Programming in C

112 Chapter 7 Working with Arrays
As an example of the const attribute, the line
const double pi = 3.141592654;
declares the const variable pi.This tells the compiler that this variable will not be modified
by the program. If you subsequently wrote a line like this in your program:
pi = pi / 2;
the gcc compiler would give you a warning message similar to this:
foo.c:16: warning: assignment of read-only variable 'pi'
Returning to Program 7.7, the character array baseDigits is set up to contain the 16
possible digits that will be displayed for the converted number. It is declared as a const
array because its contents will not be changed after it is initialized. Note that this fact
also aids in the program’s readability.
The array convertedNumber is defined to contain a maximum of 64 digits, which
holds the results of converting the largest possible long integer to the smallest possible
base (base 2) on just about all machines.The variable numberToConvert is defined to be
of type long int so that relatively large numbers can be converted if desired. Finally, the
variables base (to contain the desired conversion base) and index (to index into the
convertedNumber array) are both defined to be of type int.
After the user keys in the values of the number to be converted and the base—note
that the scanf call to read in a long integer value takes the format characters %ld—the
program then enters a do loop to perform the conversion.The do was chosen so that at
least one digit appears in the convertedNumber array even if the user keys in the number
zero to be converted.
Inside the loop, the numberToConvert modulo the base is computed to determine
the next digit.This digit is stored inside the convertedNumber array, and the index in
the array is incremented by 1. After dividing the numberToConvert by the base, the conditions
of the do loop are checked. If the value of numberToConvert is 0, the loop terminates;
otherwise, the loop is repeated to determine the next digit of the converted
number.
When the do loop is complete, the value of the variable index is the number of digits
in the converted number. Because this variable is incremented one time too many inside
the do loop, its value is initially decremented by 1 in the for loop.The purpose of this
for loop is to display the converted number at the terminal.The for loop sequences
through the convertedNumber array in reverse sequence to display the digits in the correct
order.
Each digit from the convertedNumber array is in turn assigned to the variable
nextDigit.For the numbers 10 through 15 to be correctly displayed using the letters A
through F, a lookup is then made inside the array baseDigits, using the value of
nextDigit as the index. For the digits 0 through 9, the corresponding location in the
array baseDigits contains nothing more than the characters '0' through '9' (which as
you recall are distinct from the integers 0 through 9). Locations 10 through 15 of the
array contain the characters 'A' through 'F'.So, if the value of nextDigit is 10, for