A pointer is a variable just like other variable. The only difference from other variables is that it stores the memory address other variables.

Transcription

1 Lecture 9 Pointers

2 A pointer is a variable just like other variable. The only difference from other variables is that it stores the memory address other variables. This variable may be of type int, char, array, structure, function or any other.

3 Normal variables Contain a specific value (direct reference) Pointer variables Contain memory addresses as their values countptr count 7 count 7

4 Pointer declarations * used with pointer variables int *myptr; float *Ptr; Char *strptr; Multiple pointers require using a * before each variable declaration int *myptr1, *myptr2;

5 Initialize pointers to: 0 or NULL, points to nothing (NULL preferred) an address points to a certain place/address in the memory int y = 5; int *yptr; yptr = &y; // yptr is the address of y // yptr points to y

6 int a=5; int * ptr; ptr=&a; About variable a: 1. Name of variable : a 2. Value of variable: 5 3. Address: 1025 (assume) About variable ptr: 4. Name of variable: ptr 5. Value of variable: Address: 5000 (assume)

7 Call by reference with pointer arguments Pass address of argument using & operator Arrays are not passed with & because the array name is already a pointer * operator Used as nickname for variable inside of function void double( int *number ) { *number = 2 * ( *number ); }

8 1 /* Example (2) 2 Cube a variable using call-by-reference Notice that the function prototype takes a pointer to an integer (int *). 3 with a pointer argument */ 4 5 #include <stdio.h> 6 Notice how the address of number 7 void cubebyreference( int * ); /* is given prototype - cubebyreference */ 8 expects a pointer (an address of a variable). 9 int main() 10 { 11 int number = 5; printf( "The original value of number is %d", number ); 14 cubebyreference( &number ); 15 printf( "\nthe new value of number is %d\n", number ); 16 Inside cubebyreference, *nptr is used (*nptr is number). 17 return 0; 18 } void cubebyreference( int *nptr ) 21 { 22 *nptr = *nptr * *nptr * *nptr; /* cube number in main */ 23 } Outline Function prototype 1 Initialize variables 2. Call function 3. Define function The original value of number is 5 The new value of number is 125 Program Output

9 #include<stdio.h> int main() { int *ptr=( int *)1000; ptr=ptr+1; printf(" %d",ptr); return 0; } #include<stdio.h> int main() { double *p=(double *)1000; p=p+3; printf(" %d",p); return 0; } Output: 1004 Output: 1024

10 #include<stdio.h> int main() { int *p=(int *)1000; int *temp; temp=p; p=p+2; printf("%d %d\n",temp,p); printf("difference= %d",ptemp); return 0; } Output: Difference= 2 #include<stdio.h> int main() { float *p=(float *)1000; float *q=(float *)2000; printf("difference= %d",q-p); return 0; } Output: Difference= 250

11 Arithmetic operations can be performed on pointers Increment/decrement pointer (++ or --) Add an integer to a pointer ( + or +=, - or -=) Pointers may be subtracted from each other Operations meaningless unless performed on an array

12 operation p++, p-- p+i (p-i) p[i] p + n (integer) Description Increment (decrement) p to point to the next element, it is equivalent to p+=1 (p -=1) Point to i-th element beyond (in front of) p but value of p is fixed Equivalent to p + i n must be an integer, its meaning is offset p - q Offset between pointer p and pointer q p+q, p*q, p/q, p%q Relational operator of two pointers p, q invalid valid, including p > q, p < q, p == q, p >= q, p <= q, p!= q

13 Pointer comparison ( <, ==, > ) See which pointer points to the higher numbered array element (index) Also, see if a pointer points to 0

14 #include <stdio.h> int my_array[] = {1,23,17,4,-5,100}; int *ptr; int main(void) { int i; ptr = &my_array[0]; /* point our pointer to the first element of the array */ printf("\n\n"); for (i = 0; i < 6; i++) { printf("my_array[%d] = %d ",i,my_array[i]); printf("ptr + %d = %d\n",i, *(ptr + i)); } return 0; }

15 #include <stdio.h> int main() { char line[20]; char *part = "hello"; do { printf("\nenter new String: "); gets(line); if(strcmp(line, part) == 0) printf("the same string %s\n", line); } while(strlen(line)!= 0) ; return 0; }

16 #include <stdio.h> void printchars (char *string) { int count; for(count = 0; count < strlen(string); count ++) { printf ( \n char no: %d is %c, count, string [count]) ; string[count] = a + count ; } } int main() { char Arr[]= This is a test ; puts (Arr); printchars ( Arr ); printf ( \n %s, Arr); return 0; }

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18 Note a same &a[0] a is a pointer only to the first element not the whole array. Note The name of an array is a pointer constant; it cannot be used as an lvalue.

