An array is a collection of data items, all of the same type, accessed using a common name.
A one-dimensional array is like a list; A two dimensional array is like a table; The C language places no limits on the number of dimensions in an array, though specific implementations may.
Some texts refer to one-dimensional arrays as vectors, two-dimensional arrays as matrices, and use the general term arrays when the number of dimensions is unspecified or unimportant.

Declaring Arrays:

Array variables are declared identically to variables of their data type, except that the variable name is followed by one pair of square [ ] brackets for each dimension of the array.
Uninitialized arrays must have the dimensions of their rows, columns, etc. listed within the square brackets.
Dimensions used when declaring arrays in C must be positive integral constants or constant expressions. ( Space is allocated only once, at the time the array is declared. The array does NOT change sizes later if the variable used to declare it changes. )

Examples:

int i, j, intArray[ 10 ], number;
float floatArray[ 1000 ];
int tableArray[ 3 ][ 5 ]; /* 3 rows by 5 columns

Initializing Arrays:

Arrays may be initialized when they are declared, just as any other variables.
Place the initialization data in curly {} braces following the equals sign. Note the use of commas in the examples below.
An array may be partially initialized, by providing fewer data items than the size of the array. The remaining array elements will be automatically initialized to zero.
If an array is to be completely initialized, the dimension of the array is not required. The compiler will automatically size the array to fit the initialized data.
Examples:

int i = 5, intArray[ 6 ] = { 1, 2, 3, 4, 5, 6 }, k;
float sum = 0.0f, floatArray[ 100 ] = { 1.0f, 5.0f, 20.0f };
double piFractions[ ] = { 3.141592654, 1.570796327, 0.785398163 };

Designated assignment:
This method can be mixed in with traditional iniitalization
For example:

int numbers[ 100 ] = { 1, 2, 3, [10] = 10, 11, 12, [60] = 50, [42] = 420 };

In this example,the first three elements are initialized to 1, 2, and 3 respectively.
Then element 10 ( the 11th element ) is initialized to 10
The next two elements ( 12th and 13th ) are initialized to 11 and 12 respectively.
Element number 60 ( the 61st ) is initialized to 50, and number 42 ( the 43rd ) to 420.
( Note that the designated initializers do not need to appear in order. )
As with traditional methods, all uninitialized values are set to zero.
If the size of the array is not given, then the largest initialized position determines the size of the array.

Using Arrays:

Elements of an array are accessed by specifying the index ( offset ) of the desired element within square [ ] brackets after the array name.
Array subscripts must be of integer type. ( int, long int, char, etc.
Arrays are commonly used in conjunction with loops, in order to perform the same calculations on all ( or some part ) of the data items in the array.

Sample Programs Using 1-D Arrays

The first sample program uses loops and arrays to calculate the first twenty Fibonacci numbers. Fibonacci numbers are used to determine the sample points used in certain optimization methods.

/* Program to calculate the first 20 Fibonacci numbers. */

#include
#include

int main( void ) {

int i, fibonacci[ 20 ];

fibonacci[ 0 ] = 0;
fibonacci[ 1 ] = 1;

for( i = 2; i < 20; i++ )
fibonacci[ i ] = fibonacci[ i - 2 ] + fibonacci[ i - 1 ];

for( i = 0; i < 20; i++ )
printf( "Fibonacci[ %d ] = %f\n", i, fibonacci[ i ] );

} /* End of sample program to calculate Fibonacci numbers */

Exercise: What is the output of the following program:

/* Sample Program Using Arrays */

#include
#include

int main( void ) {

int numbers[ 10 ];
int i, index = 2;

for( i = 0; i < 10; i++ )
numbers[ i ] = i * 10;

numbers[ 8 ] = 25;
numbers[ 5 ] = numbers[ 9 ] / 3;
numbers[ 4 ] += numbers[ 2 ] / numbers[ 1 ];
numbers[ index ] = 5;
++numbers[ index ];
numbers[ numbers[ index++ ] ] = 100;
numbers[ index ] = numbers[ numbers[ index + 1 ] / 7 ]–;

for( index = 0; index < 10; index++ )
printf( "numbers[ %d ] = %d\n" index, numbers[ index ] );

} /* End of second sample program