pointers

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C/C++ Training
Programming C/C++ on Eclipe
Trình bày : Ths HungNM
Pointer and Pointer with Arrays
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Introduction Pointer.
Declaring pointer variable.
Pointer with operator
Pointer Assignment
Pointer as arguments
Pointers to Constants
Array and pointer
Function pointer.
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Introduction Pointer
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Variables that can store addresses are called pointers.
The address that’s stored in a pointer is usually that of
another variable
int number = 5;
int *P = &number;
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Declaring pointer variable
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Using statement:
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Datatype *namePointer;
Example :
int *p ; // pointer to only integers
 double *p ; // pointer to only doubles
 char *p ; // pointer to only characters

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The address and indirection Operators
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The address operators
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Declaring a pointer variable set aside space for pointer.
• int *p;
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Must be init p before we use it.
• p = &i;
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The address and indirection Operators
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The indirection Operators.
Use the *(indirection) operator to access what’s stored
in the object.
 Example.
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int i = 5;
int *p = &i;
printf(“%d”,*p);
printf will be display value of i, not address of i.
As long as p point to i.
 *p is an alias for i.

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Pointer Assignment
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Use of the assignment operator to copy pointers.
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int i, *p, *q;
p = &i;
• The address of i is copied into p;
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q = p;
• This statement copies the content of p (address of i) into q
• *p = 1;
• *q = 2;
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Pointer Assignment
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Example 1
p = &i;
 q = &j;
 i = 1;
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Example 2
*p = *q;
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Copy value that p pointers to (the value i) into the
object that q point to (the variable j)
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Using a pointer with scanf()
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Example.
int value = 0;
 int *pvalue = NULL;
 pvalue = &value; /* Set pointer to refer to value */
 printf ("Input an integer: ");
 scanf(" %d", pvalue); /* Read into value via the pointer */
 printf("\nYou entered %d\n", value);
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Pointer as arguments
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Using a pointer use to argument for function.
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Example.
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Prototype of decompose
• void decompose(double, long *, double *)
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Call decompose
• decompose(3.14159, &i, &d);
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Pointers to Constants
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Use the const keyword when you declare a pointer to
indicate that the value pointed to must not be changed.
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long value = 9999L;
const long *pvalue = &value; /* Defines a pointer to a constant */
*pvalue = 8888L; /* Error - attempt to change const location */
You have only asserted that what pvalue points to must
not be changed.
You are quite free to do what you want with value:
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value = 7777L;
long number = 8888L;
pvalue = &number; /* OK - changing the address in pvalue */
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Constant Pointers
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The address stored in a pointer cannot be changed.
using the const keyword slightly differently in the
declaration of the pointer
int count = 43;
 int *const pcount = &count; /* Defines a constant */
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Can’t changed address of a pcount.
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int item = 34;
pcount = &item; /* Error - attempt to change a constant pointer */
Can be change the value that pcount points
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*pcount = 345; /* OK - changes the value of count */
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Arrays and Pointers
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Pointer Arithmetic
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Use pointer can point to array element.
• int a[10], *p;
• p = &a[0];
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Use can store 5 into a[0] by writing.
• *p = 5;
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Arrays and Pointers
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Adding an integer value to a pointer
Example :
 int a[10], *p; *q;
 p = &a[2];
 q = p + 3;
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p += 6;
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Arrays and Pointers
