הגדרת משתנים

1. הגדרת משתנים Department of Computer Science-BGU

2. typedef השימוש ב - typedef typedef משמשת כדי להגדיר טיפוסים חדשים בשפה .  התחביר : typedef <known type> <identifier>;   מיקום הגדרות ה - typedef הינו בד"כ מעל הפונקציה main. לדוגמא: typedef int distance ; typedef double weight ;  משלב זה בתוכנית הצהרות על משתנים כ - weight יהיו שקולות להצהרות כ - double . כנ"ל לגבי distance ו- int. Department of Computer Science-BGU

3. Example #include <stdio.h>  typedef double distance ;  void main(void){ distance miles , kmeters ;   scanf("%lf", &miles); kmeters = miles * 1.609 ; printf("%.2f miles = %.2f kmeters\n", miles , kmeters );  } עבור קלט 2 נקבל את הפלט :  2.00 miles = 3.22 kilometers Department of Computer Science-BGU

4. Example אפשר להשתמש ב - typedef בכדי להגדיר טיפוסים חדשים כמערכים: #define N 10 #define M 20  typedef double vector[M] ; typedef double matrix[N][M] ;  הגדרנו שני טיפוסים חדשים : vector ו - matrix . vector הוא טיפוס של מערך חד-ממדי בגודל Mשכל איבר בו הוא double ו-matrix הוא טיפוס של מערך דו-ממדי בגודל NM שכל איבר בו הוא double .  משלב זה נוכל להגדיר מערכים חד-ממדיים כנ"ל באמצעות הטיפוס vector ומערכים דו-ממדיים כנ"ל באמצעות הטיפוס matrix . לדוגמא : vector a , b ; /* a & b are both arrays dimension M */ matrix mat ; /* mat is a two-dimensions array NM */ Department of Computer Science-BGU

5. Structures Department of Computer Science-BGU

6. Structures • Often we want to be able to manipulate ‘logical entities’ as a whole • For example, complex numbers, dates, student records, etc’ • Each of these must be composed of more than one variable, but are logically units • A struct (short for structure) is a collection of variables of different types, gathered into one super-variable • It is used to define more complex data types • Variables in a struct are called members or fields Department of Computer Science-BGU

7. Example – complex numbers. The following is the definition of a new ‘variable’ of type complex number: • struct complex { int real; int img; }; • Once we define a structure, we can treat it as any type. • In a program, we can then write: struct complex num1, num2, num3; Department of Computer Science-BGU

8. Example – complex numbers. The following is the definition of a new ‘variable’ of type complex number: • struct complex { int real; int img; }; • Once we define a structure, we can treat it as any type. • In a program, we can then write: struct complex num1, num2, num3; Department of Computer Science-BGU

9. Example 2 מבנים הינם טיפוסים נגזרים לייצוג קבוצה של טיפוסים הומוגניים או הטרוגניים. לדוגמא ההגדרה הבאה : struct triple { double x ; int y ; char c ; } ; בהגדרה זו הגדרנו טיפוס חדש בשם struct triple שהינו מורכב משלושה משתנים x , y ו- c . המילה struct הינה מילה שמורה . הגדרת משתנים מהטיפוס החדש שיצרנו מתבצעת struct triple first , second ; Department of Computer Science-BGU

10. Example 2 (cont.) כך הגדרנו שני משתני חדשים בשם first ו- second להיות משתנים מטיפוס struct triple . הגישה לאיברי (שדה) המבנה מתבצעת באמצעות האופרטור '.' ( נקודה ) . למשל :   first.x הינו השדה x במשתנה first . first.c הינו השדה c במשתנה first . second.y הינו השדה y במשתנה second . דוגמא לשימוש במבנה : void main() { struct triple a ;   scanf("%lf%d%c",&a.x,&a.y,&a.c); printf("x=%.2lf , y=%d , c=%c\n",a.x,a.y,a.c);  } Department of Computer Science-BGU

11. Using typedef struct person { char name[N] , address[M]; long id ; int age , status ; } ; typedef struct person _t{ char name[N] , address[M]; long id ; int age , status ; } person; typedef struct person_ t person; struct person _t{ char name[N] , address[M]; long id ; int age , status ; } ; המכללה האקדמית להנדסה סמי שמעון

