lecture 26: structures / struct s/. 2 lecture contents: t basics of structs t struct type definition...

Lecture 26:Structures /structs/

2

Lecture Contents:

Basics of structs Struct type definition (struct reserved word) Struct type definition (typedef statement) Structs and functions Structs as input parameters and return values Arrays of structs Pointers to structs Demo programs Exercises

3

Reminder on data structs

Data structure: composite of related data items stored in memory under the same name.

Typical data structures available in most PL:– Array: already discussed – Structure: to discuss today (same as record in Pascal)

– Class: structure with functions /methods/ (OOP)– File (I/O processing aside keyboard and screen)

4

Reminder on arrays

Array: a collection of data items of the same type.

Reminder:

Simple data type: Data type used to store a single value.

scalar variable stores a single value.

Therefore array may store many data items of the same type.

5

Basics of structures

Definition: A collection of simple variables.

A data type for a record composed of multiple components. These components may be the same data type or may be not the same data type.

6

Struct definition (declaration)

struct specifier – applying struct reserved word.

struct Date

{

int day, month, year;

};

This is a template only which does not occupy storage /memory cells/.

7


OR

8


Applying typedef statement

typedef struct { int day, month, year; } Date;

This is a template only which does not occupy storage /memory cells/.

9

The struct template

The struct template has to be globally defined.

It is recommended The struct template to be typed outside any function body.

Globally defined struct template makes it visible in all functions that follow the definition.

10

Basics of structs

Arrays are data structures to store a collection of data items of the same type.

Structs as opposed to arrays, are data structures to store collections of related data items that have different types.

The individual component of a struct is called a member.

11


struct specifier – no storage occupying sample

struct Person {char name[20];char gender;int age;float grade;

};

12


OR

13


Applying typedef reserved word

typedef struct

{

char name[20];

char gender;

int age;

float grade;

} Person;

14


Member of the Person struct

char name[20]

can be replaced to

char *name or

string name

15

Referencing a struct type

How to define a struct as a variable to be a scalar, an array or a pointer?

Date a, b;

Date c[20];

Date *ptr1;

16

Referencing a struct type

How to define a struct as a variable to be a scalar, an array or a pointer?

Person John, Mary;

Person ClassCos120a[20];

Person ClassCos120b[17];

Person *ptr2;

17

Initializing struct variables

Date today = { 28, 11, 2014 };

Person peter = {“Miller”, ‘m’, 19, 3.48 };

18

Accessing struct members

Date a, b, c[20], *ptr1;

Direct component selection operator . (period)

a.day=28;

b.month=11;

c[10].year=2014;

Indirect component selection operator ─>

ptr1 = &a; ptr1─>month = 4;

19

Structs within structs

struct Dimension{ int feet; float inches; };

struct Room { Dimension length, width; };

Room kitchen; kitchen.length.feet = 12; kitchen.length.inches = 2.4; kitchen.width.feet = 10; kitchen.width.inches = 1.8;

20

Functions returning structs struct Point { int x; int y; };

Point makepoint(int xvar, int yvar) { Point temp; temp.x = xvar;temp.y = yvar; return temp;

}

void main() { Point a, b;

a = makepoint(0,0);b = makepoint(350,200);. . .

}

21

Passing struct variables as arguments to functions

struct Point { int x; int y; };

struct Rect { Point pt1, pt2; };

Pt1 – top left corner

Pt2 – bottom right corner

bool isPtinRect(Point p, Rect r)

{

return p.x>=r.pt1.x && p.x<=r.pt2.x &&

p.y>=r.pt1.y && p.y<=r.pt2.y ;

}

22

Functions with structs as arguments returning structs

struct Point { int x; int y; }; Point addpoint( Point p1, Point p2) {

Point temp; // alternate versiontemp.x = p1.x + p2.x; // p1.x = p1.x + p1.y;temp.y = p1.y + p2.y; // p1.y += p2.y;return temp; // return p1;

