C programming---Pointers The first step: visualizing what pointers represent at the machine level. In most modern computers, main memory is divided into bytes. Each byte can hold 8 bits of information: 1 0 1 0 0 0 1 Each byte has a unique address in memory.

1

Address 0 1 2 3 Address Contents

00110011

10101010 00000111 11111000 n-1 10000100

Address int i = 9; /* suppose sizeof(int) = 4 The address of variable i is 0xFA83 0xFA83 0xFA84 0xFA85 0xFA86

Pointer Variables Use a pointer variable p to store the address of a variable i, and we say p “points to” i 0xFA83 0xFA84 0xFA85 0xFA86

p

0xFA83

Declaring Pointer Variables int *p; // p is a pointer variable capable of pointing to

objects

of type int char *str; double *q; C requires that every pointer variable point to objects of a particular type(the

referenced type

) There are no restrictions on what referenced type may be. In fact, a pointer variable can even point to another pointer.

The Address and Indirection Operators int i, *p; …….

p = &i; int i; int *p = &i; int i, p = &i;

The Address and Indirection Operators printf(“%d\n”, *p); j = *&i;

The Address and Indirection Operators int i, *p = &i; i = 1; printf(“i = %d\n”, i); printf(“*p = %d\n”, *p); *p = 4; printf(“i = %d\n”, i); printf(“*p = %d\n”, *p);

Something to remember Never apply the indirection operator to an uninitialized pointer variable.

int *p; printf(“%d”, *p); Unless you know where a pointer points to, do not make an assignment to the pointer

Pointer Assignment int i, j, *p, *q; p = *i; q = p; p q i

Pointer Assignment int i, j, *p, *q; p = &i; q = &j; *q = *p; p q i

Example 2.c

Pointer as Arguments

Using const to Protect Arguments When we call a function and pass it a pointer to a variable, we normally assume that the function will modify the variable. Sometimes we just want to examine the value of a variable, not change it.

Using pointer might be efficient: time and memory space } { void f(const int *p) *p = 0; // wrong: p is a pointer to a “constant integer”

Pointers as Return Values } { int *max(int *a, int *b) if(*a > *b) return a; else return b; int *p = max(&a, &b);

} { int *f(void) int a; …… return &a; Be careful

Pointers and Arrays Pointer Arithmetic int a[10], *p; p = &a[0]; a p

Pointer Arithmetic p p = &a[2]; q = p + 3; p += 4; q

Comparing Pointers Using the relational operators( <, <=, >, >=) and the equality operator (== and !=) int *p = &a[5]; int *q = &a[1]; The value of p <= q is 0(false) The value of p >= q is 1(true)

Using Pointers for Array Processing #define N 10 int a[N], sum, *p; sum = 0; for(p = &a[0]; p < &a[N]; p++) sum += *p;

Combining the * and ++ Operators *p++ or *(p++) (*p)++ *++p or *(++p) ++*p or ++(*p) The postfix version of ++ takes precedence over * See 3.c

Using an Array Name as a Pointer int a[10]; *a = 7; // modify a[0] *(a + 2) = 13; // modify a[2] while( *a != 0 ) a++; // wrong

Array Arguments } { int find_largest(int a[], int n) { } int find_largest(int *a, int n)

Using a Pointer as an Array Name #define N 10 …..

int a[N], i, sum = 0, *p = a; …..

for(i =0; i < N; i++) sum += p[i];

Pointers and Multidimensional Arrays int a[NUM_ROWS][NUM_COLS]; int row, col; for(row = 0; row < NUM_ROWS; row++) for(row = 0; row < NUM_ROWS; row++) a[row][col] = 0; int *p; for(p = &a[0][0]; p <= &a[NUM_ROWS 1][NUM_COLS]; p++) *p = 0;