Assoc. Prof. Dr. Tansu FİLİK

Transcription

1 Assoc. Prof. Dr. Tansu FİLİK

2 Computer Programming Previously on Bil 200 Introduction to pointers String (character array) Static and automatic variables

3 static and automatic variables, and scope

4 Rem: Static variables We have seen that local variables are lost when exitting a function. Static variables are re-formed every time when entering the same function. They are different from automatic variables. These variables are formed at the beginning of the program, and their values are set. Obviously, they are valid only inside its declared function.

5 void fonk_yaz(void) { int a = 0; printf("deger = %d\n", a); a++; void main() { int i; for( i = 0 ; i < 3 ; i++ ) { fonk_yaz(); Automatic variables A new a is formed for each enterence to the function. This is the default way.

6 Example: without using static void fonk_yaz(void) { int a = 0; printf("deger = %d\n", a); a++; void main() { int i; A new a variable is formed at each enterence to the function. The variable a is destroyed whenever we quit the function. for( i = 0 ; i < 3 ; i++ ) { fonk_yaz(); So, on the screen, we would see three 0 (zero) values.

7 void fonk_yaz(void) { static int a = 0; Example re-formed every time when entering the same function printf("deger = %d\n", a); a++; void main() { int i; On the screen, you see deger = 0 deger = 1 deger = 2 for( i = 0 ; i < 3 ; i++ ) { fonk_yaz();

8 Example void fonk_yaz(void) { static int a = 0; The variable a is formed at the beginning of the program, and its value is kept. printf("deger = %d\n", a); a++; void main() { int i; The variable is not destroyed when quitting the program. for( i = 0 ; i < 3 ; i++ ) { fonk_yaz();

9 Enumerated types variables that are formed by the keyword: enum

10 Enumerated types Gives you a chance to form your own type of variables. Its usage improves intelligibility of the program. For example, we may create a type to contain week days: enum Gunler { Pazartesi, Sali, Carsamba, Persembe, Cuma, Cumartesi, Pazar ;

11 Enumerated types We may make variables of the created type enum Gunler gun; switch(gun) { case Pazartesi: printf("pazartesi"); break; case Sali: printf("sali"); break;...

12 Enumerated Types Actually, each element in an enum type is a number list starting from 0. enum Gunler { Pazartesi, /* 0 */ Sali, /* 1 */ Carsamba, /* 2 */ Persembe, /* 3 */ Cuma, /* 4 */ Cumartesi, /* 5 */ Pazar /* 6 */ ;

13 Enumerated Types Since enum elements are numerical, they can be used in comparisons of if expressions. enum Gunler gun1, gun2; gun1 = (enum Gunler) 2; /* carsamba */ gun2 = "Pazartesi"; if(gun2 < gun1) { printf("gun2 daha onceki gun\n");

14 Scope How long do the variables stay valid?

15 Scope of Variables The variable scope is defined as the program segment in which the variable can be used. The variable name is an alias for the address of the variable in the memory. For example, if the variable is defined right after "void main {", then it is valid only in main(). It cannot be used anywhere else.

16 Scope of Variables If a variable is defined inside a block ( { and ), then it is valid until the end of block. If the variable is defined before all the functions, they are called global variables, and they are valid throughout the whole file.

17 Scope of Variables No two variables may have the same name if they are defined in the same scope. In different scopes, you can use variables with the same name. Since they correspond to different memory places, there will be no ambiguity. Yet, try to avoid using the same variable names under any circumstances.

18 Example - 1 x variable is not defined yet. Compile error!!! #include <stdio.h> void main ( ) { if ( x == 3 ) { float x = 4.5; x was only in the block, and now not defined any more... printf ( "blok içinde x=%f", x); printf( "bloktan sonra x=%d\n", x);

19 Example - 2 x variable is defined. OK!! #include <stdio.h> void main ( ) { int x = 3; Here, the nearestly defined one is valid. if ( x == 3 ) { float x = 4.5; printf ( "blok içinde x=%f", x); printf( bloktan sonra x=%d\n", x);

20 Scope of Variables In the previous example, there is an x variable defined right after main(). But in if block, there is another variable called x. Inside the block, that new variable will be valid, and the outer x is not valid. Since the two variables correspond to two different memory addresses, they will not be confused.

21 Would they? Run the program and see the following output: blok içinde x=4.5 bloktan sonra x=3

22 Example (Scope) Question #include <stdio.h> void main(void){ int x = 5, y = 7, z; x += y++; if (y > 7){ int x = 2; x += y; printf("x = %d\n", x); printf("x = %d\n", x); x = 10 X = 12

23 Computer Programming Pointers Pointers Variables whose addresses are specified.

24 Computer Programming Pointers (we used them to access parameters that are sent by reference-address) A pointer points to something. They require using & and * symbols. A variable, e.g. 'int yil' is a name of memory place. A pointer keeps the address of this variable. The variable corresponds to whatever the address contains. Pointers are methods to reach to the values contained inside addresses.

25 Computer Programming Pointers Consider a variable: 'int yillar'. Then, &yillar corresponds to the address of the variable. Remember that we used & symbol before the variables used in scanf(). scanf("%d", &yillar); &yillar is actually a constant for the pointer. & means "adress of". A line such as printf("%d", & yillar) writes someting like This is the memory address of the variable yillar.

