1. ECE 103 Engineering Programming Chapter 49 Structures Unions, Part 1 Herbert G. Mayer, PSU CS Status 6/4/2014 Initial content copied verbatim from ECE 103 material developed by Professor Phillip Wong @ PSU ECE

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

3. 2 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. 3 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. 4 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. 5 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. 6 Example: Declare an array of structures for sixty 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. 7 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. 8 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. 9 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. 10 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. 11 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. 12 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. 13 Example: #include #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: Joe Enter age: 21 Enter first name: Sara Enter age: 19 Enter first name: Frank Enter age: 20 Joe 21 Sara 19 Frank 20

15. 14 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. 15 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. 16 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 struct data { int x; int v[2]; }; int main (void) { struct data x = {1, {5, 2}}; return 0; } // C99 version #include 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. 17 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. 18 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. 19 Example: #include #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] = 0 P.x[1] = 4 Q.x[1] = 0 P.x[2] = 6 Q.x[2] = 0 P.x[0] = 1 Q.x[0] = 1 P.x[1] = 4 Q.x[1] = 4 P.x[2] = 6 Q.x[2] = 6

21. 20 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. 21 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. 22 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. 23 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. 24 Example: #include #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], * 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: Joe Enter age: 21 Enter first name: Sara Enter age: 19 Enter first name: Frank Enter age: 20 Joe 21 Sara 19 Frank 20