1. ECE 103 Engineering Programming Chapter 50 Structures Unions, Part 2 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 Nested Structures Structures and Functions Unions Examples

3. 2 Nested Structures A structure definition may include, among its members, a structure variable of a different type. To access a structure within a structure, use the member operator (. or -> ) multiple times.

4. 3 Example: typedef struct point2D /* Coordinates of a 2-D point */ { double x, y; } Point2D; struct triangle /* Triangle's vertices are points */ { Point2D vertex1, vertex2, vertex3; }; struct triangle tri[3]; /* Array of three triangles */ struct triangle * ptr; /* Pointer to a triangle */ /* Initialize vertex1 of second triangle (index 1) */ tri[1].vertex1.x = 0; tri[1].vertex1.y = 0; ptr = &tri[2]; /* Point to third triangle */ ptr->vertex1.x = 5;

5. 4 Self-Referential Structures It is illegal to define a structure with a member variable of the same type as the structure. It is perfectly legal to define a structure that contains a pointer to the same type as itself. These are called “self-referential structures” and are used in data structures such as linked lists and trees.

6. 5 Examples: /* This is NOT legal in C */ struct list_node { int value; struct list_node next; /* WRONG */ } /* These are legal in C */ struct list_node { int value; struct list_node * next; /* Uses pointer, so OK */ }; struct tree_node { int value; struct tree_node * left, * right; /* Uses pointers, so OK */ };

7. 6 Structures and Functions Multiple values can be stored in a structure and passed to a function. This reduces the number of individual arguments that need passing. Passing a structure to a function by value:  A copy of the structure variable’s contents is passed to the function.  The function can change the copy but not the original argument.  If the passed structure contains an array, a complete copy of the array is passed.

8. 7 Passing a structure to a function by reference:  Pass by reference can be simulated by passing a pointer to the structure as the function argument.  It is more efficient to pass large structures to a function by reference.  If the pointed-to structure should not be changed, then use the const modifier. Multiple values can also be returned from a function by storing them in a structure and returning the structure.

9. 8 Example: typedef struct point2D { double x, y; } Point2D; void fun_v (Point2D p) { p.x = 10.5; p.y = 21.0; } void fun_p (Point2D * p) { p->x = 31.0; p->y = 500.0; } int main (void) { Point2D point; point.x = 1.0; point.y = 5.0; fun_v(point); /* After return from call: 1.0 5.0 */ fun_p(&point); /* After return from call: 31.0 500.0 */ return 0; }

10. 9 Example: #include #define MAXSIZE 100 typedef struct /* Encapsulate array data in a structure */ { int maxsize; /* Maximum array size */ int numel; /* Current number of elements */ int ele[MAXSIZE]; } array; void DispArray (const array * p) { int k; for (k = 0; k numel; k++) printf("%d\n", p->ele[k]); } int main (void) { array A1, A2; A1.maxsize=MAXSIZE; A1.ele[0]=5; A1.ele[1]=-3; A1.numel=2; A2 = A1; DispArray(&A2); return 0; }

11. 10 Example: #include typedef struct /* Use this to hold multiple return values */ { double mag, phase; /* Magnitude & phase of complex number */ } MagPhase; MagPhase calculate_magphase (double a, double b) { MagPhase mp; /* Hold calculated values for return */ mp.mag = sqrt(a*a + b*b); mp.phase = atan2(b,a); return mp; } int main (void) { MagPhase Q = {0, 0}; printf("Pre : mag = %f phase = %f\n", Q.mag, Q.phase); Q = calculate_magphase(4, 5); printf("Post: mag = %f phase = %f\n", Q.mag, Q.phase); return 0; } Actual Output: Pre : mag = 0.000000 phase = 0.000000 Post: mag = 6.403124 phase = 0.896055

12. 11 Unions Unions offer a way to economize on storage. Like a structure, a union is a collection of variables of different types. However, only a single field can actually hold information at any given time.

13. 12 A union definition is similar to a structure definition. Example: /* Define a union */ union UData { char c_number; int i_number; float f_number; double d_number; };

14. 13 To define a variable of type union UData : union UData num; To assign values to num: num.c_number = 'A'; num.f_number = 3.14; num.i_number = 2; Assigning 3.14 to num cancels the original assignment of 'A' to num. Likewise, storing the integer value 2 in num will cancel the assignment of 3.14.

15. 14 For a union variable, the system allocates an amount of storage large enough to hold its largest member. This same storage is used for all the smaller union members as well. A member of a union can be:  A variable or array of any legal data type  A structure or another union A union can be used to view a single piece of data as different types, as specified in the union declaration.

16. 15 Example: struct SData { int x; double y; char ch; }; struct SData snum; snum.x = 256; snum.y = -6.73; snum.ch = 'A'; x y ch snum 256 -6.73 'A'

17. 16 Example: union UData { int x; double y; char ch; }; struct UData unum; unum.x = 256; unum.y = -6.73; unum.ch = 'A'; unum 256 x y -6.73 'A' ch