Ch7Structs.pptx

Copyright © 2017, 2014 Pearson Education, Inc.
Chapter 7: Introduction to Classes and
Objects
Starting Out with C++
Early Objects
Ninth Edition
by Tony Gaddis, Judy Walters,
and Godfrey Muganda

Topics
7.12 Structures
7-2

Copyright © 2017, 2014 Pearson Education, Inc. 3
Structures
• Recall that elements of arrays must all be
of the same type
• In some situations, we wish to group
elements of different types
scores : 85 79 92 57 68 80 . . .
0 1 2 3 4 5 98 99
employee R. Jones 123 Elm 6/12/55 14.75

7.12 Structures
• Structure: A programmer-defined data type
that allows multiple variables to be grouped
together
• Structure declaration format:
struct structure name
{
type1 field1;
type2 field2;
…
typen fieldn;
};
7-4

Example struct Declaration
struct StudentStruct
{
int studentID;
string name;
short year;
double gpa;
};
7-5
structure name
structure members
Note the
required
;

struct Declaration Notes
• struct names commonly begin with an
uppercase letter, because they are a
type
• Multiple fields of same type can be
declared in a comma-separated list
string name,address;
• Fields in a structure are all public by
default
7-6

Defining Structure Variables
• A struct declaration does not allocate
memory or create variables, because they
are a type
• To define variables, use structure tag as the
type name
StudentStruct s1;
7-7
studentID
name
year
gpa
s1

Accessing Structure Members
• Use the name of the struct variable
the name of the member
separated by a dot .
getline(cin, s1.name);
cin >> s1.studentID;
s1.gpa = 3.75;
• The dot is called the member selector
7-8

Aggregate Operations with Structures
• Recall that arrays had no whole array
operations (except passing reference to
parameter)
• Structures DO have aggregate operators
– assignment statement =
– parameter (value or reference)
– return a structure as a function type

Aggregate Operations with Structures
• Limitations on aggregate operations
– no I/O
– no arithmetic operations
– no comparisons
cout << old_part;
cin >> new_part;
old_part = new_part + old_part;
if (old_part < new_part)
cout << ...;

Aggregate Operations with
Structures
• struct variables must be initialized,
compared, written one member at a time.

Displaying struct Members
To display the contents of a struct
variable, you must display each field or
member separately, using the dot operator
Wrong:
cout << s1; // won’t work!
Correct:
cout << s1.studentID << endl;
cout << s1.name << endl;
cout << s1.year << endl;
cout << s1.gpa;
7-12

Comparing struct Members
• Similar to displaying a struct, you
cannot compare two struct variables
directly:
if (s1 >= s2) // won’t work!
• Instead, compare member variables:
if (s1.gpa >= s2.gpa) // better
7-13

Initializing a Structure
Structure members cannot be initialized in
the structure declaration, because no
memory has been allocated yet
struct Student // Illegal
{ // initialization
int studentID = 1145;
string name = "Alex";
short year = 1;
float gpa = 2.95;
};
7-14

Initializing a Structure (continued)
• Structure members are initialized at the time
a structure variable is created
• You can initialize a structure variable’s
members with either
– an initialization list
– a constructor
7-15

Initialization List Example
Structure Declaration Structure Variable
struct Dimensions
{ int length,
width,
height;
};
Dimensions box = {12,6,h};
7-16
box
length 12
width 6
height 3

Using a Constructor to Initialize
Structure Members
• This is similar to a constructor for a class:
– the name is the same as the name of the
struct
– it has no return type
– it is used to initialize data members
• It is normally written inside the struct
declaration
7-17

A Structure with a Constructor
struct Dimensions
{
int length,
width,
height;
// Constructor
Dimensions(int L, int W, int H)
{length = L; width = W; height = H;}
};
7-18

Nested Structures
A structure can have another structure as a
member.
struct PersonInfo
{ string name,
address,
city;
};
struct Student
{ int studentID;
PersonInfo pData;
short year;
double gpa;
};
7-19

Members of Nested Structures
Use the dot operator multiple times to access
fields of nested structures
Student s5;
s5.pData.name = "Joanne";
s5.pData.city = "Tulsa";
Reference the nested structure’s fields by the
member variable name, not by the structure
name
s5.PersonInfo.name = "Joanne"; //no!
7-20

Structures as Function Arguments
• You may pass members of struct
variables to functions
computeGPA(s1.gpa);
• You may pass entire struct variables to
functions by value or by reference
void CopyPart (PartStruct partOrig, PartStruct & partNew);
7-21

Notes on Passing Structures
• Using a value parameter for structure can
slow down a program and waste space
• Using a reference parameter speeds up
program execution, but it allows the function
to modify data in the structure
• To save space and time while protecting
structure data that should not be changed,
use a const reference parameter
void showData(const StudentStruct &s)
// header
7-22

Returning a Structure from a Function
• A function can return a struct
StudentStruct getStuData(); // prototype
s1 = getStuData(); // call
• The function must define a local structure
variable
– for internal use
– to use with return statement
7-23

Returning a Structure Example
Student getStuData()
{ Student s; // local variable
cin >> s.studentID;
cin.ignore();
getline(cin, s.pData.name);
getline(cin, s.pData.address);
getline(cin, s.pData.city);
cin >> s.year;
cin >> s.gpa;
return s;
}
7-24

Contrast Arrays and structs

Arrays of structs
• First declare a struct (such as
PartStruct)
• Then specify an array of that type
• Access elements of the array, elements of
the struct
typedef PartStruct PartsArray [50];
for (x = 0; x <50; x++)
cout << _______________________;
How do we
print all the
descrip fields?
parts [x].descript;

structs with Arrays
• Example
const ARRAYSIZE = 1000;
struct LISTTYPE
{
int list [ARRAYSIZE]; //array containing the list
int listLength; //length of the list
}

Hierarchical structures
• Defn => structs where at least one of the
components is, itself, a struct
• Example:
struct InventoryStruct {
PartStruct part;
int qty_sold, re_order_qty;
VendorStruct vendor; };

Choosing Data Structures
• Strive to group logical elements of a
structure together
– calls for hierarchical structures
• Push details of entities down to lower levels
of the structure
• Data Abstraction <=> separation of logical
peoperties of a data type from its
implementation

Testing and Debugging Hints
• Declaration of a struct type must end with
a semicolon ;
• Be sure to specify the full member selector
when referencing a component of a struct
variable
– don’t leave out the struct name

Testing and Debugging
• When using an array in a struct, the index
goes at the end
studentRecord.scores[x]
• When using an array of struct, the index
goes after the struct name
parts[x].qty

• Process struct members separately … the
only aggregate operations will be
• Assignment =
• Parameter passing
void do_it (PartStruct part);
• Function return
PartStruct ProcessThings ( );

• Be careful using same member names in
different struct types
• Compiler keeps them separate OK
• Human readers can easily confuse them
• struct PartStruct {
int qty;
. . .
} ;
• struct ScoresStruct {
int qty;
. . .
} ;`

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