Structures. What is a structure? It is a collection of possibly different types Name the collection Name the components

Transcription

1 Structures What is a structure? It is a collection of possibly different types Name the collection Name the components For example: a student record: beckmann component names In C beckmann.hw is 54 name "Jane Beckmann" id hw 54 exam 98 grade 3.4 beckmann.name is "Jane Beckmann" 2*beckmann.exam is 196 collection name 142 S -1

2 Defining Structures #define MAX_NAME 40 typedef struct { char name[max_name + 1]; int id; int hw,exam; double grade; } student_record; define a new data type just after the preprocessor commands (# commands) Not a variable declaration Terminology: A "struct" is called a "record" or "structure" Its components are sometimes called "fields" or "members" leads to classes in C S -2

3 Declaring structure variables Write the typedef definition at the beginning of the program (after the # commands) In a function, write: int i1; int count = 0; student_record s1; a type a variable student_record beckman= {"Jane Beckmann", ,54,98,3.4}; can initialize structures like arrays 142 S -3

4 Note Don't need to use typedef (but convenient) Instead, could write in the function struct {char name[max_name+1]; int id; int hw, exams; double grade;} beckmann; Or use a structure tag (just after # commands) tag struct student_info{char name[max_name+1]; int id; int hw, exams; double grade;}; And in a function write struct student_info beckmann; 142 S -4

5 Using struct variables How to access a member of a struct? use the. operator dot e.g s1.hw A struct can be used wherever a regular variable of the same type can be used Examples: s1.hw = 80; s1.exam = 90; s1.grade=(double)(s1.hw + s1.exam)/50.; printf("%s %f", beckmann.name,beckmann.id); scanf("%d",&s1.id); length = strlen(s1.name); 142 S -5

6 What we can and can't do CAN use = with structs s1 = beckmann same as strcpy(s1.name,beckmann.name); s1.id = beckmann.id; s1.hw = beckmann.hw; s1.exam = beckmann.exam; s1.grade = beckmann.grade; CAN'T use == with structs if (s1 == beckmann) { }; /*ERROR*/ CAN'T use printf and scanf directly No placeholders for structs Write your own functions 142 S -6

7 When to use structures Collect together values that are treated as a unit Examples: better program organization easier to maintain typedef struct {int dollars, cents;} money; typedef struct {int hours,minutes; double seconds;} time; Functions can return structs: another way to have multiple return values. Files and databases: collection of structs 142 S -7

8 Examples Find the midpoint between two points Without structures void midpoint(double x1, double y1, double x2, double y2, double * midxp, double * midyp) { *midxp = (x1 + x2)/2.0; *midyp = (y1 + y2)/2.0; }... double ax, bx, ay, by, mx,my;... midpoint(ax, ay, bx, by, &mx, &my); in some other function 142 S -8

9 Define the structure point: typedef struct {double x,y;} point; And use it as point a={0.0,0.0}, b={5.0,10.0}; point m; m.x = (a.x + b.x)/2.0; m.y = (a.y + b.y)/2.0; Could also have a function point midpoint(point pt1, point pt2) { point mid; mid.x = (pt1.x + pt2.x)/2.0; mid.y = (pt1.y + pt2.y)/2.0; return mid; }... point a={0.0,0.0}, b={5.0,10.0}, m; m = midpoint(a,b); struct assignment 142 S -9

10 Comparing structures Recall: can't use == Instead use a function e.g to check if two points are the same int points_equal(point pt1,point pt2) { return(pt1.x==pt2.x && pt1.y==pt2.y); }... if (points_equal(a,b)) {...} /*OK*/ 142 S- 10

11 Input/Output with structures Recall: no placeholders for structs Instead use a function void print_point(point p) { printf("%f %f",p.x,p.y); } void scan_point(point * pp) { point temp; scanf("%lf%lf",&temp.x,&temp.y); *pp = temp; }... point a; scan_point(&a); print_point(a); parentheses are important (. precedes *) Could have written void scan_point(point *pp) { scanf("%lf%lf",&(*pp).x,&(*pp).y); } 142 S- 11

12 Pointers and Structures typedef struct{double x,y;} point;... point a; point *pp; pp = &a; In the memory a a.x a.y pp &(*pp).x &(*pp).y Expression Type pp point * *pp point (*pp).x double &(*pp).x double * 142 S 12

13 Shorthand: (*pp).x same as pp->x indirect component selection operator Example: void scan_point(point *pp) { scanf("%lf%lf",&pp->x,&pp->y); } 142 S 13

14 Structures within structures Example: How to define a rectangle struct start from one corner point struct has a height and width dimension struct in C typedef struct{double x,y;} point; typedef struct {double width,height;} dimension; typedef struct {dimension size; point lower_left; int line_color, fill_color;} rectangle; 142 S 14

15 Using these structures rectangle a ={{1.0,2.0},{2.0,3.0}, BLUE, CYAN}; rectangle b; b.fill_color = RED; b.lower_left.y = 15.0; S 15

16 typedef struct{double x,y;} point; typedef struct {double width,height;} dimension; typedef struct {dimension size; point lower_left; int line_color, fill_color;} rectangle; rectangle R; rectangle * rp; Type Quiz R.size R.lower_left R.fill_color R.lower_left.x &R.lower_left.x rp->size &rp->lower_left *rp.line_color R->size R->size->width dimension point int double double * dimension point * error! error! error! 142 S16