C programming---Pointers

1. 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: Each byte has a unique address in memory.

2. Address Address Contents 0 1 2 3 n-1

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

4. 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

5. 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.

6. The Address and Indirection Operators • int i, *p; • ……. • p = &i; • int i; • int *p = &i; • int i, p = &i;

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

8. 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);

9. 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

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

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

12. Pointer as Arguments Example 2.c

13. 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” }

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

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

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

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

18. 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)

19. 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;

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

21. 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

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

23. 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];

24. 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;