Arrays In C

An array is a group of related data items that share a common name or An array is a data structure consisting of a group of elements that are accessed by indexing. In most programming languages each element has the same data type and the array occupies a contiguous area of storage.

#include <stdio.h>
/* count digits, white space, others */
main()
{
int c, i, nwhite, nother;
int ndigit[10];
nwhite = nother = 0;
for (i = 0; i < 10; ++i)
ndigit[i] = 0;
while ((c = getchar()) != EOF)
if (c >= '0' && c <= '9')
++ndigit[c-'0'];
else if (c == ' ' || c == '\n' || c == '\t')
++nwhite;
else
++nother;
printf("digits =");
for (i = 0; i < 10; ++i)
printf(" %d", ndigit[i]);
printf(", white space = %d, other = %d\n",
nwhite, nother);
}

The output of this program on itself is
digits = 9 3 0 0 0 0 0 0 0 1, white space = 123, other = 345
The declaration

int ndigit[10];

declares ndigit to be an array of 10 integers. Array subscripts always start at zero in C, so the elements are ndigit[0], ndigit[1], ..., ndigit[9]. This is reflected in the for loops that initialize and print the array.
A subscript can be any integer expression, which includes integer variables like i, and integer constants.
This particular program relies on the properties of the character representation of the digits. For example, the test

if (c >= '0' && c <= '9')

determines whether the character in c is a digit. If it is, the numeric value of that digit is
c - '0'
This works only if '0', '1', ..., '9' have consecutive increasing values. Fortunately, this is true for all character sets.
By definition, chars are just small integers, so char variables and constants are identical to ints in arithmetic expressions. This is natural and convenient; for example c-'0' is an integer expression with a value between 0 and 9 corresponding to the character '0' to '9' stored in c, and thus a valid subscript for the array ndigit.