Structs, Enums, Unions Rudra Dutta CSC 230 - Spring 2007, Section 001.

Structs, Enums, UnionsStructs, Enums, Unions

Rudra DuttaCSC 230 - Spring 2007, Section 001

Copyright Rudra Dutta, NCSU, Spring, 2007 2

Derived Data TypesDerived Data Types

Arrays Pointers Structs Enums Unions


structstruct: A Single Variable Type: A Single Variable Typewith Multiple Componentswith Multiple Components

Example: a person has a– Weight– Height– Gender– ID number– Age in years– Etc.

To indicate these are all part of the same “entity”, we define a struct data type


ExampleExample

Makes more sense than simply defining these fields individually, not indicating how they are related

struct person { int weight; int height; char gender; int idnum; short age; …};struct person person1;


Compared with JavaCompared with Java

instance fields of a class in Java are about the same thing as members of a struct in C– Except there is no access specifier (public, private) in C for struct members – they are all public

Syntax for referring to both is the same

person1.height = 72; person1.weight = 180; person1.gender = ‘M’; …


Initializing StructuresInitializing Structures

Unitialized struct person person1;

struct person person1 = {72, 180, ‘M’, 123456789, 20};

struct person person1 = {72, 180, ‘M’};

struct person person1 = {.height = 72, .gender = ‘M’, .idnum = 123456789};

Fully initialized

Partly initialized by member order

Partly initialized by member name


Referring to Referring to structsstructs and and membersmembers

Simple assignment to a struct memberperson3.weight = person2.weight;

Assignment to an entire struct

person2 = person1;

person4 = (struct person) {person2.height, person3.weight, person1.gender, 987654321, person1.age+65};


structstructs as Input Parameterss as Input Parameters

Structs are passed by value, as usual– i.e., a copy is made and passed to the function

void printperson ( struct person );

int main () { struct person person1 = {…}; … (void) printperson (person1); …}

void printperson ( struct person p ){ …}


structstructs as Return Valuess as Return Valuesstruct persongetperson ( void ){ struct person np = {0, 0, ‘ ‘, 0}; (void) printf (“Enter person characteristics: “); scanf(“%d %d %c %d”, &(np.height), &(np.weight), &(np.gender), &(np.age);

return (np); /* function returns multiple values * in a single struct */}

int main () { struct person person1; … person1 = getperson (); …}

Parentheses needed?


Arrays containing Arrays containing structstructss

Example…int main () { struct person persons[100]; … persons[1] = getperson (); persons[2].age = persons[1].age; …}

Parentheses needed, i.e., should it be…… (persons[2]).age = (persons[1]).age;…


Initializing Arrays of StructsInitializing Arrays of Structs

Example struct person persons[100] = { { 72, 180, ‘M’, 20 }, { 66, 100, ‘F’, 19 }, { 76, 240, ‘M’, 25}, [10] = { .height = 70, .gender = ‘M’} };


structstructs Containing s Containing structstructss

Are parentheses needed, e.g., (person1.phone).areacode?

struct phonenumber { short countrycode; short areacode; short exchange; short number; }; struct person { int height; int weight; struct phonenumber phone; … } person1;… if (person1.phone.areacode == 919) …

struct phonenumber { short countrycode; short areacode; short exchange; short number; }; struct person { int height; int weight; struct phonenumber phone; … } persons[100];… if (persons[5].phone.areacode == 919) …


structstructs Containing Arrayss Containing Arrays

struct person { int weight, height; char gender; int idnum; short age; short coursenums[6]; }; struct person persons[100];


structstructs Containing Arrays s Containing Arrays (cont’d)(cont’d)

How query if course number of first class taken by third person is 230?

How assign 246 to 4th class of 10th person? How initialize 12th person’s data?


struct person { int weight, height; char gender; int idnum; short age; short coursenums[6]; }; struct person persons[100]; struct person *p_array;

p_array = persons; /* point to first struct */ (*p_array).weight = 175; p_array->height = 170;

Pointers toPointers to struct structss

Pointer access to structs is very common– Special notation defined for the purpose


The The constconst Keyword Keyword

Indicates to the compiler that a value should not change during program execution– Statements that attempt to change this value will

cause a compiler warning– The only way to set variable: initialize during

definition

const int twopowfive = 32; const float pi;

twopowfiv = 64; /* ERROR */ pi = 3.14159; /* ERROR */


The The constconst Keyword (cont’d) Keyword (cont’d)

