Class Prep

1
Class Prep

• Bring to Class
• In-Class Exercises
• Homework 6
• Paper for Classroom Printer
• Copy Programs and CD Folder
• Run MATLAB

2
Grab Files

• Copy Clue_script.m, students.m, (CDs.m, makeCD.m,
  filterCDs.m, printAll.m) in CD folder from
  70week06 of the csc070 course folder into your
  sub-folder
• Run MATLAB and set the Current Directory to your
  sub-folder.

3
HW 5d

• function ltrs firstLetters(str) by Dave
  Hannay
• find the first letters in this string
• where regexp(str,'\lt\w')
• ltrs str(where)

4
Week 06-b(7.3-7.5)

• Structures

5
Creating a Structure

• Lets create a simple structure
• person.firstname 'George'
• person.lastname 'Orwell'
• person.address1 '123 Sesame St.'
• person.city 'Any City'
• person.state 'NY'
• person.zip '12345-6789'

6
Structures are Universal Containers

• Any type of data can be loaded into a structure
  array
• Different data types can be loaded and will
  overwrite whatever is already there
• Structure addressing is simpler that Cell Arrays
• fieldnames( ) will return the field names in a
  structure
• You should get used to using the dot notation
  because it is widely used in programming,
  especially with objects

7
Structures The Big Picture

• Structures can hold elements in fields, which in
  turn contain data.

text1
'Hello'
numb1
1 2 3 4
mystruc
AE6382 Fall 2004
text2
5 6 7 8
numb2
8
Terminology

• A structure array is a collection of records.
• A record is a set of related fields where each
  field may contain a different data type.
• A field is an array of data that defines a
  particular attribute of an object.
• i.e. The collection of fields comprise a record
  the collections of records comprise a structure
  array.
• For example
• Structure Array employees
• Record employees(3)
• Fields name, address, date of employment,
  salary, etc.

9
Hands-On DEMO Sample of Direct Assignment(see
Clue_script.m)

• Clue struct Set up "Clue" as an empty struct
• Structure fields can be
• created and assigned dynamically.
• Clue.who 'Prof Plum'
• Clue.where 'Study'
• Clue.what 'Candlestick'
• disp(Clue)
• who 'Prof Plum'
• where 'Study'
• what 'Candlestick'

10
More Samples

• Even though no index was used in creating the
• first record in 'Clue', another set can be
• appended by using an index.
• Clue(2).who 'Ms. Scarlet'
• Clue(2).where 'Library'
• Clue(2).what 'Rope'
• disp(Clue(2))
• who 'Ms. Scarlet'
• where 'Library'
• what 'Rope'

11
Adding Fields

• New fields can be added after structures
• have been created.
• Clue(2).turns 15 "turns" is a new field
• disp(Clue(2)) used 1st in record 2
• who 'Ms. Scarlet'
• where 'Library'
• what 'Rope'
• turns 15

12
Adding Fields

• Adding new fields will cause a 'null' field in
  the
• cells already defined.
• disp(Clue(1))
• who 'Prof Plum'
• where 'Study'
• what 'Candlestick'
• turns turns was not defined originally

13
The struct Function

• All fields (a complete record) can be assigned
• with one statement.
• All text values are enclosed in quotes
• numerical data are not.
• Clue(3) struct('who', 'Col Mustard', ...
  'where', 'kitchen', ...
  'what', 'revolver', ...
  'turns', 22)
• disp(Clue(3))
• who 'Col Mustard'
• where 'kitchen'
• what 'revolver'
• turns 22

14
Using Array Functions

• Clue(1).turns is null, we can assign it a
  value.
• Clue(1).turns 17
• Below, notice the lack of an index for Clue and
  the square brackets
• around Clue.turn. This combination returns a
  vector of 'turns'
• to the MATLAB function, sum, which calculates
  the sum of that
• vector.
• The length function returns the number of
  records in Clue.
• Dividing the sum by the number of records
  gives us the average
• of the numbers stored in turns.
• avg_turns sum(Clue.turns)/length(Clue)
• mean(Clue.turns) would also
  work!
• Then we display our results.
• fprintf('Average number of turns
  0.1f\n',avg_turns)
• 18.0

15
Hands-On DEMO people Structure

• people(1).name 'Elmo'
• people(1).city 'Schenectady'
• people(1).state 'NY'
• people(2).name 'Oscar'
• disp(people(2))
• Also try
• Disp(people)

16
DEMO Adding Fields to Structures

• You add a field to every structure in an array by
  adding the field to a single structure. For
  example, to add a social security number field to
  the person array
• people(2).ssn '555-12-6666'
• Now people(2).ssn has the assigned value.
• NOTE Every other structure in the array now also
  has the ssn field, but these fields contain the
  empty matrix until you explicitly assign a value
  to them.

17
Manipulating Field Names

• To determine the names of the fields in a
  structure, the built-in function fieldnames(...)
  returns a cell array containing the field names
  as strings.
• gtgt fieldNames fieldnames(people)
• fieldNames
• 'name'
• 'item2'...
• Fields can also be accessed indirectly by
  setting a variable to the name of the field, and
  then using parentheses to indicate that the
  variable contents should be used as the field
  name
• gtgt fn fieldNames1
• gtgt people(1).(fn) people(1).(fn) 'xxx'
• people
• name 'Elmoxxx'
• city Schenectady
• ...

