(u)
% double is a dedicated function.
% performs the opposite function.
• Exercise
» a = double('a')
» char(a)
• Questions: What is the numerical value of ‘a’ and
what does it mean?
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Manipulating String Arrays
• Strings can be manipulated like arrays.
• Exercises
»
»
»
»
»
»
»
»
u = T(16:24)
u = T(24:-1:16)
u = T(16:24)’
v = 'I can''t find the manual!’ % Note quote in string
u ='If a woodchuck could chuck wood,';
v = 'how much wood could a woodchuck chuck?';
w = [u,v] % string concatenation in Matlab
disp(u)
% works just like for arrays
• Lessons?
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Character Strings in Multiple Rows
» v = ['Character strings having more than'
'one row must have the same number ‘
'of columns just like arrays!
']
• Exercise
» lawyers = char('Cochran','Shapiro','Clark','Darden');
» lawyers(3,:)
• Lesson?
• Exercise
» help char
» help str2mat
» help strvcat
• Lesson?
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String Construction / Manipulation
>> t = ‘How about this character string?’
>> size (t) % What’s this?
>> whos
• Lesson?
>> u = double (t)
>> char(u)
• Lesson?
>>u = t(16:24)
>>u = t(24:-1:16)
>>u = t(16:24)’
• Lesson?
Page 129-131
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What are Cell Arrays?
• Cell arrays are containers for “collections” of data of
any type stored in a common container.
• Cell arrays are like a wall of PO boxes, with each PO
box containing its own type of information.
• When mail is sent to a PO box the PO box number is
given. Similarly each cell in a cell array is indexed.
• Cell arrays are created using cell indexing in the same
way that data in a table or an array is created and
referenced,.
• The difference is the use of curly braces { }.
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Cell Array Access
• Cell arrays look a lot like Matlab arrays but they cannot
generally be manipulated the same way.
• Cell arrays should be considered more as data “containers”
and must be manipulated accordingly. (Cell arrays cannot
be used in arithmetic computations like arrays can, e.g., + * / ^)
• Addressing Cell Arrays:
A(i,j) = {x} this is called CELL INDEXING
A{i,j} = x
this is called CONTENT ADDRESSING
• either can be used, but be careful…
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Cell Array Examples
first = ‘Hello’;
second = {‘hello’,’world’,’from’,’me’};
third (1,1) = {‘happy’}; % Cell indexing
third {2,1} = ‘birthday’; % Content addressing
third {3,1} = 40;
• What do you observe? Lesson?
>>
>>
>>
>>
third
third (1,1), third {1,1}
third (2,1), third {2,1}
third (3,1), third {3,1}
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Cell Arrays of Strings
• All rows in a string array MUST have the same number of
columns … this is a pain.
• Solution?
• Cell arrays!!!! Next slide … but just try the following!
• Exercise
» C = {'How';'about';'this for a';'cell array of strings?'}
• Question: What is different from what you have been doing
before?
• Exercises
»
»
»
»
size(C)
C(2:3)
C([4,3,2,1])
[a,b,c,d] = deal(C{:})
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Lessons?
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Cell Array Examples … contd.
• Exercise
» C = cell(2,3) % Defines C to be a cell array
» C(1,1) = {'This does work'} % ( ) refer to PO Box
» C{2,3} = 'This works too’
% { } refers to contents
• Lesson?
» A = cell(1,3) % Note 1 x 3
» A = {'My' , 'name', 'is' , ‘Burdell'} %
» A = {'My'; 'name'; 'is' ; ‘Burdell'}
Note 1 x 4
• Lessons?
• Exercise … Important
» help lists
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Cell Array Examples … contd.
• Exercise
» A = {'My'; 'name'; 'is' ; 'Farrokh'}
» iscellstr(A) % logical test for a cell array of strings
» ischar(A) % logical test for a string array
• Lesson?
• Exercise
»
»
»
»
»
»
help cell
help cellstr
help celldisp
B = cell(2,4)
B = {'My', 'name', 'is', Burdell; 10, 20, 30, 40}
celldisp(B)
• Lesson?
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Cell Arrays are Simple…
The basics of working with cell arrays are well
explained in the Matlab documentation.
Additional information can be found at the Mathworks
web site:
http://www.mathworks.com/
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What are Structures?
• Numeric, character and cell arrays all reference the
individual elements by number.
• Structures reference individual elements within each
row (called “fields”) by name.
• To access these fields, the dot “.” notation is used.
• Assignment is as follows:
structurename.fieldname = datatype;
Text Page 115
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Creating a Structure…
• Let’s create a simple structure:
–
–
–
–
–
–
person.firstname = ‘George’;
person.lastname = ‘Burdell’;
person.address1 = ‘803 Tech Parkway’;
person.city = ‘Atlanta’;
person.state = ‘GA’;
person.zip = ‘30332-0001’;
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Structures: A Bigger Picture…
• Structures can hold elements in fields, which in
turn contain data types.
‘Hello’
text1
numb1
[1 2 3 4]
text2
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numb2
5 6
7 8
mystruc
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More on Structures…
• A structure can have a field that is a structure itself.
• A structure array is that which contains more than one
record for each field name.
• As the structure array is expanded (more records are
created), all unassigned fields are filled with an empty
matrix.
• All structures have the same number of fields and
elements in each field.
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Example of a Structure ...:
student.name.first = ‘Joe’;
student.name.last = ‘Smith’;
student.score = 82.39;
student.grade = ‘B’;
student(2).name.first = ‘Anna’;
student(2).name.last = ‘Lee’;
student(2).score = 94.50;
Student(3).name.first = ‘Jerry’;
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• NOTE: There
are other
spaces to fill,
but we haven’t
assigned any
values to
these fields,
so they remain
as an empty
matrix.
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Picture of Student Grade Structure
student(1) student(2) student(3)
first
Joe
Anna
Jerry
last
Smith
Lee
[]
score
82.39
94.50
[]
grade
B
[]
[]
name
student
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Structure Resources…
• Structures are explained in the Matlab online
documents.
• You can find tutorials on the web:
http://www.mathworks.com/
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Summary
Learning
Matlab can store
and manipulate
much more than
simply numbers
AE6382 Design Computing
Action Items
• Review the lecture
• Review the material on the websites
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