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10

C Structures, Unions, Bit Manipulations and Enumerations

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But yet an union in partition.

William Shakespeare

The same old charitable lie�Repeated as the years scoot by�Perpetually makes a hit—

“You really haven’t changed a bit!”

Margaret Fishback

I could never make out what those�damned dots meant.

Winston Churchill

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OBJECTIVES

In this chapter you will learn:

To create and use structures, unions and enumerations.
To pass structures to functions by value and by reference.
To manipulate data with the bitwise operators.
To create bit fields for storing data compactly.

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10.1 Introduction

10.2 Structure Definitions

10.3 Initializing Structures

10.4 Accessing Members of Structures

10.5 Using Structures with Functions

10.6 typedef

10.7 Example: High-Performance Card Shuffling and Dealing Simulation

10.8 Unions

10.9 Bitwise Operators

10.10 Bit Fields

10.11 Enumeration Constants

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10.1 Introduction

Structures

Collections of related variables (aggregates) under one name

Can contain variables of different data types

Commonly used to define records to be stored in files
Combined with pointers, can create linked lists, stacks, queues, and trees

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10.2 Structure Definitions

Example

struct card {

char *face;

char *suit;� };

struct introduces the definition for structure card
card is the structure name and is used to declare variables of the structure type
card contains two members of type char *

These members are face and suit

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Common Programming Error 10.1

Forgetting the semicolon that terminates�a structure definition is a syntax error.

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10.2 Structure Definitions

struct information

A struct cannot contain an instance of itself
Can contain a member that is a pointer to the same structure type
A structure definition does not reserve space in memory

Instead creates a new data type used to define structure variables

Definitions

Defined like other variables:

card oneCard, deck[ 52 ], *cPtr;

Can use a comma separated list:

struct card {

char *face;

char *suit;

} oneCard, deck[ 52 ], *cPtr;

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Good Programming Practice 10.1

Always provide a structure tag name when creating a structure type. The structure tag name is convenient for declaring new variables of the structure type later in the program.

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Good Programming Practice 10.2

Choosing a meaningful structure tag name helps make a program self-documenting.

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10.2 Structure Definitions

Valid Operations

Assigning a structure to a structure of the same type
Taking the address (&) of a structure
Accessing the members of a structure
Using the sizeof operator to determine the size of a structure

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Common Programming Error 10.2

Assigning a structure of one type to a structure of a different type is a compilation error.

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Common Programming Error 10.3

Comparing structures is a syntax error.

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Fig. 10.1 | Possible storage alignment for a variable of type struct example showing an undefined area in memory.

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Portability Tip 10.1

Because the size of data items of a particular type is machine dependent and because storage alignment considerations are machine dependent, so too is the representation of a structure.

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10.3 Initializing Structures

Initializer lists

Example:

card oneCard = { "Three", "Hearts" };

Assignment statements

Example:

card threeHearts = oneCard;

Could also define and initialize threeHearts as follows:

card threeHearts;

threeHearts.face = “Three”;

threeHearts.suit = “Hearts”;

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10.4 Accessing Members of Structures

Accessing structure members

Dot operator (.) used with structure variables

card myCard;

printf( "%s", myCard.suit );

Arrow operator (->) used with pointers to structure variables

card *myCardPtr = &myCard;

printf( "%s", myCardPtr->suit );

myCardPtr->suit is equivalent to

( *myCardPtr ).suit

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Error-Prevention Tip 10.1

Avoid using the same names for members of structures of different types. This is allowed, but it may cause confusion.

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Good Programming Practice 10.3

Do not put spaces around the -> and .�operators. Omitting spaces helps emphasize that the expressions the operators are contained in are essentially single variable names.

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Common Programming Error 10.4

Inserting space between the - and > components of the structure pointer operator (or between the components of any other multiple keystroke operator except ?:) is a syntax error.

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Common Programming Error 10.5

Attempting to refer to a member of a structure by using only the member’s name is a syntax error.

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Common Programming Error 10.6

Not using parentheses when referring to�a structure member that uses a pointer and the structure member operator�(e.g., *cardPtr.suit) is a syntax error.

