Pointers
CSE 109
Prepared By : Mashiat Mustaq
Edited By: Md. Nurul Muttakin
Introduction to Pointers
● A variable that holds the memory address of another object
● Example :
○ type *var_name;
○ Here type/base type specified the type of the object the
pointer can point to
○ Such as: int *p; here p is a pointer to an integer
Introduction to Pointers
Pointer Operators
● & : returns the address of the variable it precedes
● * : returns the value stored at the address it precedes
int *p, q;
q = 199;
p = &q;
printf("%d", *p);
Introduction to Pointers : Indirection
When a variable’s value is referenced through a pointer
int *p, q;
p = &q;
*p = 199; Indirection
printf("%d", q);
printf(“%p”, p);
Introduction to Pointers
Why base type is important?
→ Compiler determines the number of bytes to copy when an
indirect assignment is made based on base type
int q;
double *fp; Problem?
fp = &q;
*fp = 100.23;
Introduction to Pointers
Why base type is important?
→ Compiler determines the number of bytes to copy when an
indirect assignment is made based on base type
int q;
double *fp; Problem?
fp = &q; Overwrites the adjacent additional
*fp = 100.23; 4 bytes to q
Introduction to Pointers
Why should we initialize a pointer as NULL always?
→ If we access an unassigned pointer, it points to a garbage
value and it is dangerous.
int *p;
*p = 10; Unassigned pointer,
program will crash
Example
What will happen with the following code?
int *p;
double q, temp;
temp = 1234.34;
p = &temp;
q = *p;
printf("%f", q);
Pointer Operators
● Arithmetic operators : +, ++, -, --
● Only integer quantities can be added / subtracted
● Performs relative to the base type
int *p;
p = 200 (address)
p++;
p = 204 (address)
Pointer Operators
● Arithmetic operators : +, ++, -, --
● Only integer quantities can be added / subtracted
● Performs relative to the base type
int *p;
P points to the 200th integer
p = p + 200; past the one to which
previously pointed
Pointer Operators
● Difference between *p++ and (*p)++ ?
Pointer Operators
● Difference between *p++ and (*p)++ ?
● *p++ first increments p then obtains the value at the new
location
● (*p)++ increments the value through indirection
Pointer Operators
char *cp, ch;
int *ip, i;
float *fp, f;
double *dp, d;
cp = &ch; ip = &i; fp = &f; dp = &d;
printf("%p %p %p %p\n", cp, ip, fp, dp);
cp++; ip++; fp++; dp++;
printf("%p %p %p %p\n", cp, ip, fp, dp);
Pointer with Arrays
● Array name without index → pointer to the start of the array
● Function only accepts pointers for arrays
int a[5] = {10, 20, 30, 40, 50};
int *p;
p = a;
printf( "%d %d %d\n" , *p, *(p+1), *(p+2));
printf("%d %d %d\n" , a[0], a[1], a[2]);
Pointer with Arrays
● Multidimensional array
● float balance[10][5] → how to access balance[3][1] using pointer?
Pointer with Arrays
● Multidimensional array
● float balance[10][5] → how to access balance[3][1] using pointer?
float *p;
p = (float *) balance; p = balance might
not work in all
*(p + (3 * 5) + 1) compilers as p
might point to the
first row and p+1
will point to the
second row
Pointer with Arrays : Indexing
char str[] = "Pointers are fun";
char *p;
p = str;
for(int i = 0; p[i]; i++)
printf("%c", p[i]);
Pointer with Arrays : Problems
● Print a string in reverse order using pointers
Arrays of pointers
Declaration : type *array_name[size];
int var[] = {10, 100, 200};
int i, *ptr[3];
for ( i = 0; i < 3; i++) {
ptr[i] = &var[i]; /* assign the address of integer. */
}
for ( i = 0; i < 3; i++) {
printf("Value of var[%d] = %d\n", i, *ptr[i] );
}
Arrays of pointers
int i, j; printf("Using Pointer:");
int M = 3, N = 4; for ( i = 0; i < M * N; i++) {
int *p; if ( i%N == 0) printf("\n");
int a[3][4] = {1, 2, 4, 5, 3, 0, 6, 8, 7, 9, 11, 10}; printf("%d ", *(p+i));
}
printf("Using Array:\n"); printf("\n");
for ( i = 0; i < M; i++) {
for ( j =0; j < N; j++ ) { p = a[0];
printf("%d ", a[i][j]); printf("Using Pointer2:\n");
} for ( i = 0; i < M; i++) {
printf("\n"); for ( j =0; j < N; j++ ) {
} printf("%d ", * ( p + N * i + j ) );
//p = a; // not recommended }
// p = (int *) a ; // better than p = a printf("\n");
// p = a[0]; }
p = &a[0][0];
Pointer to an array VS array of pointers
● int* arr[3]
○ arr is an array of 3 pointers to integers.
○ Each element of arr is a pointer that can point to an integer.
○ It's often used when you need an array where each element is a
pointer, and each pointer can point to a different memory location.
