in most contexts in later use, a mention of the variable array is converted to a pointer to the...

8/9/2019 In Most Contexts in Later Use, A Mention of the Variable Array is Converted to a Pointer to the First Item in the Arra

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which defines an array named array to hold 100 values of the primitive type int. If declared within a

function, the array dimension may also be a non-constant expression, in which case memory for the

specified number of elements will be allocated. In most contexts in later use, a mention of the variable

array is converted to a pointer to the first item in the array. The sizeof operator is an exception: sizeofarray yields the size of the entire array (that is, 100 times the size of an int). Another exception is the &

(address-of) operator, which yields a pointer to the entire array (e.g. int (*ptr_to_array)[100] =

&array;).[edit] Accessing elements

The primary facility for accessing the values of the elements of an array is the array subscript operator.To access the i-indexed element of array, the syntax would be array[i], which refers to the value stored

in that array element.

Array subscript numbering begins at 0. The largest allowed array subscript is therefore equal to thenumber of elements in the array minus 1. To illustrate this, consider an array a declared as having 10

elements; the first element would be a[0] and the last element would be a[9]. C provides no facility for

automatic bounds checking for array usage. Though logically the last subscript in an array of 10elements would be 9, subscripts 10, 11, and so forth could accidentally be specified, with undefined

results.

Due to arraypointer interchangeability, the addresses of each of the array elements can be expressed

in equivalent pointer arithmetic. The following table illustrates both methods for the existing array:

Array subscripts vs. pointer arithmetic Element index 1 2 3 nArray subscript array[0] array[1] array[2] array[n-1]

Dereferenced pointer *array *(array + 1) *(array + 2) *(array + n-1)

Similarly, since the expression a[i] is semantically equivalent to *(a+i), which in turn is equivalent to*(i+a), the expression can also be written as i[a] (although this form is rarely used).

[edit] Dynamic arrays

A constant value is required for the dimension in a declaration of a static array. A desired feature is the

ability to set the length of an array dynamically at run-time instead:

int n = ...;

int a[n];

a[3] = 10;


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This behavior can be simulated with the help of the C standard library. The malloc function provides asimple method for allocating memory. It takes one parameter: the amount of memory to allocate in

bytes. Upon successful allocation, malloc returns a generic (void *) pointer value, pointing to the

beginning of the allocated space. The pointer value returned is converted to an appropriate typeimplicitly by assignment. If the allocation could not be completed, malloc returns a null pointer. The

following segment is therefore similar in function to the above desired declaration:

#include /* declares malloc */

int *a;

a = malloc(n * sizeof(int));a[3] = 10;

The result is a "pointer to int" variable (a) that points to the first of n contiguous int objects; due toarraypointer equivalence this can be used in place of an actual array name, as shown in the last line.

The advantage in using this dynamic allocation is that the amount of memory that is allocated to it can

be limited to what is actually needed at run time, and this can be changed as needed (using the standardlibrary function realloc).

When the dynamically-allocated memory is no longer needed, it should be released back to the run-

time system. This is done with a call to the free function. It takes a single parameter: a pointer topreviously allocated memory. This is the value that was returned by a previous call to malloc. It is


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considered good practice to then set the pointer variable to NULL so that further attempts to access the

memory to which it points will fail. If this is not done, the variable becomes a dangling pointer, and

such errors in the code (or manipulations by an attacker) might be very hard to detect and lead to

obscure and potentially dangerous malfunction caused by memory corruption.

free(a);

a = NULL;

Standard C-99 also supports variable-length arrays (VLAs) within block scope. Such array variables

are allocated based on the value of an integer value at runtime upon entry to a block, and aredeallocated at the end of the block.

float read_and_process(int sz)

{float vals[sz]; // VLA, size determined at runtime

for (int i = 0; i < sz; i++)vals[i] = read_value();

return process(vals, sz);

}

[edit] Multidimensional arrays


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In addition, C supports arrays of multiple dimensions, which are stored in row-major order.

Technically, C multidimensional arrays are just one-dimensional arrays whose elements are arrays. The

syntax for declaring multidimensional arrays is as follows:

int array2d[ROWS][COLUMNS];


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(where ROWS and COLUMNS are constants); this defines a two-dimensional array. Reading the

subscripts from left to right, array2d is an array of length ROWS, each element of which is an array of

COLUMNS ints.

To access an integer element in this multidimensional array, one would use

array2d[4][3]

Again, reading from left to right, this accesses the 5th row, 4th element in that row (array2d[4] is an

array, which we are then subscripting with the [3] to access the fourth integer).

Higher-dimensional arrays can be declared in a similar manner.

A multidimensional array should not be confused with an array of references to arrays (also known as

Iliffe vectors or sometimes array of arrays). The former is always rectangular (all subarrays must be the

same size), and occupies a contiguous region of memory. The latter is a one-dimensional array ofpointers, each of which may point to the first element of a subarray in a different place in memory, and

the sub-arrays do not have to be the same size. The latter can be created by multiple use of malloc.

[edit] Strings

In C, string literals (constants) are surrounded by double quotes ("), e.g. "Hello world!" and are

compiled to an array of the specified char values with an additional null terminating character (0-

valued) code to mark the end of the string.

String literals may not contain embedded newlines; this proscription somewhat simplifies parsing of

the language. To include a newline in a string, the backslash escape \n may be used, as below.

There are several standard library functions for operating with string data (not necessarily constant)organized as array of char using this null-terminated format; see below.

C's string-literal syntax has been very influential, and has made its way into many other languages,such as C++, Perl, Python, PHP, Java, Javascript, C#, Ruby. Nowadays, almost all new languages adoptor build upon C-style string syntax. Languages that lack this syntax tend to precede C.

[edit] Backslash escapes

If you wish to include a double quote inside the string, that can be done by escaping it with a backslash

(\), for example, "This string contains \"double quotes\".". To insert a literal backslash, one must double

it, e.g. "A backslash looks like this: \\".


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Backslashes may be used to enter control characters, etc., into a string:

Escape Meaning

\\ Literal backslash

\" Double quote\' Single quote

\n Newline (line feed)

\r Carriage return\b Backspace

\t Horizontal tab

\f Form feed\a Alert (bell)

\v Vertical tab

\? Question mark (used to escape trigraphs)

\nnn Character with octal value nnn\xhh Character with hexadecimal value hh


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