#### A.5.1 Computing the Width of an Integer Data Type

The most common reason that a program needs to know how many bits are in an integer type is for using an array of `long int` as a bit vector. You can access the bit at index n with

         vector[n / LONGBITS] & (1 << (n % LONGBITS))

provided you define `LONGBITS` as the number of bits in a `long int`.

There is no operator in the C language that can give you the number of bits in an integer data type. But you can compute it from the macro `CHAR_BIT`, defined in the header file `limits.h`.

`CHAR_BIT`
  This is the number of bits in a `char`—eight, on most systems. The value has type `int`.

  You can compute the number of bits in any data type type like this:

                sizeof (type) * CHAR_BIT