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Outline

fig10_02.c

(1 of 2 )

Structure definition

Structure definition must end with semicolon

Dot operator accesses members of a structure

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Outline

fig10_02.c

(2 of 2 )

Arrow operator accesses members of a structure pointer

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10.5 Using Structures with Functions

Passing structures to functions

Pass entire structure

Or, pass individual members

Both pass call by value

To pass structures call-by-reference

Pass its address
Pass reference to it

To pass arrays call-by-value

Create a structure with the array as a member
Pass the structure

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Common Programming Error 10.7

Assuming that structures, like arrays, are automatically passed by reference and trying to modify the caller’s structure values in the called function is a logic error.

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Performance Tip 10.1

Passing structures by reference is more efficient than passing structures by value (which requires the entire structure to be copied).

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10.6 typedef

typedef

Creates synonyms (aliases) for previously defined data types
Use typedef to create shorter type names
Example:

typedef struct Card *CardPtr;

Defines a new type name CardPtr as a synonym for type struct Card *
typedef does not create a new data type

Only creates an alias

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Good Programming Practice 10.4

Capitalize the first letter of typedef names to emphasize that they are synonyms for other type names.

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Portability Tip 10.2

Use typedef to help make a program�more portable.

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10.7 Example: High-Performance Card Shuffling and Dealing Simulation

Pseudocode:

Create an array of card structures
Put cards in the deck
Shuffle the deck
Deal the cards

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Outline

fig10_03.c

(1 of 3 )

Each card has a face and a suit

Card is now an alias for struct card

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Outline

fig10_03.c

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Constant pointer to modifiable array of Cards

Fills the deck by giving each Card a face and suit

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Outline

fig10_03.c

(3 of 3 )

Each card is swapped with another, random card, shuffling the deck

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Outline

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Common Programming Error 10.8

Forgetting to include the array subscript when referring to individual structures in an array of structures is a syntax error.

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10.8 Unions

union

Memory that contains a variety of objects over time
Only contains one data member at a time
Members of a union share space
Conserves storage
Only the last data member defined can be accessed

union definitions

Same as struct

union Number {

int x;

float y;

};

union Number value;

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10.8 Unions

Valid union operations

Assignment to union of same type: =
Taking address: &
Accessing union members: .
Accessing members using pointers: ->

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Common Programming Error 10.9

Referencing data in a union with a�variable of the wrong type is a logic error.

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Portability Tip 10.3

If data is stored in a union as one type and referenced as another type, the results are implementation dependent.

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Software Engineering Observation 10.1

As with a struct definition, a union definition simply creates a new type. Placing a union or struct definition outside any function does not create a global variable.

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Common Programming Error 10.10

Comparing unions is a syntax error.

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Portability Tip 10.4

The amount of storage required to store a union is implementation dependent but will always be at least as large as the largest member of the union.

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Portability Tip 10.5

Some unions may not port easily to other computer systems. Whether a union is portable or not often depends on the storage alignment requirements for the union member data types on a given system.

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Performance Tip 10.2

Unions conserve storage.

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Outline

fig10_05.c

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Union definition

Union definition must end with semicolon

Note that y has no value

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Outline

fig10_05.c

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Giving y a value removes x’s value

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10.9 Bitwise Operators

All data is represented internally as sequences of bits

Each bit can be either 0 or 1
Sequence of 8 bits forms a byte

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Portability Tip 10.6

Bitwise data manipulations are machine dependent.

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Fig. 10.6 | Bitwise operators.

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Outline

fig10_07.c

(1 of 2 )

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Outline

fig10_07.c

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displayMask is a 1 followed by 31 zeros

Bitwise AND returns nonzero if the leftmost bits of displayMask and value are both 1, since all other bits in displayMask are 0s.

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Common Programming Error 10.11

Using the logical AND operator (&&) for the bitwise AND operator (&) and vice versa is an error.

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Fig. 10.8 | Results of combining two bits with the bitwise AND operator &.

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Fig. 10.11 | Results of combining two bits with the bitwise inclusive OR operator |.

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Fig. 10.12 | Results of combining two bits with the bitwise exclusive OR operator ^.