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Subtructing an integer from a pointer.
If p pointer to the array element a[i], then p-j pointer
to a[i-j];
 Example.
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• int *p = &a[8];
• q = p-3;
• p -= 6;
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Example Pointer.
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Arrays and Pointers
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Pointer to compound literals
A pointer to point to an element with in an array
created by compound literals.
 Example.
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• int *p = (int []){3,0,3,4,1};
• p point to first element of five element array {3,0,3,4,1}.
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Other way:
• int a[] = {3,0,3,4,1};
• p = &a[0];
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Using pointers for array processing
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Combining the * and ++ Operators
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Using combine the * (inderection) and ++ operators in
statement in process array elements.
• a[i++] = j;
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Using p is pointing to an array element.
• *(p++) = j;
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Notes :
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Using pointers for array processing
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Combining the * and ++ Operators
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Example :
• Total array with using pointer.
– int a[] = {3,5,7,10,23};
– int *p;
» Using for
for(p=&a[0]; p<&a[N]; p++)
{
sum += *p;
}
Using while
p = &a[0];
while (p<&a[N])
sum += *p++;
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Using array name as a pointer
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The name of an array can be use as a pointer to
the first element in the array.
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Example :
• int a[10]; *a = 7; /* store 7 in a[0] */.
• *(a+1) = 12 /* store 12 in a[1]*/;
• *(a+n) = k /* store k in a[n] */
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Using for, while to view data with array.
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Example :
• for(p = &a[0]; p<&a[n]; p++)
{
sum +=*p;
}
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Example Using pointer with array
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Reversing a series of numbers.
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Pointer with multidimensional array.
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Processing the elements of multidimension arrays.
#define NUM_ROWS 10;
# define NUM_COLS 10;
int a[NUM_ROWS][NUM_COLS];
int row, col;
for(int row=0; row< NUM_ROWS ; row++)
for(int col=0; col < NUM_ROWS ; col++)
a[row][col] = 0;
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Pointer with multidimensional array.
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Using pointer with multidimensional array.
#define NUM_ROWS 10;
# define NUM_COLS 10;
int a[NUM_ROWS][NUM_COLS];
int *p;
for(p=&a[0][0]; p< &a[NUM_ROWS-1][NUM_COLS]; p++)
*p= 0;
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Processing the rows of a multidimensional array
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Example.
Using pointer with multidimensional array.
#define NUM_ROWS 10;
# define NUM_COLS 10;
int a[NUM_ROWS][NUM_COLS];
int *p, i;
for(i = 0; i<NUM_ROWS; i++)
for(p=a[i]; p<a[i] + NUM_COLS; p++)
*p= 0;
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Processing the colum of a multidimensional array
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Example.
#define NUM_ROWS 10;
# define NUM_COLS 10;
int a[NUM_ROWS][NUM_COLS];
int (*p)[NUM_COLS], i;
for(i = 0; NUM_COLS; i++)
for(p=&a[0]; p<&a[NUM_ROWS]; p++)
(*p)[i] = 0
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Using the name of multidimension array as a pointer
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Example.
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Not using array name.
• for(p=&a[0]; p<&a[NUM_ROWS]; p++)
(*p)[i] = 0;
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Using array name.
• for(p = a; p<a + NUM_ROWS; p++)
– (*p)[i] = 0;
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Example Multidimentional array with pointer
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Function Pointer
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Declaration function Pointer.
Datatype (*name function)();
 Example :
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• int
(*cmp)(); // function pointer has integer type
• float (*fcmp)(); // function pointer has float type.
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Example function pointer.
#include <stdio.h>
int add( int a, int b) { return(a+b);}
int subtruct( int a, int b) { return(a-b);}
int multiable( int a, int b) { return(a*b);}
int modul( int a, int b) { return(a%b);}
void main(void) {
int (*cmp)(); /* function pointer has int type*/
int a, b;
printf(“\n Input a=”); scanf(“%d”, &a);
printf(“\n Input b=”); scanf(“%d”, &b);
cmp=add; // Using function pointer
printf(“\n Addition a + b =%d”, cmp(a,b));
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Example function pointer.
cmp = subtruct;
printf(“\n Subtruction a - b =%d”, cmp(a,b));
cmp = multiable;
printf(“\n Multiable a * b =%d”, cmp(a,b));
cmp = modul;
printf(“\n modul(a, b) =%d”, cmp(a,b));
getch();
}
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End
• Thank You
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