18
DEMO Deleting Fields from Structures

• You can remove a given field from every structure
  within a structure array using the rmfield
  function. Its most basic form is
• struct2 rmfield(struct1, 'field')
• newpeople rmfield(people,'state')
• disp(newpeople)
• disp(people)
• disp(newpeople(1))

19
Hands-On DEMO Examining Structure Values

• Note that you can double-click variables in the
  Workspace to examine their values
• For structure arrays, you can double-click
  individual records (in the grid) to examine
  individual field values
• Try this with the people and newpeople structures

20
Nesting Structures

• A structure field can contain another structure,
  or even an array of structures.
• Once you have created a structure, you can use
  the struct function or direct assignment
  statements to nest structures within existing
  structure fields.

21
Nested Structures with the struct Function

• For example, create a 1-by-1 structure array
• A struct('data', 3 4 7 8 0 1, 'nest',...
• struct('testnum', 'Test 1', 'xdata', 4 2
  8,...
• 'ydata', 7 1 6) )
• gtgt disp(A(1).data)
• gtgt disp(A(1).nest.xdata)
• Try double-clicking on the A variable in the
  Workspace

22
Nested Structures using Direct Assignment

• These statements add a second element to the
  array
• A(2).data 9 3 2 7 6 5
• A(2).nest.testnum 'Test 2'
• A(2).nest.xdata 3 4 2
• A(2).nest.ydata 5 0 9

23
Indexing Nested Structures

• To index nested structures, append nested field
  names using dot notation.
• The first text string in the indexing expression
  identifies the structure array, and subsequent
  expressions access field names that contain other
  structures.
• To access all the xdata vector in the nested
  structure in A(2), use A(2).nest.xdata.
• To access element 2 of the ydata field in A(1),
  use A(1).nest.ydata(2).

24
Another Nested Structure

• student.name.first 'Joe'
• student.name.last 'Smith'
• student.score 82.39
• student.grade 'B'
• student(2).name.first 'Jane'
• student(2).name.last 'Lee'
• student(2).score 94.50
• Student(3).name.first 'Jerry'

• NOTE There are other spaces to fill, but we
  havent assigned any values to these fields, so
  they remain as an empty matrix.

25
Picture of Student Grade Structure
student(2)
student(3)
student(1)
first
Joe
Jane
Jerry
name
last
Smith
Lee

student
score
82.39
94.50

grade
B


26
Hands-On DEMO Student Structure(see students.m)

• Creates a Structure Array called student with
  fields name and grade (grade will be an array
  with three exam grades in it)
• Fills in the structure with made up data for
  three students
• Averages the grades of all three students and
  displays the result in the command window

27
7.5 Engineering ExampleAssembling a Structure

• Given this structure, determine the order of
  assembly starting from A.

28
Hands-On DEMO CD Albums

• Simplified version of Engineering Example 7.5
• Each entry in the array will be a structure
  describing the CD
• Genre
• Artist
• Title
• Tracks
• Year
• See CD folder

29
Custom Constructor Functions

• See makeCD.m
• gtgt CD makeCD('Blues', 'Charles, Ray', ...
• 'Genius Loves Company', 18, 4.5, 2004 )
• CD
• genre 'Blues'
• artist 'Charles, Ray'
• title 'Genius Loves Company'
• tracks 18
• stars 4.5000
• year 2004

30
Building Structure Arrays with makeCD()

• See CDs.m
• album(1) makeCD( 'rap', 'P Diddy', 'Doofus',
  14, 4, 1996)
• album(2) makeCD( 'rock', 'Lincoln Park',
  'Metea', 16, 3, 2003)
• album(3) makeCD( 'classic', 'Yanni',
  'Dreaming', 26, 0.6, 1999)
• album(4) makeCD('country', 'Garth Brooks',
  'Sevens',20, 4.5, 1995)

31
Getting at ALL the Fields

• function printAll(CDs)
• flds fieldnames(CDs(1))
• for CD CDs
• str '-gt'
• for index 1length(flds)
• attrib fldsindex
• str str ' ' strValue(CD.(attrib))
• end
• disp(str)
• end

32
Hands-On DEMO Filter by Year

• filterCDs should display the titles
• of all CDs since some given year
• year input('List CDs since what year? ')
• filterCDs(album, year)
• Try 1999

33
The filterCDs function

• function c filterCDs( a, yr )
• extract the titles of CDs recorded since this
  year
• for ix 1length(a)
• thisCD a(ix)
• if thisCD.year gt yr
• disp(thisCD.title)
• end
• end

34
SUMMARY Cell Array vs. Structure
35
Further Study

• There are only 6 different operations you can
  possibly perform on a collection of data items.
• Each operation requires a function to specify how
  that operation should be performed
• The operations are
• Build create the collection from some source
• Traverse touch each item and summarize
• Map change the values in each item
• Filter remove some items from the collection
• Search determine whether a specific item exists
• Sort re-order the collection

36
What else could we do with our catalog of CDs?

• Display each item as a catalog
• Delete items you have sold or discarded
• Add items as you buy new ones
• Update all the records to add a field (like price
  or perhaps its position in the top 100)
• Find a particular CD by ANY fieldname
• Sort them by artist or genre

37
Homework 6 Structures File I/O

• 6a) Problem 9, page 182.
• 6b) Problem 13, page 184.
• 6c) Problem 1, page 200.
• 6d) Problem 5, page 201.

38
In-Class Exercise 6b Structured Data(Problem
10, Page 182)

• Create a library as an array of structures. Each
  structure has the following fields Title,
  Author, ISBN.
• Using MATLAB structures, create the first three
  entries for the library structure array, using
  data of your choosing.
• Write a script that will display the titles of
  all the books in the library structure array.
• You should be able to deal with libraries of all
  different sizes.

39
END