char Constants

    A char constant is written with single quotes (') like 'A' or 'z'. The char constant 'A' is really just a synonym for the ordinary integer value 65 which is the ASCII value for uppercase 'A'. There are special case char constants, such as '\t' for tab, for characters which are not convenient to type on a keyboard.

int Constants

    Numbers in the source code such as 234 default to type int. They may be followed by an ‘L’ (upper or lower case) to designate that the constant should be a long such as 42L. An integer constant can be written with a leading 0x to indicate that it is expressed in hexadecimal -- 0x10 is way of expressing the number 16. Similarly, a constant may be written in octal by preceding it with “0” -- 012 is a way of expressing the number 10.

Floating point Constants

    Constants in the source code such as 3.14 default to type double unless the are suffixed with an ‘f’ (float) or ‘l’ (long double). Single precision equates to about 6 digits of precision and double is about 15 digits of precision. Most C programs use double for their computations. The main reason to use float is to save memory if many numbers need to be stored. The main thing to remember about floating point numbers is that they are inexact. For example, what is the value of the following double expression?

    (1.0/3.0 + 1.0/3.0 + 1.0/3.0)    // is this equal to 1.0 exactly?

    The sum may or may not be 1.0 exactly, and it may vary from one type of machine to another. For this reason, you should never compare floating numbers to each other for equality (==) -- use inequality (<) comparisons instead. Realize that a correct C program run on different computers may produce slightly different outputs in the rightmost digits of its floating point computations.