1. Introduction to C Programming CE00312-1
Lecture 16
Pointers in C

2. Pointers Pointers are necessary for understanding and use of:
Passing Parameters by Reference
Arrays (especially strings)
Dynamic Allocation of Memory
Dynamic Data Structures
A Pointer is a reference to (or address of) a memory location.

3. 1) Passing Parameters by Reference
Function swap exchanges the values of its 2 parameters:
#include “stdio.h”
void swap(int *, int *); // prototype
int main(void) {
int a=3, b=7;
swap(&a, &b);
// addresses of values to be swapped
printf(“1st is %d, 2nd is %d”, a, b);
return 0; }

4. Function swap void swap(int *x, int *y)
// x, y are copies of the addresses a, b {
int temp;
temp = *x; // using pointers to access
*x = *y; // originals (not copies)
*y = temp; // and swap them via temp }

5. 2) Arrays and Strings Consider the declaration:
char x[10]; // a string of size 10
x[3] is the character in element 3 of array x
&x[3] is the address of element 3 of array x
x is the address of array x
(actually the same as &x[0])
*x is the character that x points to
(ie same as x[0])

6. Strings as pointers
Strings should always be declared as arrays inside the main function, so that required storage can be allocated.
Thus char x[10] allocates 10 bytes.
Strings can be declared explicitly as pointers
– typically inside functions,
e.g. char *s;
declares variable, s, as a pointer to a character –
(ie string pointer).
It can point to any string –or any part of a string –
or to examine all the characters within a string.

7. Arrays as parameters
Arrays and strings, such as x above, passed to functions as parameters are already addresses.
Example: int list [100];
sort(list, n);
The address of array, list, is passed to the sort function.
void sort(int a[], int n)
So that array, a, also refers to the original elements of list.

8. Example of a String Function
The function, print_reverse,
given a string (as a parameter)
prints the string in reverse
by dealing with one character at a time, using a pointer
starting with the last and continuing while it has not yet
reached the start of the string.

9. Main function Main function reads, reverse and writes a string.
#include “stdio.h”
int main(void) {
char line[81] ; // for string storage
gets(line); // whole line is the string
printf(“Reversed line is\n”);
print_reverse(line); // line is address
return 0; }

10. print_reverse function
void print_reverse(char *s) //s is a string {
char *ptr;
ptr = s + strlen(s) - 1; // last char
while ( ptr != s) // not start of string
printf(“%c”, *ptr); // the character
ptr--; // previous char }
printf(“%c”, *ptr);// ptr == s
// first character of s