12. איתחול של מבנה ניתן לבצע איתחול של מבנהץ כדוגמא במבנה : struct person { char name[N] , address[M]; long id ; int age , status ; } ; איתחול: struct person person1= {“Haim Cohen”,”Dimona”, 012345678, 34, 2}; struct person class[2] = {{“Yaakov Cohen”,”Lahavim”, 012000678, 56, 1}, {“Cohen OneTwo”,”Meitar”, 012333678, 44, 2}}; המכללה האקדמית להנדסה סמי שמעון

13. Exercise • Implement the MultComplex function – • Input - two complex numbers • Output – their multiplication • Note - If x=a+ib and y=c+id then: z = xy = (ac-bd)+i(ad+bc) • Write a program that uses the above function to multiply two complex numbers given by the user Department of Computer Science-BGU

14. פתרון typedef struct complex{ double x,y; }complex; complex * mult_comp(complex a, complex b){ complex * z = (complex *)malloc(sizeof(complex)); z->x = a.x * b.x – a.y * b.y; z->y = a.x * b.y + a.y * b.x; return z; } Department of Computer Science-BGU

15. Miscellaneous structure trivia • Structure members may be ordinary variable types, but also other structures and even arrays ! • Structures can therefore be rather large and take up a lot of space ! • Many times we prefer to pass structures to functions by address, and not by value. • Thus a new copy of the structure is not created – just a pointer to the existing structure Department of Computer Science-BGU

16. Nested structures typedef struct{           char           name[30];      char           address[50];           char           phone_num[15];           } Supplier ; typedef struct {           int max;           int min;           int curr; }Inventory;   typedef struct {           char           name[30];           Supplier   supplier;           char           department[15];           Inventory inventory;    } Item ; Department of Computer Science-BGU

17. More trivia • Structures cannot be compared using the == operator • They must be compared member by member • Usually this will be done in a separate function • Structures can be copied using the = operator • Member-wise copy Department of Computer Science-BGU

18. Exercise • Write a struct that represents a date (day, month, year) • Write a function that increments the datevoid IncDate(Date *d); • For example – 31.12.05 -> 1.1.06 Department of Computer Science-BGU

19. Structures containing arrays • A structure member that is an array does not ‘behave’ like an ordinary array. • When copying a structure that contains a member which is an array, the array is copied element by element • Not just the address gets copied. • Reminder – ordinary arrays can’t be copied simply by using the ‘=‘ operator. • They must be copied using a loop. Department of Computer Science-BGU

20. Structures containing arrays • The same happens when passing the structure to a function • Changing the array inside the function won’t change it in the calling function • Reminder – when passing an ordinary array to a function, all that gets passed is the address of its first element • Hence every change to the array within the function, changes the array in the calling function Department of Computer Science-BGU

21. Pointer to structure מבנה הוא סוג משתנה רגיל מרגע הגדרתו ולכן ניתן ליצור משתנה בצורה דינמית, נניח : typedef struct stam{ int value; }item; ניתן להגדיר : item * ptr; ptr = (item *) malloc(sizeof(item); עכשיו אנו משנים את האופרטור להגיע לכל שדה בתוך המבנה: scanf*(“%d”, &(ptr->value)); Department of Computer Science-BGU

22. Access structure members • If A is of some structure with a member named x, then A.x is that member of A • struct complex C;C.real = 0; • If A is a pointer to a structure with a member x, then A->x is that member of the variable pointed by A. • This is simply shorthand for - (*A).x • struct complex *pc = &C;pc->real = 1; Department of Computer Science-BGU

23. Pointers are another matter • If the member is a pointer, for example to a dynamically allocated array, all that gets copied is the pointer (the address) itself • Hence, we should take extra carewhen manipulating structures that contain pointers Department of Computer Science-BGU

24. Pointers in structure typedef struct stam{ char name[80]; char * p_name; int value; }item; • Problems: • Must malloc; • Copy , only the address • Delete , first the allocations Department of Computer Science-BGU