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Introduction to Computing Lecture 12 Pointers

Introduction to Computing Lecture 12 Pointers. Dr. Bekir KARLIK Yasar University Department of Computer Engineering bekir. karlik@ yasar.edu.tr. Topics. Introduction to Pointers Pointers and Function Parameters. ch:. ’A’. 0x2000. Memory Address of a Variable. char ch = ’A’;.

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Introduction to Computing Lecture 12 Pointers

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  1. Introduction to Computing Lecture 12Pointers • Dr. Bekir KARLIK • Yasar University • Department of Computer Engineering • bekir.karlik@yasar.edu.tr

  2. Topics • Introduction to Pointers • Pointers and Function Parameters

  3. ch: ’A’ 0x2000 Memory Address of a Variable char ch = ’A’; The memory address of the variable ch. The value of the variable ch.

  4. char ch = ’A’; ’A’ 0x2000 &ch yields the value 0x2000 The &Operator • Gives the memory address of an object • Also known as the “address operator.”

  5. “conversion specifier” for printing a memory address Example: char ch; printf(“%p”, &ch);

  6. 0x3A15 0x2000 chPtr Pointers A variable which can store the memory address of another variable. 0x1FFE 0x1FFF 0x2000 0x2001 0x2002 etc. ‘B’ ch

  7. Pointers • A pointer is a variable. • Contains a memory address. • Points to a specific data type. • Pointer variables are usually named varPtr.

  8. Example: char* cPtr; cPtr: 0x2004 Can store an address of variables of type char • We say cPtris a pointer to char.

  9. c: cPtr: 0x2000 A 0x2004 Pointers and the & Operator Example: char c = ’A’; char *cPtr; cPtr = &c; Assigns the address ofc to cPtr. 0x2000

  10. We can have pointers to any data type. • The * can be anywhere between the type and the variable. Example: int* numPtr; float* xPtr; Example: int *numPtr; float * xPtr; Notes on Pointers

  11. You can assign the address of a variable to a “compatible” pointer using the &operator. int aNumber; int *numPtr; numPtr = &aNumber; Example: • You can print the address stored in a pointer using the %pconversion specifier. Example: printf(“%p”, numPtr); Notes on Pointers (cont.)

  12. Notes on Pointers (cont.) Beware of pointers which are not initialized! int *numPtr; ??? numPtr

  13. Notes on Pointers (cont.) • When declaring a pointer, it is a good idea to always initialize it toNULL (a special pointer constant). int *numPtr = NULL; NULL numPtr

  14. The *Operator • Allows pointers to access to variables they point to. • Also known as “dereferencing operator.” • Should not be confused with the * in the pointer declaration.

  15. cPtr: c: 0x2000 0x2004 A Pointers and the  Operator Example: char c = ’A’; char *cPtr = NULL; Changes the value of the variable which cPtr points to. cPtr = &c; *cPtr = ’B’; B 0x2000

  16. Easy Steps to Pointers int num; char ch = ‘A’; float x; • Step 1: Declare the variable to be pointed to. num: ch: ‘A’ x:

  17. Easy Steps to Pointers int num; char ch = ‘A’; float x; • Step 2: Declare the pointer variable. numPtr: int* numPtr = NULL; char *chPtr = NULL; float * xPtr = NULL; chPtr: xPtr: num: ch: ‘A’ x:

  18. Easy Steps to Pointers int num; char ch = ‘A’; float x; • Step 3: Assign address of variable to pointer. numPtr: addr of num int* numPtr = NULL; char *chPtr = NULL; float * xPtr = NULL; chPtr: addr of ch xPtr: addr of x numPtr = # chPtr = &ch; xPtr = &x; num: ch: ‘A’ A pointer’s type has to correspond to the type of the variable it points to. x:

  19. Easy Steps to Pointers int num; char ch = ‘A’; float x; • Step 4: De-reference the pointers. numPtr: addr of num int* numPtr = NULL; char *chPtr = NULL; float * xPtr = NULL; chPtr: addr of ch xPtr: addr of x numPtr = # chPtr = &ch; xPtr = &x; num: 65 *xPtr = 0.25; *numPtr = *chPtr; ch: ‘A’ x: 0.25

