Slides adopted from T. Ferguson Spring 2016

Transcription

1 CSE3 Introduction to Programming for Science & Engineering Students Mostafa Parchami, Ph.D. Dept. of Comp. Science and Eng., Univ. of Texas at Arlington, USA Slides adopted from T. Ferguson Spring 06

2 Pointers Introduction to Pointers Pointers and Function Parameters

3 Memory and Variables char ch = A ; Video: 0x000 ch: A The memory address of the variable ch The value of the variable ch 3

4 & Operator Gives the memory address of an object char ch = A ; 0x000 A &ch yields the value 0x000 Also known as the address operator 4

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

6 Pointers A variable which can store the memory address of another variable 0x3A5 0x000 chptr 0xFFE 0xFFF 0x000 0x00 0x00 B ch etc 6

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

8 Example: cptr: Pointers char* cptr; 0x004 Can store an address of variables of type char We say cptr is a pointer to char 8

9 Pointers and, & Operator Example: char c = A ; char *cptr; cptr = &c; Assigns the address of c to cptr c: A cptr: 0x000 0x000 0x004 9

10 Pointers We can have pointers to any data type Example: int* numptr; float* xptr; The * can be anywhere between the type and the variable Example: int *numptr; float * xptr; 0

11 Pointers You can assign the address of a variable to a compatible pointer using the & operator Example: int anumber; int *numptr; numptr = &anumber; You can print the address stored in a pointer using the %p conversion specifier Example: printf( %p, numptr);

12 Asterisk Operator Allows pointers to access variables they point to Also known as dereferencing operator Should not be confused with the * in the pointer declaration

13 Pointers and * Operator Example: char c = A ; char *cptr = NULL; cptr = &c; *cptr = B ; Changes the value of the variable which cptr points to c: cptr: B A NULL 0x000 0x000 0x004 3

14 Steps Step : Declare the variable to be pointed to int num; char ch = A ; float x; num: ch: A x: 4

15 Steps Step : Declare the pointer variable int num; char ch = A ; float x; int* numptr = NULL; char *chptr = NULL; float * xptr = NULL; numptr: chptr: xptr: num: ch: x: NULL NULL NULL A 5

16 Steps Step 3: Assign address of variable to pointer int num; char ch = A ; float x; int* numptr = NULL; char *chptr = NULL; float * xptr = NULL; numptr = # chptr = &ch; xptr = &x; numptr: addr of num chptr: xptr: num: ch: addr of ch addr of x A A pointer s type has to correspond to the type of the variable it points to x: 6

17 Steps Step 4: De-reference the pointers int num; char ch = A ; float x; int* numptr = NULL; char *chptr = NULL; float * xptr = NULL; numptr = # chptr = &ch; xptr = &x; *xptr = 0.5; *numptr = *chptr; numptr: addr of num chptr: xptr: addr of ch addr of x num: 65 ch: A x: 0.5 7

18 Pointers int *numptr; Beware of pointers which are not initialized!??? numptr 8

19 Pointer Initialization When declaring a pointer, it is a good idea to always initialize it to NULL (a special pointer constant) int *numptr = NULL; NULL numptr 9

20 Pointers and Function Parameters Example: Function to swap the values of two variables x: y: swap x: y: 0

21 Pointers and Function Parameters #include <stdio.h> void swap(int a, int b) int tmp; Bad swap tmp = a; a = b; b = tmp; return; int main() int x =, y = ; swap(x, y); printf( %d %d\n, x, y); return 0;

22 Pointers and Function Parameters #include <stdio.h> void swap(int a, int b) int tmp; Bad swap tmp = a; a = b; b = tmp; return; int main() int x =, y = ; swap(x, y); printf( %d %d\n, x, y); return 0; x: y:

23 Pointers and Function Parameters #include <stdio.h> Bad swap void swap(int a, int b) int tmp; tmp: tmp = a; a = b; b = tmp; return; a: b: int main() int x =, y = ; swap(x, y); printf( %d %d\n, x, y); return 0; x: y: 3

24 Pointers and Function Parameters #include <stdio.h> Bad swap void swap(int a, int b) int tmp; tmp: tmp = a; a = b; b = tmp; return; a: b: int main() int x =, y = ; swap(x, y); printf( %d %d\n, x, y); return 0; x: y: 4

25 Pointers and Function Parameters #include <stdio.h> Bad swap void swap(int a, int b) int tmp; tmp: tmp = a; a = b; b = tmp; return; a: b: int main() int x =, y = ; swap(x, y); printf( %d %d\n, x, y); return 0; x: y: 5

26 Pointers and Function Parameters #include <stdio.h> Bad swap void swap(int a, int b) int tmp; tmp: tmp = a; a = b; b = tmp; return; a: b: int main() int x =, y = ; swap(x, y); printf( %d %d\n, x, y); return 0; x: y: 6

27 Pointers and Function Parameters #include <stdio.h> Bad swap void swap(int a, int b) int tmp; tmp: tmp = a; a = b; b = tmp; return; a: b: int main() int x =, y = ; swap(x, y); printf( %d %d\n, x, y); return 0; x: y: 7

28 Pointers and Function Parameters #include <stdio.h> void swap(int* a, int* b) int tmp; Good swap tmp = *a; *a = *b; *b = tmp; return; int main() int x =, y = ; swap(&x, &y); printf( %d %d\n, x, y); return 0; 8

29 Pointers and Function Parameters #include <stdio.h> void swap(int* a, int* b) int tmp; Good swap tmp = *a; *a = *b; *b = tmp; return; int main() int x =, y = ; swap(&x, &y); printf( %d %d\n, x, y); return 0; x: y: 9

30 Pointers and Function Parameters #include <stdio.h> Good swap void swap(int* a, int* b) int tmp; tmp: tmp = *a; *a = *b; *b = tmp; return; a: b: addr of x addr of y int main() int x =, y = ; swap(&x, &y); printf( %d %d\n, x, y); return 0; x: y: 30

31 Pointers and Function Parameters #include <stdio.h> Good swap void swap(int* a, int* b) int tmp; tmp: tmp = *a; *a = *b; *b = tmp; return; a: b: addr of x addr of y int main() int x =, y = ; swap(&x, &y); printf( %d %d\n, x, y); return 0; x: y: 3

32 Pointers and Function Parameters #include <stdio.h> Good swap void swap(int* a, int* b) int tmp; tmp: tmp = *a; *a = *b; *b = tmp; return; a: b: addr of x addr of y int main() int x =, y = ; swap(&x, &y); printf( %d %d\n, x, y); return 0; x: y: 3

33 Pointers and Function Parameters #include <stdio.h> Good swap void swap(int* a, int* b) int tmp; tmp: tmp = *a; *a = *b; *b = tmp; return; a: b: addr of x addr of y int main() int x =, y = ; swap(&x, &y); printf( %d %d\n, x, y); return 0; x: y: 33

34 Pointers and Function Parameters #include <stdio.h> void swap(int* a, int* b) int tmp; Good swap tmp = *a; *a = *b; *b = tmp; return; int main() int x =, y = ; swap(&x, &y); printf( %d %d\n, x, y); return 0; x: y: 34

35 Pointers and Function Parameters Change the value of an actual parameter variable scanf demystified char ch; int numx; float numy; scanf( %c %d %f, &ch, &numx, &numy); 35

36 Reading Deitel and Deitel Chapter 7 (7.-7.4) 36