# Pointers and Arrays in C

By: Priya Emailed: 1769 times Printed: 2515 times

 By: rohit kumar - how this program is work By: Kirti - Hi..thx for the hadoop in By: Spijker - I have altered the code a By: ali mohammed - why we use the java in ne By: ali mohammed - why we use the java in ne By: mizhelle - when I exported the data By: raul - no output as well, i'm ge By: Rajesh - thanx very much... By: Suindu De - Suppose we are executing

In C, there is a strong relationship between pointers and arrays, strong enough that pointers and arrays should be discussed simultaneously. Any operation that can be achieved by array subscripting can also be done with pointers. The pointer version will in general be faster but, at least to the uninitiated, somewhat harder to understand.

The declaration

```   int a[10];
```
defines an array of size 10, that is, a block of 10 consecutive objects named a[0], a[1], ...,a[9].

The notation a[i] refers to the i-th element of the array. If pa is a pointer to an integer, declared as

```   int *pa;
```
then the assignment
```   pa = &a[0];
```
sets pa to point to element zero of a; that is, pa contains the address of a[0].

Now the assignment

```   x = *pa;
```
will copy the contents of a[0] into x.

If pa points to a particular element of an array, then by definition pa+1 points to the next element, pa+i points i elements after pa, and pa-i points i elements before. Thus, if pa points to a[0],

```   *(pa+1)
```
refers to the contents of a[1], pa+i is the address of a[i], and *(pa+i) is the contents of a[i].

These remarks are true regardless of the type or size of the variables in the array a. The meaning of ``adding 1 to a pointer,'' and by extension, all pointer arithmetic, is that pa+1 points to the next object, and pa+i points to the i-th object beyond pa.

The correspondence between indexing and pointer arithmetic is very close. By definition, the value of a variable or expression of type array is the address of element zero of the array. Thus after the assignment

```   pa = &a[0];
```
pa and a have identical values. Since the name of an array is a synonym for the location of the initial element, the assignment pa=&a[0] can also be written as
```   pa = a;
```
Rather more surprising, at first sight, is the fact that a reference to a[i] can also be written as *(a+i). In evaluating a[i], C converts it to *(a+i) immediately; the two forms are equivalent. Applying the operator & to both parts of this equivalence, it follows that &a[i] and a+i are also identical: a+i is the address of the i-th element beyond a. As the other side of this coin, if pa is a pointer, expressions might use it with a subscript; pa[i] is identical to *(pa+i). In short, an array-and-index expression is equivalent to one written as a pointer and offset.

There is one difference between an array name and a pointer that must be kept in mind. A pointer is a variable, so pa=a and pa++ are legal. But an array name is not a variable; constructions like a=pa and a++ are illegal.

When an array name is passed to a function, what is passed is the location of the initial element. Within the called function, this argument is a local variable, and so an array name parameter is a pointer, that is, a variable containing an address. We can use this fact to write another version of strlen, which computes the length of a string.

```   /* strlen:  return length of string s */
int strlen(char *s)
{
int n;

for (n = 0; *s != '\0', s++)
n++;
return n;
}
```
Since s is a pointer, incrementing it is perfectly legal; s++ has no effect on the character string in the function that called strlen, but merely increments strlen's private copy of the pointer. That means that calls like
```   strlen("hello, world");   /* string constant */
strlen(array);            /* char array[100]; */
strlen(ptr);              /* char *ptr; */
```
all work.

As formal parameters in a function definition,

```   char s[];
```
and
```   char *s;
```
are equivalent; we prefer the latter because it says more explicitly that the variable is a pointer. When an array name is passed to a function, the function can at its convenience believe that it has been handed either an array or a pointer, and manipulate it accordingly. It can even use both notations if it seems appropriate and clear.

It is possible to pass part of an array to a function, by passing a pointer to the beginning of the subarray. For example, if a is an array,

```   f(&a[2])
```
and
```   f(a+2)
```
both pass to the function f the address of the subarray that starts at a[2]. Within f, the parameter declaration can read
```   f(int arr[]) { ... }
```
or
```   f(int *arr) { ... }
```
So as far as f is concerned, the fact that the parameter refers to part of a larger array is of no consequence.

If one is sure that the elements exist, it is also possible to index backwards in an array; p[-1], p[-2], and so on are syntactically legal, and refer to the elements that immediately precede p[0]. Of course, it is illegal to refer to objects that are not within the array bounds.

 Sponsored Links If this tutorial doesn't answer your question, or you have a specific question, just ask an expert here. Post your question to get a direct answer.

Be the first one to add a comment

Your email(required, will not be shown to the public):

Your sites URL (optional):

More Tutorials by Priya
 Getting started with ASP EJB Entity Beans Error Handling - Stderr and Exit - sample program in C Pointers and Arrays in C Data Types and Sizes in C virtual inheritance example in C++ ctime() sample program in C++ TextBox sample program in J2ME Advantages of Java Beans list() contents of a Directory - sample program in Java File example program in Java continue and break statements in C++ Use of 'throw' in Java Passing parameters to a function by value in C++ Demonstrating global and local variables in C++

More Tutorials in C
 Sum of the elements of an array in C Printing a simple histogram in C Sorting an integer array in C Find square and square root for a given number in C Simple arithmetic calculations in C Command-line arguments in C Calculator in C Passing double value to a function in C Passing pointer to a function in C Infix to Prefix And Postfix in C while, do while and for loops in C Unicode and UTF-8 in C Formatting with printf in C if, if...else and switch statements in C with samples Statements in C

More Latest News
Most Viewed Articles (in C )
 Character Pointers and Functions in C Symbolic Constants using #define in C Assignment Operators and Expressions in C Pointers and Arrays in C Printing a simple histogram in C The Birth and history of C Programming Language Basics of C Getting Started with C Variables and Arithmetic Expressions in C The for statement in C Functions in C Relational and Logical Operators in C Type Conversions in C (String to Integer, isdigit() etc) Multi-dimensional Arrays in C (Explained using date conversion program) Basics of Structures in C
Most Emailed Articles (in C)
 Increment and Decrement Operators in C Pointer Arrays and Pointers to Pointers in C union example program in C Getting Started with C Word Counting sample program in C Arguments - Call by Value in C External Variables and Scope in C Constants and escape sequences in C Declarations in C Type Conversions in C (String to Integer, isdigit() etc) The if else statement in C Do while Loops in C goto and labels in C File Inclusion in C #if, #elif, #ifndef, #ifdef in C (Conditional Inclusion)