} void main() { Point a, b, c; a=makepoint(0,0);

b=makepoint(40, 20); c=addpoint(b,makepoint(20, 40));. . . }

23

Parallel arrays and arrays of structs

Problem: data base for COS120 students

Solution based on parallel arrays:

Alternate solution: based on array of structures:

24


Problem: data base for COS120 students Solution based on parallel arrays:

char *name[20]; int id[20]; double gpa[20];

Three parallel arrays because data items with same subscript pertain the same student name[I] = “Peter”;

id[I] = 0100245678;gpa[I] = 3.68;

25


Problem: data base for COS120 students Alternate solution: based on array of structs:

struct Stud{char *name; int id; double gpa;};

Stud ar[20];

. . .

ar[I].name = “Peter”;

ar[I].id = 0100245678;

ar[I].gpa = 3.68;

26

Pointers to structs

struct Person { char *name, gender; int age;};

Person Mary, family[5], *ps; for (ps=family; ps<family+5; ps++) // or for (ps=&family[0]; ps<=&family[4]; ps++) {

(*ps).name = … or ps->name = …(*ps).gender = … or ps->gender = …(*ps).age = … or ps->age = …

}

27

Pointers to Structs

struct Electricity

{

string current;

int volts;

};

Electricity *p, *q;

p and q are pointers to a struct of type Electricity

28

Pointers to Structs

p = new Electricity; Allocates storage for struct of type

Electricity and places address into pointer p

Struct access operator .

? ?current voltsp

29

Assignments

*p.current = “AC”;*p.volts = 115;

Statements above can also be writtenp -> current = “AC”;p -> volts = 115;

AC 115current voltsp

30

Struct Member Access via Pointers

Form: p -> m Example: p -> volts

Example:– cout << p->current << p->volts <<endl;

Output– AC115

31

Pointers to Structs

q = new Electricity; Allocates storage for struct of type Electricity and

places address into pointer q Copy contents of p struct to q struct

*q = *p;

AC 115

q->current q->voltsq

32

Pointers to Structs

q->volts = 220;

q = p;

AC 220

q->current q->voltsq

AC 115

AC 220

q

p q->current q->voltsp->current p->volts

33

Unions

A data structure that overlays components in memory, allowing one chunk of memory (the same memory cell or memory cells with same initial address) to be interpreted in multiple ways.

34

More on structs

Extract from Friedman/Koffman, chapter 9

Data Structures Arrays and Structs

Chapter 9

36

9.7 The Struct Data Type

struct used to store related data items Individual components of the struct are

called its members Each member can contain different types of

data Employee example

37

Struct Employee

// Definition of struct employeestruct employee{ int id; string name; char gender; int numDepend; money rate; money totWages;};

38

Employee variables

// Definition of two struct employee variables

employee organist, janitor;

39

Anonymous struct

// Definition of anonymous struct

struct

{

int id;

string name;

char gender;

int numDepend;

money rate;

money totWages;

}

organist, janitor;

40

Accessing Members of a struct

Members are accessed using the member access operator, a period

For struct variable s and member variable m to access m you would use the following:– cout << s.m << endl;

Can use all C++ operators and operations on structs

41


organist.id = 1234;

organist.name = “Noel Goddard”;

organist.gender = ‘F’;

organist.numDepend = 0;

organist.rate = 6.00;

organist.totWages += organist.rate * 40.0;

42

9.8 Structs as Operands and Arguments

How to do arithmetic and other operations using structs

Process entire struct using programmer defined functions

Often better to pass an entire struct rather than individual elements

43

Struct Copy or Assignment

struct copies

Given employee struct


organist = janitor;

44

Passing struct as an Argument

Grading program example Keep track of students grades Prior to our learning structs we needed to

store each item into a single variable Group all related student items together Pass struct by const reference if you do not

want changes made

45

ExamStat.h

// FILE: ExamStat.h

struct examStats

{

string stuName;

int scores[3];

float average;

char grade;