● int (*arr)[3]
○ arr is a pointer that can point to the beginning of an array of 3
integers.
○ It's often used to represent a 2D array, and pointer arithmetic can be
used to navigate through rows and columns.
Pointer to an array example
int a[2][3] = {
{1, 2, 3},
{4, 5, 6}
};
int (*p)[3]; // Declare a pointer to an array of 3 integers
p = a; // This assigns the address of the first row of the array to p
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 3; j++) {
printf("%d ", *(*(p + i) + j));
}
printf("\n");
}
Pointer with Strings
● Variable length strings
● A char pointer indicates to the address of the string
in string table of the program.
char *p = "one two three";
printf(p);
Arrays of pointers
Declaration : type *array_name[size];
// Declaring an array of pointers to characters
char *names[] = {"John", "Jane", "Doe"};
// Accessing and printing the second name
printf("%s\n", names[1]); // Outputs: Jane
// names[1][1] → a
Arrays of pointers
There are 10 students in a class. Each student has a
roll number (starting from 1) and a name. You need to
build a program that outputs the name of each student
if the given input is the roll number using array of
pointers.
Passing pointer to a function
double getAverage(int *arr, int size);
double getAverage(int *arr, int size) {
int main () {
int i, sum = 0;
double avg;
/* an int array with 5 elements */
int balance[5] = {1000, 2, 3, 17, 50};
for (i = 0; i < size; ++i) {
double avg;
sum += arr[i];
}
/* pass pointer to the array as an
argument */
avg = (double)sum / size;
avg = getAverage( balance, 5 ) ;
return avg;
}
/* output the returned value */
printf("Average value is: %f\n", avg );
return 0;
}
Passing pointer to a function
int main() {
void sort(int *a, int n){
int *a, i, j, n;
// Sorting the array using pointer // Input the number of elements to store in the
int tmp; array
for (int i = 0; i < n; i++) { scanf("%d", &n);
for (int j = i + 1; j < n; j++) { // Input elements into the array
if (*(a + i) > *(a + j)) { for (i = 0; i < n; i++) {
// Swap elements if they are in the printf(" element - %d : ", i + 1);
wrong order scanf("%d", a + i);
}
tmp = *(a + i);
*(a + i) = *(a + j); sort(a, n);
*(a + j) = tmp;
} // Displaying the sorted array elements
} for (i = 0; i < n; i++) {
printf(" element - %d : %d \n", i + 1, *(a + i));
}
}
} }
Passing pointer to a function
int main() {
// Function to process a 2D array
int a[2][3] = {
void processArray(int (*arr)[3], int rows,
{1, 2, 3},
int cols) { {4, 5, 6}
for (int i = 0; i < rows; i++) { };
for (int j = 0; j < cols; j++) {
printf("%d ", arr[i][j]); // Pass the 2D array to the function
} processArray(a, 2, 3);
for (int i = 0; i < 2; i++) {
printf("\n");
for (int j = 0; j < 3; j++) {
}
printf("%d ", a[i][j]);
}
arr[1][1] = 11; printf("\n");
} }
}
Return pointer from a function
int* fun()
{
static int A = 10;
return (&A);
}
int main()
{
int* p;
p = fun();
printf( "%p\n" , p);
printf( "%d\n" , *p);
return 0;
}
Return pointer from a function
int* fun()
{
If we used ‘int A’ (local
static int A = 10;
return (&A); variable) inside the
} function, it would have
gone outside scope
once the function exits
int main()
→ Segmentation fault
{
int* p;
p = fun();
printf( "%p\n" , p);
printf( "%d\n" , *p);
return 0;
}
Pointer to Pointer
Pointer to
Pointer Variable
pointer
char **mp, *p, ch;
p = &ch;
mp = &p;
**mp = 'A';
Pointer to Structure
struct Point {
int x;
int y;
};
int main() {
struct Point p1 = {10, 20};
struct Point *ptr = &p1;
printf("x = %d, y = %d\n", ptr->x, ptr->y);
ptr->x = 30;
ptr->y = 40;
printf("Updated x = %d, y = %d\n", ptr->x, ptr->y);
return 0;
}
Pointer to Function
A pointer to a function in C is used to store the address of a
function.
- It allows dynamic function calls.
- Useful for callbacks and passing functions as arguments.
- Syntax: `return_type (*pointer_name)(parameter_list);`
Pointer to Function
Example Declaration:
```
int (*funcPtr)(int, int);
```
- `int`: Return type of the function.
- `(*funcPtr)`: Pointer to a function.
- `(int, int)`: Parameters of the function.
Pointer to Function
```
#include <stdio.h>
int add(int a, int b) {
return a + b;
}
int (*funcPtr)(int, int);
int main() {
funcPtr = add;
printf("Sum: %d\n", funcPtr(5, 3));
return 0;
}
```
- Output: `Sum: 8`
- `funcPtr` points to `add` and calls it dynamically.
Resources for study
● https://www.tutorialspoint.com/cprogramming/c_pointers
.htm
● https://www.scaler.com/topics/c/array-of-pointers-in-c/
