5 EME Fall 2006
Discussion 6 1. Reasons for using pointers People say that one of the more important and distinguish characteristics of C as a programming language is its
sophisticated use of pointers. People also say that the correct understanding and use of pointers is critical to successful C programming, since every real-world C program uses pointers. So, here are four main reasons for using pointers in our programs. (1) Pointers provide the way by which functions can modify their calling arguments. This means that by using pointers, we can pass addresses of variables to functions so that functions can change the value of these variables. (2) Pointers support C's dynamic allocation routines. This means that by using pointers, we can work with memory that has been allocated dynamically. (3) Pointers can improve the efficiency of certain routines. (4) Pointers can directly access the computer's hardware.
2. What are pointers? A pointer is a variable that holds the address of a memory location. Whenever we declare a variable, as in int i; the computer reserves a memory location for it. The
computer might, for example, set aside memory location 1000 for i. Memory address 1000 Computer memory Variable name i
When we subsequently refer to i, we are indeed referring to this location. Assigning 5 to i with i = 5; places a 5 in memory location 1000. Memory address 1000 Computer memory 5 Variable name i
We say that 1000 is the address of i.
3. Pointer variables In C, the straightforward way to access a variable is
through its name. Another way to access a variable, which is sometimes more convenient and efficient, is through a pointer that holds the address of that variable.
The general form for declaring a pointer variable is type *name; where type is the base type of the pointer and can be any valid C type. The name of the pointer variable is specified by name.
int i; i = 5; // Access a variable i by its name. int *p;
Memory address 1000
Computer memory 5
Variable name i
2000
p
p = &i; // The address of i is assigned to p. // We say that the variable p points to i.
Note:
The
address
operator
&
gives
the
address
of
a
variable. The expression &i represents the address of i. Memory address 1000 Computer memory 5 Variable name i
2000
1000
p
*p = 50; // Access a variable i by a pointer that points // to it. Here we assign 50 to i through p. The // expression is equivalent to i = 50;.
Note: The dereference (indirection) * operator gives the contents of an object pointed to by a pointer. The
expression *p represents the value stored in the memory pointed to by p.
Memory address 1000
Computer memory 50
Variable name i
2000 4. char a[6] = "hello"; Memory address 1000
1000
p
Computer memory h e l l o \0
Variable name a
char *p; p = a; Memory address 1000 Computer memory h e l l o \0 Variable name a
2000
1000
p
char q[6]; Memory address 3000 Computer memory Variable name q
q = a; // It is a wrong statement to copy the contents of a // to q, since q is not a pointer variable. Memory address 1000 Computer memory h e l l o \0 Variable name a
3000
q
/* correct way to copy the contents of a to q */ int i; while(a[i] != `\0') { q[i] = a[i]; i = i + 1; } q[i] = `\0';
5. int a[2][3];
Address of the 1
st
Value of the 1st element in each row
element
in each row
a[0] = *a a[1] = *(a+1)
a[0][0] = **a a[1][0] = **(a+1)
a[0][0] a[0][1] a[0][2] a[1][0] a[1][1] a[1][2]
Address of the the (i, j) element
Value of the the (i, j) element
&(a[i][j]) = *(a+i)+j
a[i][j] = *(*(a+i)+j)
6. Write a function that sorts an array with n elements in ascending order.
void sortArray(int n, int *a); /* ex1.c */ #include <stdio.h> #define N 2 #define M 3 void sortArray(int, int *); int main() { int a[N][M] = {3, 1, 6, 4, 2, 5}; int i, j; sortArray(N*M, a); for(i=0; i<N; i++) { for(j=0; j<M; j++) { printf("%d ", a[i][j]); } printf("\n"); } return 0; } void sortArray(int n, int *a) { int i, j, temp; for(i=0; i<(n-1); i++) { for(j=i+1; j<n; j++) { if(a[i] > a[j]) { temp = a[i]; a[i] = a[j]; a[j] = temp; } } } }
7. Write a program containing a function that reads in the length and width of a rectangle as its arguments and returns both the area and perimeter of the rectangle. Hint: The function needs to return a pointer.
/* ex2.c */ #include <stdio.h> #include <stdlib.h> // This header file defines the // prototype of malloc(). double *func(double l, double w) { double *val; val = (double *)malloc(2*sizeof(double)); val[0] = l*w; val[1] = (l+w)*2; return val; } int main() { double l, w; double *retval; printf("Please enter the length : "); scanf("%lf", &l); printf("Please enter the width : "); scanf("%lf", &w); retval = func(l, w); printf("The area is %f\n", retval[0]); printf("The perimeter is %f\n", retval[1]); free(retval); return 0; }