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21 Note To access an array, any pointer to the first element can be used instead of the name of the array.

22 Compiler determine length of string and then size of amessage pmessage: string constant, cannot modified n o w i s t h e t i m e \0 Modifiable character array amessage: n o w i s t h e t i m e \0

23 #include <stdio.h> int my_array[] = {1,23,17,4,-5,100}; int *ptr; int main(void) { int i; ptr = &my_array[0]; /* point our pointer to the first element of the array */ printf("\n\n"); for (i = 0; i < 6; i++) { printf("my_array[%d] = %d ", i, my_array[i] ); printf( \t ptr + %d = %d\n", i, *(ptr + i) ); } return 0; }

24 #include <stdio.h> int countnchar (char *string, char ch) { char *p; int count = 0; for(p = string; *p!= '\0'; p++) { if(*p == ch) count++; } return count; } int main() { char f = A ; int x ; char Arr[]= This is A test for letter A ; x = countnchar ( Arr, f ); printf ( Letter A exist : %d times, x ); return 0; }

25 #include <stdio.h> # define N 5 void swap (int *a, int *b){ int temp = *a; *a = *b; *b = temp; } int main() { int i, j; int Arr[N]={1,2,3,4,5}; for (i=0; i < N/2 ; i++) swap ( & Arr [ i ], & Arr [ (N-1) i ] ); return 0; }

26 #include <stdio.h> # define N 5 void swap (int *a, int *b){ int temp = *a; *a = *b; *b = temp; } int main() { int i, j; int Arr[N]={1,2,3,4,5}; for (i=0; i < N/2 ; i++) swap ( Arr + i, Arr + (N-1) i ); return 0; }

27 #include <stdio.h> void reverse(char *mysrt) { char * lp = mysrt; /* left pointer */ char *rp = &mysrt[strlen(mysrt)-1]; /* right pointer */ char tmp; int main() while(lp < rp) { { char Arr[]="This is a test"; tmp = *lp; puts(arr); *lp = *rp; reverse (Arr); *rp = tmp; printf("\n"); lp++; puts(arr); rp--; } return 0; } }

28 Arrays can contain pointers to..

29 int x = 4; int *y = &x; int *z[4] = {NULL, NULL, NULL, NULL}; int a[4] = {1, 2, 3, 4}; z[0] = a; // same as &a[0]; z[1] = a + 1; // same as &a[1]; z[2] = a + 2; // same as &a[2]; z[3] = a + 3; // same as &a[3]; for (x=0;x<4;x++) printf("\n %d --- %d ",a[x],*z[x]);

30 Arrays can contain pointers to (array)

31 For example: an array of strings char *suit[ 4 ] = { "Hearts", "Diamonds", "Clubs", "Spades" }; Strings are pointers to the first character Each element of suit is a pointer to a char The strings are not actually stored in the array suit, only pointers to the strings are stored suit array has a fixed size, but strings can be of any size

32 char * each element of suit is a pointer to a char The strings are not actually stored in the array suit, only pointers to the strings are stored suit array has a fixed size, but strings can be of any size suit[0] H e a r t s \0 suit[1] D i a m o n d s \0 suit[2] C l u b s \0 suit[3] S p a d e s \0

33 char *suit[ 4 ] = { "Hearts", "Diamonds", "Clubs", "Spades" }; int main() { int x ; for (x = 0; x < 4 ; x++) printf("\n %s --- %d ", suit[x],strlen(suit[x])); return 0; }

34 We can use pointer to struct: struct MyPoint {int x, int y}; MyPoint point, *ptr; point.x = 0; point.y = 10; ptr = &point; ptr->x = 12; same as (*ptr).x ptr->y = 40; same as (*ptr).y

35 #include <stdio.h> struct inven { char code; float cost; int pieces ; } ; int main() { struct inven part; read (&part); write (part); return 0; }

36 void read (struct inven *in) { printf ("\n Enter Product Data. \n") ; printf (" Enter part code: "); scanf ("%c",&in->code); printf (" Enter part cost: "); scanf ("%f",&in->cost); printf (" Enter no of pieces: "); scanf ("%d",&in->pieces); } void write (struct inven out) { printf (" part code: %c \n", out.code); printf (" part cost: %f \n", out.cost); printf (" no of pieces: %d \n", out.pieces); }

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