ECE 103 Engineering ProgrammingChapter 49

Structures Unions, Part 1

Herbert G. Mayer, PSU CSStatus 6/4/2014

Initial content copied verbatim fromECE 103 material developed by

Professor Phillip Wong @ PSU ECE

Syllabus Structure Definition Accessing a Structure Initializing a Structure Pointer to struct Examples

3

Structure Definitions

Simple variables are declared with a single data type and can hold values only of that type.

Example:

/* Arrays for up to sixty students */char Student_Name[60][100];int ID_Number[60];int HW_Scores[60][6];double HW_average[60];

4

A structure allows multiple variables to be grouped together and be referenced as an aggregate variable.

struct tag{

/* Variable declarations listed here */};

tag is the name assigned to the structure. Braces { } delimit the body of the structure. A semicolon is needed after the ending brace. Variables are declared within the structure body.

Each variable is a member of the structure. Structures can have either block or file scope.

5

Example:struct Student_Info{ char Student_Name[100]; int ID_Number; int HW_Scores[6]; float HW_Average;};

Note: This only defines the “layout” of the structure. It does not actually allocate storage for the member variables.

6

Once the structure is defined, a variable can be declared to be of that structure type.

Example:Declare a structure variable for a single student:

struct Student_Info SData;

Student_Name

ID_Number

HW_Scores

SData

HW_Average

…When SData is declared, storage is now allocated to hold the structure’s member variables.

7

Example:Declare an array of structures forsixty students:

struct Student_Info SData[60];

Student_Name

ID_Number

HW_Scores

SData[0]

HW_Average

…

Student_Name

ID_Number

HW_Scores

SData[1]

HW_Average

…

Student_Name

ID_Number

HW_Scores

SData[59]

HW_Average

…

.

.

.

8

Defining a structure can be combined with declaring a variable of that structure type.

Example:struct point3D{ double x, y, z; int color;

} point[1000];

9

The structure tag can be omitted if a named structure is not required.

Example:struct{ double x, y, z; int color;

} point[1000];

Note: If there is no structure tag, it cannot be used for later variable declarations.

10

A typedef can alias away the “struct” part.Example:/* This has separate structure and typedef definitions */struct point3D{ double x, y, z; int color;};typedef struct point3D Point3D;

Point3D pt;

Example:/* This combines the structure and typedef definitions */typedef struct point3D

{double x, y, z;int color;

} Point3D;

Point3D pt;

11

If a typedef is combined with a structure definition, the tag is optional.

Example:typedef struct

{double x, y, z;int color;

} Point3D;

/* Variable declarations */Point3D pt;

Note: If the tag is omitted, then structure variable declarations must also omit the struct keyword.

Example:struct Point3D pt; /* Causes compiler error */

12

Accessing a Structure

To access a member of a structure variable, use the member operator (a period) :

struct_name.member_varname

struct_name is the name of the structure. member_varname is the name of the member

variable within the structure.

To access an array of structures:

struct_varname[index].member_varname

13

Example:/* Structure definition */struct window{ int win_id; int win_ulc_x, win_ulc_y; int win_width, win_height; int win_color; char win_title[80];};

/* Variable declarations */struct window w;struct window win[10];

/* Change the values of some member variables */w.win_id = 5;w.win_id++;strcpy(w.win_title, "Main_Window");

win[3].win_width = 300;win[3].win_height = 250;

14

Example:#include <stdio.h>#define NAMELEN 80#define NUM 3

typedef struct ID_info{

char FirstName[NAMELEN];int Age;

} ID_info_t;

int main (void){

ID_info_t friend[NUM];int k;for (k = 0; k < NUM; k++)

{printf("Enter first name: "); scanf("%s",

friend[k].FirstName);printf("Enter age: "); scanf("%d", &friend[k].Age);

}for (k = 0; k < NUM; k++){

printf("%s %d\n", friend[k].FirstName, friend[k].Age);}return 0;

}

Actual Output:Enter first name: JoeEnter age: 21Enter first name: SaraEnter age: 19Enter first name: FrankEnter age: 20Joe 21Sara 19Frank 20

15

Initializing Structures

A structure variable can be initialized when it is declared. Initial values for the members are enclosed in braces.