};

466

PrintStats.cpp

// File: printStats.cpp Prints the exam statistics// Pre: The members of the struct variable// stuExams are assigned values.// Post: Each member of stuExams is displayed.

void printStats(examStats stuExams) { cout << "Exam scores for " <<stuExams.stuName << ":\n“; cout << stuExams.scores[0] << ' ' <<

stuExams.scores[1] << ' ' << stuExams.scores[2] << endl;

cout << "Average score: " <<stuExams.average << endl; cout << "Letter grade : " << stuExams.grade << endl;}

47

Common Programming Errors

No prefix to reference a struct member Incorrect prefix reference to a struct

member Missing ; following definition of struct Initialization of struct members

48

Exercise 26.1-26.7

Build programs based on structs: Create a struct named Date to include three components all of int type – Day, Month and Year. Define

three Date variables – D1, D2 and D3. D1 to be initialized. Load values for D2 using assignment statements and values for D3 entering input data from the keyboard. Display on the screen all members of the struct variables D1, D2, D3.

Create a struct named Time to include three components all of int type – Hrs, Mins and Scnds. Define three Time variables – T1, T2 and T3. T1 to be initialized. Load values for T2 using assignment statements and values for T3 entering input data from the keyboard. Display on the screen all members of the struct variables T1, T2, T3.

Create a struct named Person with data members for name, gender and age. Define struct variables describing data for your family.

Create a struct named Employee with data members for name, position and salary. Define struct variables describing data for five officers.

Create a struct named Student with data members for name, id# and e-mail address. Define struct variables describing data for your friends.

Create a struct to describe a 2D point in polar coordinates. Create a struct to describe a 3D point in rectangular coordinates.

49

Homework 6

Paper record to be presented before the end of next class

Tasks:– See .doc file.

50

Before lecture end

Lecture:

Structs

More to read:

Friedman/Koffman, Chapter 09

Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Chapter 9:Data Structs: Arrays and Structs

Problem Solving,

Abstraction, and Design using C++ 5e

by Frank L. Friedman and Elliot B. Koffman

Copyright © 2007 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 52

9.7 The struct Data Type

• struct used to store related data items • Individual components of the struct are called its

members• Each member can contain different types of data• Three parts

– declaration of struct

– use of struct to define a new variable

– access of members of the structd variable

• Typically declare struct as global


struct Type Declaration

struct struct-type

{

type1 id-list1;

type2 id-list2;

. . .

typen id-listn;

};

struct employee

{

int id;

string name;

char gender;

int numDepend;

float rate;

float totWages;

};

. . .




organist.id = 1234;

organist.name = “Noel Goddard”;

organist.gender = ‘F’;

organist.numDepend = 0;

organist.rate = 12.00;

organist.totWages = 480.0;

. . .

organist.totWages += (organist.rate * 40.0);


9.8 structs as Operands and Arguments

• struct copy or assignment

organist = janitor; // copy janitor to organist

• Copies each member from janitor struct to organist struct


Passing a struct as an Argument

• Actual and formal parameters must be same type struct

• Using structs as arguments can shorten argument list

• Passing structs by value can be inefficient, since it duplicates values of all members


Passing struct by Reference

• Same as other reference parameters– use & to identify in parameter list

• Can also use constant reference– precede parameter with reserved word const– E.g.

void printStats(const examStats& stuExams)


Listing 9.11 Function printStats


Listing 9.12 Function readEmployee


9.10 Common Programming Errors

• structs– References to a struct member with no prefix– Reference to a struct member no incorrect

prefix– Missing semicolon following a struct

declaration

61

Thank You

For

Your Attention

lecture 26: structures / struct s/. 2 lecture contents: t basics of structs t struct type definition...

Documents

simple data type

collection of data items

data structsdata structure

definition declarationmember

int feet

room kitchen kitchen

b date c20 date

int day