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Structures

Structures. ANSI-C. Review: Data Structures. Functions give us a way to organize programs. Data structures are needed to organize data, especially: 1. large amounts of data 2. variable amounts of data 3. sets of data where the individual pieces are related to one another

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Structures

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  1. Structures ANSI-C

  2. Review: Data Structures • Functions give us a way to organize programs. • Data structures are needed to organize data, especially: • 1. large amounts of data • 2. variable amounts of data • 3. sets of data where the individual pieces are related to one another • Arrays helped with points 1 and 2, but not with point 3, as arrays must contain the same type of values. • – Example: the data describing one pixel: x, y,color • – Example: data about one student: name, ID, gpa, etc.

  3. structs: Heterogeneous Structures • Collection of values of possibly differing types. • Used to unify several values of possibly different types. Result seen as one unit. • When using structs you must: • Name the collection; • name the components: calledfields. • Example: a student record collects under one name various pieces of information about a student: • Give name to the collection: studentRecord • Give type and name to each of the fields. • Example: a date collects under one name: day, month, year.

  4. Defining structs typedef struct { /* typedefs go at the top of the program */ char name [20] ; int id ; int hw; int exams ; double grade ; } studentRecord ; • Defines anew data type calledstudentRecord. • Doesnot declare (create) a variable.No storage is allocated.

  5. Terminology • A “struct” is sometimes called a “record” or “structure”. • Its “components” are often called “fields” or "members".

  6. Declaring struct variables • With structs, we’re moving toward a way for programmers to define their own types. • Having defined studentRecord at the top of the program: • studentRecord s1 ; • studentRecord harvey ; • /*studentRecord is a type; s1 and harvey are variables. */

  7. Selection operator: . Variable name. Must be of struct type field name Field selector operator • Having declared: studentRecord s1 ; • To access any of the fields: s1.fieldName

  8. Things YouCan andCan’t Do • Youcan • use = to assign whole struct variables • Youcan • have a struct as a function return type • Youcan’t use == to directly compare struct variables;can compare fields directly • Youcan’t directly scanf or printf structs; • can read the individual fields

  9. struct initializers …/*typedef structs go at top*/ int i1 ; int count = 0 ; char c1[5] = { ‘h’, ‘e’,’l’, ‘p’, ‘\0’}; char pet[] = “lamb” ; studentRecord harvey = {“Harvey S.”, 9501234, 87, 74, 3.1} ;

  10. Using Components of struct Variables studentRecord s1; ... scanStatus = scanf(“%d”, &s1.id) ; s1.hw = 90 ; s1.exams= 80 ; s1.grade = (double)(s1.hw + s1.exams) / 50.0 ; printf(“%d: %f”, s1.id, s1.grade) ;

  11. Assigning Whole structs s1 = harvey ; • equivalent to s1.id = harvey.id ; s1.hw = harvey.hw ; s1.exams = harvey.exams ; s1.grade= harvey.grade ; strcpy(s1.name, harvey.name) ; /* string copy – covered in slides on strings */

  12. Why use structs? • Collect together values that are treated as a unit (for compactness, readability,maintainability). • Functions can return structs • another way to have “multiple” return values. • Files and databases: collections of structs e.g., 300 (or 30,000) student records. typedef struct { int hours; int minutes ; int seconds ; } time ; typedef struct { int dollars; int cents ; } money ;

  13. Example: /* Given 2 endpoints of a line, “return” coordinates of midpoint */ void midpoint( double x1, double y1, /* 1st endpoint */ double x2, double y2, /* 2nd endpoint */ double *midxp, double *midyp ) /* Pointers to midpoint */ { *midxp = (x1 + x2) / 2.0; *midyp = (y1 + y2) / 2.0; } double ax, ay, bx, by, mx, my; ... midpoint(ax, ay, bx, by, &mx, &my);

  14. Points as structs typedef struct { double xcoord; double ycoord; } point ; ... point p1 = {0.0, 0.0}, p2 = {5.0, 10.0} ; point middle ; middle.xcoord = (p1.xcoord + p2.xcoord ) / 2.0 ; middle.ycoord = (p1.ycoord + p2.ycoord ) / 2.0 ;

  15. Midpoint Function via structs point midpoint (point pt1, point pt2) { point mid; mid.xcoord = ( pt1.xcoord + pt2.xcoord ) / 2.0 ; mid.ycoord = ( pt1.ycoord + pt2.ycoord ) / 2.0 ; return (mid); } …/* a call */ /* point struct declaration and initialization */ point p1 = {0.0, 0.0}; point p2 = {5.0, 10.0}; point middle; ... middle = midpoint (p1, p2) ; /* struct assignment */

  16. Midpoint with references void setMidpoint (point pt1, point pt2, point *mid) { (*mid).x = ( pt1.xcoord + pt2.xcoord ) / 2.0 ; (*mid).y = ( pt1.ycoord + pt2.ycoord ) / 2.0 ; } /* . has high precedence than * */ pointa = {0.0, 0.0}; point b = {5.0, 10.0}; point m; setMidpoint (a, b, &m) ; • Structs behave like all non-array types whenused as parameters.

  17. Pointer Shorthand: -> void set_midpoint (point pt1, point pt2, point *mid) { mid->x = ( pt1.xcoord + pt2.xcoord ) / 2.0 ; mid->y = ( pt1.ycoord + pt2.ycoord ) / 2.0 ; } • structp - > component means (*structp).component • -> is called the “indirect component selection operator”

  18. Testing Equality of structs • if (pt1 = = pt2) { ... } /* Doesn’t work */ • You need to specifically compare each field in the struct int pointsEqual(point pt1, point pt2) { return (pt1.xcoord = = pt2.xcoord && pt1.ycoord = = pt2.ycoord ) ; } if (pointsEqual(pt1, pt2)) { ... } /* OK */

  19. Do-it-yourself struct I/O void printPoint (point p) { printf (“(%f,%f)”, p.xcoord, p.ycoord) ; } void scanPoint (point *ptptr) { point temp ; scanf (“%lf %lf”, &temp.xcoord, &temp.ycoord) ; *ptptr = temp ; } point aPoint ; scanPoint (&aPoint) ; printPoint (aPoint) ;

  20. Alternative scan_point void scanPoint (point *ptptr) { scanf (“%lf %lf”, & ptptr->x, & ptptr->y) ; } point aPoint ; scanPoint (&aPoint) ; printPoint (aPoint) ;

  21. scan_point without -> void scanPoint (point *ptptr) { scanf (“%lf %lf”, &(*ptptr).x, &(*ptptr).y) ; } point aPoint ; scanPoint (&aPoint) ; print_point (aPoint) ;

  22. Hierarchical structs typedef struct { double x; double y ; } point ; typedef struct { double width; double height ; } dimension ; typedef struct { dimension size ; point lowerLeft ; int lineColor, fillColor ; } rectangle ;

  23. Using structs within structs point origin = { 0.0, 0.0 } ; rectangle a = { {5.0, 10.0}, {1.0, -2.0}, BLUE,CYAN}; rectangle b ; b.fillColor = BLUE ; b.lowerLeft = origin ; /* place at origin */ b.lowerLeft.y = 15.0 ; /* move up 15 */ ... b.size.width = 2.0 * b.size.width ; /* stretch in x */ b.size.height = 4.0 * b.size.height ; /* stretch in y */

  24. QUIZ: Calculating Types • rectangle r; • rectangle * rp; • r.size • r.lowerLeft • r.fillColor • r.lowerLeft.x • &r.lowerLeft.y • rp -> size • &rp -> lowerLeft • *rp.linecolor • r -> size • rp -> size -> width

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