  20. Pointers and Function Parameters • Example: Function to swap the values of two variables: swap x: 1 y: 2 x: 2 y: 1

  21. #include <stdio.h> void swap1(int a, int b) { int tmp; tmp = a; a = b; b = tmp; } main() { int x = 1, y = 2; swap1(x, y); printf(“%d %d\n”, x, y); } Bad swap

  22. #include <stdio.h> void swap1(int a, int b) { int tmp; tmp = a; a = b; b = tmp; } main() { int x = 1, y = 2; swap1(x, y); printf(“%d %d\n”, x, y); } Bad swap x: 1 y: 2

  23. #include <stdio.h> void swap1(int a, int b) { int tmp; tmp = a; a = b; b = tmp; } main() { int x = 1, y = 2; swap1(x, y); printf(“%d %d\n”, x, y); } Bad swap tmp: a: 1 b: 2 x: 1 y: 2

  24. #include <stdio.h> void swap1(int a, int b) { int tmp; tmp = a; a = b; b = tmp; } main() { int x = 1, y = 2; swap1(x, y); printf(“%d %d\n”, x, y); } Bad swap tmp: 1 a: 1 b: 2 x: 1 y: 2

  25. #include <stdio.h> void swap1(int a, int b) { int tmp; tmp = a; a = b; b = tmp; } main() { int x = 1, y = 2; swap1(x, y); printf(“%d %d\n”, x, y); } Bad swap tmp: 1 a: 2 b: 2 x: 1 y: 2

  26. #include <stdio.h> void swap1(int a, int b) { int tmp; tmp = a; a = b; b = tmp; } main() { int x = 1, y = 2; swap1(x, y); printf(“%d %d\n”, x, y); } Bad swap tmp: 1 a: 2 b: 1 x: 1 y: 2

  27. #include <stdio.h> void swap1(int a, int b) { int tmp; tmp = a; a = b; b = tmp; } main() { int x = 1, y = 2; swap1(x, y); printf(“%d %d\n”, x, y); } Bad swap tmp: 1 a: 2 b: 1 x: 1 y: 2

  28. #include <stdio.h> void swap2(int* a, int* b) { int tmp; tmp = *a; *a = *b; *b = tmp; } main() { int x = 1, y = 2; swap2(&x, &y); printf(“%d %d\n”, x, y); } Good swap

  29. #include <stdio.h> void swap2(int* a, int* b) { int tmp; tmp = *a; *a = *b; *b = tmp; } main() { int x = 1, y = 2; swap2(&x, &y); printf(“%d %d\n”, x, y); } Good swap x: 1 y: 2

  30. #include <stdio.h> void swap2(int* a, int* b) { int tmp; tmp = *a; *a = *b; *b = tmp; } main() { int x = 1, y = 2; swap2(&x, &y); printf(“%d %d\n”, x, y); } Good swap tmp: a: addr of x b: addr of y x: 1 y: 2

  31. #include <stdio.h> void swap2(int* a, int* b) { int tmp; tmp = *a; *a = *b; *b = tmp; } main() { int x = 1, y = 2; swap2(&x, &y); printf(“%d %d\n”, x, y); } Good swap tmp: 1 a: addr of x b: addr of y x: 1 y: 2

  32. #include <stdio.h> void swap2(int* a, int* b) { int tmp; tmp = *a; *a = *b; *b = tmp; } main() { int x = 1, y = 2; swap2(&x, &y); printf(“%d %d\n”, x, y); } Good swap tmp: 1 a: addr of x b: addr of y x: 2 y: 2

  33. #include <stdio.h> void swap2(int* a, int* b) { int tmp; tmp = *a; *a = *b; *b = tmp; } main() { int x = 1, y = 2; swap2(&x, &y); printf(“%d %d\n”, x, y); } Good swap tmp: 1 a: addr of x b: addr of y x: 2 y: 1

  34. #include <stdio.h> void swap2(int* a, int* b) { int tmp; tmp = *a; *a = *b; *b = tmp; } main() { int x = 1, y = 2; swap2(&x, &y); printf(“%d %d\n”, x, y); } Good swap x: 2 y: 1

  35. scanfdemystified. char ch; int numx; float numy; scanf(“%c %d %f”, &ch, &numx, &numy); Pointers and Function Arguments • Change the value of an actual parameter variable.

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