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Outline

fig10_09.c

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Bitwise AND sets each bit in the result to 1 if the corresponding bits in the operands are both 1

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Outline

fig10_09.c

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Bitwise inclusive OR sets each bit in the result to 1 if at least one of the corresponding bits in the operands is 1

Bitwise exclusive OR sets each bit in the result to 1 if only one of the corresponding bits in the operands is 1

Complement operator sets each bit in the result to 0 if the corresponding bit in the operand is 1 and vice versa

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Outline

fig10_09.c

(3 of 3 )

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Outline

fig10_10.c

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Common Programming Error 10.12

Using the logical OR operator (||) for the bitwise OR operator (|) and vice versa is an error.

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Outline

fig10_13.c

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Left shift operator shifts all bits left a specified number of spaces, filling in zeros for the empty bits

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Outline

fig10_13.c

(2 of 3 )

Right shift operator shifts all bits right a specified number of spaces, filling in the empty bits in an implementation-defined manner

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Outline

fig10_13.c

(3 of 3 )

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Common Programming Error 10.13

The result of shifting a value is undefined if the right operand is negative or if the right operand is larger than the number of bits in which the left operand is stored.

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Portability Tip 10.7

Right shifting is machine dependent. Right shifting a signed integer fills the vacated bits with 0s on some machines and with 1s on others.

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Fig. 10.14 | The bitwise assignment operators.

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Fig. 10.15 | Operator precedence and associativity. (Part 1 of 2.)

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Fig. 10.15 | Operator precedence and associativity. (Part 2 of 2.)

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10.10 Bit Fields

Bit field

Member of a structure whose size (in bits) has been specified
Enable better memory utilization
Must be defined as int or unsigned
Cannot access individual bits

Defining bit fields

Follow unsigned or int member with a colon (:) and an integer constant representing the width of the field
Example:

struct BitCard {

unsigned face : 4;

unsigned suit : 2;

unsigned color : 1;

};

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10.10 Bit Fields

Unnamed bit field

Field used as padding in the structure
Nothing may be stored in the bits

struct Example {

unsigned a : 13;

unsigned : 3;

unsigned b : 4;

}

Unnamed bit field with zero width aligns next bit field to a new storage unit boundary

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Performance Tip 10.3

Bit fields help conserve storage.

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Outline

fig10_16.c

(1 of 2 )

Bit fields determine how much memory each member of a structure can take up

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Outline

fig10_16.c

(2 of 2 )

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Portability Tip 10.8

Bit-field manipulations are machine dependent. For example, some computers allow bit fields to cross word boundaries, whereas others do not.

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Common Programming Error 10.14

Attempting to access individual bits of a bit field as if they were elements of an array is a syntax error. Bit fields are not “arrays of bits.”

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Common Programming Error 10.15

Attempting to take the address of a bit field (the & operator may not be used with bit fields because they do not have addresses).

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Performance Tip 10.4

Although bit fields save space, using them can cause the compiler to generate slower-executing machine-language code. This occurs because it takes extra machine language operations to access only portions of an addressable storage unit. This is one of many examples of the kinds of space–time trade-offs that occur in computer science.

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10.11 Enumeration Constants

Enumeration

Set of integer constants represented by identifiers
Enumeration constants are like symbolic constants whose values are automatically set

Values start at 0 and are incremented by 1
Values can be set explicitly with =
Need unique constant names

Example:

enum Months { JAN = 1, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC};

Creates a new type enum Months in which the identifiers are set to the integers 1 to 12

Enumeration variables can only assume their enumeration constant values (not the integer representations)

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Outline

fig10_18.c

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Enumeration sets the value of constant JAN to 1 and the following constants to 2, 3, 4…

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Outline

fig10_18.c

(2 of 2 )

Like symbolic constants, enumeration constants are replaced by their values at compile time

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Common Programming Error 10.16

Assigning a value to an enumeration constant after it has been defined is a syntax error.

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Good Programming Practice 10.5

Use only uppercase letters in the names of enumeration constants. This makes these constants stand out in a program and reminds you that enumeration constants�are not variables.

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