Is this better than macros?#define TWOPOWFIV 32#define PI 3.14159

How about this?struct pet { char *name; unsigned short weight; unsigned char age; unsigned char type;};const struct pet mypet =

{ “Fluffy”, 30, 5, DOG };


constconst and Pointers and Pointers

Changeable pointer to changeable character

Unchanging pointer to changeable character

Changeable pointer to unchanging character

Unchanging pointer to unchanging character

char * cp = &c;*cp++ = ‘A’; /* no problems */

char * const cp = &c;cp = dp ; /* ERROR, changes pointer */

const char * cp = &c;*cp = ‘Z’ ; /*ERROR, changes value pointed to */

const char * const cp = &c;*cp++ = ‘Z’ ; /* ERROR, changes both */


Enumerated Data TypeEnumerated Data Type

Use for variables with small set of possible values, where binary encoding of value is unimportant

enum colors {red, blue, green, purple}; enum colors mycolors;

mycolors = blue; if (mycolors == blue) printf("color == blue\n"); else if (mycolors == green) printf("color == green\n");


Enumerated Data Type (cont’d)Enumerated Data Type (cont’d)

Don’t compare variables of different enumerated types!

enum {blue, red, green, purple} newcolors;enum {black, brown, orange, yellow} morecolors;

newcolors = red;morecolors = brown;if (morecolors == newcolors) printf("Same color\n");

Although you can interpret enumerated data types as integers, this is not recommended


Enumerated Data Types (cont’d)Enumerated Data Types (cont’d)

Is this better than macros?#define BLUE 0#define RED 1#define GREEN 2#define PURPLE 3

int colors;

colors = RED;if (colors == RED) {…};


The The typedeftypedef Statement Statement Assigns an alternate name (synonym) to a C data

type– More concise, more readable

typedef char * cptr;cptr cp, dp;

typedef struct mystruct { int val; cptr name; struct mystruct *next;} llnode;llnode entries[100];


The The typedeftypedef Statement (cont’d) Statement (cont’d)

Sometimes typedefs used to provide machine portability– To retarget a program for a different architecture, just

redefine the typedefs and recompile

Usually, typedefs are collected in a header file that is #include’d in all source code modules

typedef int values[20];values tbl1, tbl2;


_Bool_Bool variables variables

Defines an integer variable that is restricted to store only the values 0 (false) and 1 (true)– Attempt to assign any non-zero value will actually

store the value 1#include <stdbool.h>…_Bool test1;

test1 = ((c = getchar()) && (c != ‘n’));

if (test1) …


Bit Fields in CBit Fields in C

Way to pack bits into a word– When useful?

Bit fields are defined like (integer) members of a structure


Bit Fields Example Bit Fields Example (http://www.cs.cf.ac.uk/Dave/C/)(http://www.cs.cf.ac.uk/Dave/C/)

Frequently devices and OS communicate by means of a single word

struct Disk_register { unsigned ready:1; unsigned error_occured:1; unsigned disk_spinning:1; unsigned write_protect:1; unsigned head_loaded:1; unsigned error_code:8; unsigned track:9; unsigned sector:5; unsigned command:5;};


Example (cont’d)Example (cont’d)struct Disk_register * dr = (struct Disk_register * ) MEMADDR;

/* Define sector and track to start read */dr->sector = new_sector;dr->track = new_track;dr->command = READ;

/* ready will be true when done, else wait */while ( ! dr->ready ) ;

if (dr->error_occured) /* check for errors */ { switch (dr->error_code) ...... }


Bit Fields (cont’d)Bit Fields (cont’d)

Restrictions– My recommendation: always make unsigned– # of bits determines maximum value– Cannot have array of bit fields– Cannot have pointer to a bit field

Danger: bit-fields are non-portable!– Order in which bit-fields stored within a word is

system dependent


The The unionunion Statement Statement

Defined like a structure, but only stores exactly one of the named members– Reason: save storage– Important?

Nothing in the union tells you which member is stored there!– Usually: programmer defines another field to indicate


unionunion Example Exampleunion info { unsigned short speed_of_flight; // bird _Bool freshwater; // fish enum {VERY, SOME, LITTLE} hairiness; // mammal};

struct { unsigned char type; char * name; union info info;} animals[10];

animals[0].type = MAMMAL;animals[0].name = "Polar Bear";animals[0].info.hairiness = VERY;

Structs, Enums, Unions Rudra Dutta CSC 230 - Spring 2007, Section 001.

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