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 1

Each byte has a unique address in memory.

Address

00110011

10101010

00000111

11111000

10000100

Address Contents

0

1

2

3

n-1

Addressint i = 9; /* suppose sizeof(int) = 4The address of variable i is 0xFA83

0xFA83

0xFA84

0xFA85

0xFA86

Pointer VariablesUse a pointer variable p to store the address of a variable i, and we say p “points to” i

0xFA83

0xFA83

0xFA84

0xFA85

0xFA86

p

Declaring Pointer Variables

int *p; // p is a pointer variable capable of pointing to objects of type intchar *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

Pointer as Arguments

Example 2.c

Using const to Protect ArgumentsWhen 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 Valuesint *max(int *a, int *b){ if(*a > *b) return a; else return b;}

int *p = max(&a, &b);

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

Pointers and ArraysPointer 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;