Example:struct point3D{ double x, y, z; int color;};

struct point3D pt = {0, 30, 15, 255};

16

Example:struct point3D{

double x, y, z;int color;

};

struct point3D pt[3] =

{{ 0, 30, 15, 255},

{10, 25, 15, 128},

{ 0, 30, 0, 16}};

17

Designated Initializers for Structures

When a structure variable is declared, C99 allows the members of the structure to be initialized by name, if desired.

/* C90 version */#include <stdio.h>

struct data{

int x;int v[2];

};

int main (void){

struct data x = {1, {5, 2}};

return 0;}

// C99 version#include <stdio.h>

struct data{

int x;int v[2];

};

int main (void){

struct data P = {.x=1, .v[0] = 5, .v[1] = 2};

return 0;}

C99

18

Assignment Operator Applied to Structures

Given two structure variables of the same type, one variable can be copied directly to the other using the assignment operator ( = ).

When copying a structure, the value of each member variable is copied, including entire arrays.

If structures can be assigned to each other, can they also be compared using ==, >, <, etc?

Unfortunately, no. This is not legal in C.

19

Example:

struct point3D{

double x, y, z;int color;

};typedef struct point3D Point3D;

Point3D pt1, pt2, pt3;

/* Initialize pt1 */pt1.x = 10; pt1.y = 20; pt1.z = 0; pt1.color = 34;

/* Copy member variables individually */pt2.x = pt1.x; pt2.y = pt1.y; pt2.z = pt1.z;pt2.color = pt1.color;

/* Copy entire structure at once */pt3 = pt1;

20

Example:#include <stdio.h>

#define SIZE 3

typedef struct

{

int x[SIZE];

} array;

int main (void)

{

array P = {{1,4,6}}, Q = {{0}};

int k;

for (k = 0; k < SIZE; k++)

printf("P.x[%d] = %d Q.x[%d] = %d\n", k, P.x[k], k, Q.x[k]);

Q = P; /* Copy entire array by copying structure */

printf("\n");

for (k = 0; k < SIZE; k++)

printf("P.x[%d] = %d Q.x[%d] = %d\n", k, P.x[k], k, Q.x[k]);

return 0;

}

Actual Output:P.x[0] = 1 Q.x[0] = 0P.x[1] = 4 Q.x[1] = 0P.x[2] = 6 Q.x[2] = 0

P.x[0] = 1 Q.x[0] = 1P.x[1] = 4 Q.x[1] = 4P.x[2] = 6 Q.x[2] = 6

21

Pointers to Structures

Variables that point to structures can be declared:

struct tag * var_name;

To dereference a structure pointer, parentheses are needed, since the member operator has higher precedence than the indirection operator. *svar.mvar is incorrectly parsed as *(svar.mvar). (*svar).mvar is the correct dereferencing syntax.

22

Example:

typedef struct point2D{

int x, y;} Point2D;

Point2D point = {0, 0};Point2D * ptr = &point;

/* Access member variable via pointer */(*ptr).x = 1;(*ptr).y = 5;

23

Because the syntax (*ptr).x is clumsy, an alternative syntax is available:

pointer_varname->member_varname

-> is the pointer operator, which consists of a hyphen followed by a greater-than sign.

No whitespace is permitted between – and >

24

Example:

Point2D point;Point2D * ptr;

point.x = 10;printf("%d\n", point.x);

ptr = &point;

(*ptr).x = 10; /* Assign 10 to member x */printf("%d\n", (*ptr).x);

ptr->x = 10; /* This does same thing */printf("%d\n", ptr->x);

25

Example:#include <stdio.h>#define NAMELEN 80#define NUM 3typedef struct ID_info{

char FirstName[NAMELEN];int Age;

} ID_info_t;

int main (void){

ID_info_t friend[NUM], * f = friend; /* f is pointer to friend */int k;for (k = 0; k < NUM; k++)

{printf("Enter first name: "); scanf("%s", (f+k)-

>FirstName);printf("Enter age: "); scanf("%d", &(f+k)->Age);

}for (k = 0; k < NUM; k++){

/* Show access using both pointer and array notation */printf("%s %d\n", (f+k)->FirstName, f[k].Age);

}return 0;

}

Actual Output:Enter first name: JoeEnter age: 21Enter first name: SaraEnter age: 19Enter first name: FrankEnter age: 20Joe 21Sara 19Frank 20