26 Computer Programming Pointers What are & and *? & : Address operator. Returns the address of whatever comes after it. * : Access value or eliminate reference operator. It returns the contained value of whatever address number comes after it. * and & are duals of each other. Example: *&a is a itself.

27 Computer Programming Pointers What are & and *? With * operator, construct a variable: /* *pd means it pd contains address. */ int *pd, d = 5; Use & to access to the address of a variable. /* Load address of d into pd variable. */ pd = &d; To access to the variable at an address, use * *pd = 10; /* Assigns 10 as a value of d */

28 Graphically pd int *pd, d = 5; pd = &d; *pd=10 &d 10 5 *pd==d==5 => d==10!!!

29 Computer Programming Pointers Pointer Variable A pointer variable contains an address. As an example, an integer pointer is defined like int *p; The variable p itself holds an integer correponding to the address of the integer *p. int a = 5, *p; p = &a; printf("%d", *p); writes 5 on the screen.

30 Computer Programming Pointers Pointer Variables Examples FILE *dosya; int *a; float *b; char *c; The star symbol before the variable name means that these variables are represented by memory addresses.

31 Example Change the value of yillar variable using pointers. int yillar, *p; yillar = 20; p = &yillar; *p = 10; adress : : : : : : : : : : variables yillar p

32 Computer Programming Pointers Pointers and Arrays The writers of the C language developed a smart method of using pointers with arrays. The name of an array is a pointer to the first element of the array. Example: int tekler[5] = {3,5,7,9,11; The name tekler corresponds to a pointer.

33 Computer Programming Pointers Example: Pointer Operations int tekler[5] = {3,5,7,9,11; int *p, n; p = tekler; /* p points to 3 */ p = & tekler[0]; /* same as above */ n = *p; /* n is now 3 */ n = p[0]; /* same as above */ p[4] = 13; /* tekler[4] becomes 13 */

34 int tekler[5] = {3,5,7,9,11; p = tekler; e = & tekler[0]; n = *p; d = p[2]; p[4] = 13; e p 1000: 1004: 1008: 1016: 1020: 1024: 1028: 1032: 1036: 1040: 1044: 1048: tekler n d

35 Computer Programming Pointers Arithmetic Operations on Pointers Pointer variables can be incremented or decremented. A numerical value can be added to or subtracted from an integer variable. Two pointers of the same type can be added or subtracted. Two pointers can be compared. A pointer which not point to any address contains 0 (NULL).

36 Computer Programming Pointers Pointer Arithmetic int tekler[5] = {3,5,7,9,11; int *p, n; p = tekler; /* p points to 3 */ ++p; /* p points to 5 */ p = (tekler + 1); /* p points to 5 */ p = (tekler + 4); /* p points to 11 */ p = & tekler[4]; /* p points to 11 */

37 Computer Programming Pointers Pointers and Array Operators Array names are constant addresses that correspond to always the same place. ++tekler; tekler = p; Pointer variables have free addresses which can be changed. ++p; p += 5; p--; ERROR VALID and USEFUL

38 Computer Programming Pointers Pointer Arithmetic int i[7]; int *j; j= i; /* j points to i[0] */ j= j+1; /* Increment j pointer*/ *j= 3; /* Value pointed to by j is 3 */ j= i; /* Same as j= &i[0] */ *j= 10; /* i[0] = 10 */ j[1]=4; /* Value pointed to by j+1 is 4 */ j= j+5; /* Same as j = &i[5] */ j[1]= 5; i= i+4; /* Error!!! */ j= j+2; /* j goes to one after last element*/ *j= 4; /* Error!!! Nothing there...*/

39 int i, arr[10]; for (i=0;i<10;i++) arr[i] = 3*i + 100; arr = &(arr[0]) arr+1 = &(arr[1]) arr[0] = *arr arr[1] = *(arr+1) *(arr+i) = arr[i] arr+i = &(arr[i]) pointer (*) representaion, is equivalent to array [] representation.

40 Computer Programming Pointers *arr = arr[0] = 100 *(arr+1) = arr[1] = 103; *(arr++) = 103 NOW arr points to the next element s address. &(arr[3]) = arr+3 for(i=1;i<10;i++) printf( %d\n,*(arr++)); /* prints out the elements of the array. However, arr will be pointing to the end of the array now, so be careful about further using it: */ printf( %d\n,*arr); /* arr[9] is printed */

41 Computer Programming Pointers Passing Arrays to Functions How do we send all elements of an array to a function? We send the address of the first element. void yaz(int *dz) { printf("%d", dz[4]); void main() { int dizi[5]; yaz(dizi); /*yaz(&(dizi[0]));*/

42 Computer Programming Pointers Passing Arrays to Functions How do we send all elements of an array to a function? We send the address of the first element. void dizi_carp(int ar1[], int katsayi, int uzunluk) { int i; for(i=0;i<uzunluk;i++) ar1[i] *= katsayi;

43 Computer Programming Pointers Passing Arrays to Functions Example: Averaging Function float ortalama(float a[], int n) { float ortalama(float a[], int n); /* prototype */ float toplam = 0.0; int i; int main (void) { float dizi[5] = {1.0, 2.0, 3.0, 4.0, 5.0; printf("%f\n", ortalama(dizi, 5)); /* call */ for (i=0; i < n; i++) toplam += a[i]; return( toplam / n );