ETE 132 Lecture 7 By Munirul Haque

Topics Defining Your Own Types Structures

Structures The structure mechanism allows us to aggregate variables of different types struct definitions are usually placed outside of functions so that they are in scope throughout the file, as in the following example:

Structures struct card_struct { int number; char suit; }; /* note the semicolon after the definition! */ void some_function() { struct card_struct a, b; a.number = 3; a.suit = ’D’; b = a; }

Structures The “.” in a.num is the “structure member operator”, which connects the structure name and the member name. A “member” is a variable in a structure. Assignment (=) works just as you would expect, as if there were a separate assignment for each structure member.

More on structures The reason a semicolon follows the struct definition is that the definition is a statement. Also, you can declare a variable of that struct type using basically the same syntax: struct card_struct { int number; char suit; } my_card;

Example: points in the plane #include <math.h> struct point { double x; double y; }; double distance( struct point p1, struct point p2 ) { double dx, dy, dist; dx = p1.x - p2.x; dy = p1.y - p2.y; dist = sqrt((dx * dx) + (dy * dy)); return( dist ); } void main() { /* here’s a convenient notation for structure initialization: */ struct point a, b; a.x = 3.5; a.y = 4.5; b.x = 6.5; b.y = 0.5; printf( "Distance: %f\n", distance( a, b )); }

What happens here? struct employee { char name[50]; int age; }; void main() { struct employee tom1, tom2; strcpy( tom1.name, "Thomas Wolfe" ); tom1.age = 104; tom2 = tom1; tom2.name[0] = ’G’; printf( "Name: %s", tom1.name ); }

A Comparison Function There’s no standard way to compare structures. You can’t try tom1 == tom2 in the previous example, or tom1 < tom2. You can always write comparison code, if you need to: int compare_employees( struct employee e1, struct employee e2 ) { return (e1.age == e2.age) && (strcmp(e1.name,e2.name) == 0); }

Structure member as Parameter void sum(double p1_x,double p1_y,double p2_x,double p2_y ) { struct point psum; psum.x = p1_x + p2_x; psum.y = p1_y + p2_y; printf(“%lf_%lf\n”, psum.x, psum.y); } void main() { struct point a, b, c; a.x = 3.5; a.y = 4.5; b.x = 6.5; b.y = 0.5; printf(“%lf_%lf_%lf_%lf\n”, a.x, a.y, b.x, b.y); sum( a.x, a.y, b.x, b.y); 10_5 3.5_4.5_6.5_0.5

Structure as Parameter Structures work seamlessly with functions. A structure is a type, so it can be the type of a function parameter (as here), or a return type: point sum( struct point p1, struct point p2 ) { struct point psum; psum.x = p1.x + p2.x; psum.y = p1.y + p2.y; return psum; } void main() { struct point a, b, c; a.x = 3.5; a.y = 4.5; b.x = 6.5; b.y = 0.5; c = sum( a, b); printf(“%lf_%lf_%lf_%lf_%lf_%lf\n”, a.x, a.y, b.x, b.y, c.x, c.y); 3.5_4.5_6.5_0.5_10_5

Array of Structure You can also define an array of structures: struct point point_list[100]; for(i=0;i<100;i++) { /* initialize point i */ point_list[i].x = (i+1)*2; point_list[i].y = (i+1)*3; } printf(“Point %d: %lf_%lf\n”, (i+1), point_list[i].x, point_list[i].y); Point 1: 2_3 Point 2: 4_6 Point 3: 6_9 … Point 100: 200_300 12

Problem Write a program to take input from console, Name (within 20 characters), Address (50 char), Telephone No (max. 9 digit), and Gender (single char) of 15 Students. Print those students information in reverse order (started with Gender, ended with Name for every student)