C Programming Part 2: Data Types, Variables, and Constants

How are data stored in C?

• Data are stored as variables or constants.
• Variables are just memory locations that hold specified values/data.
  
• Variables and constants are assigned a data type during creation.
• Data types limit the possible values the variable/constant can have.
• There are data type modifiers which will further limit or increase the range of values.
• The modifiers are short, long, signed, and unsigned. They are placed before the data type.
• Examples: short int, unsigned char, long float.
• Some data type modifiers can also be used as data types. They just act like int.

C Data Types

Type	Description	Bits	Range
char	characters	8	-128 to 127
unsigned char	small unsigned int	8	0 to 255
int	integers	32	-2,147,483,648 to +2,147,483,647
unsigned int	unsigned integers	32	0 to 4,294,467,295
short	small integers	16	-32,768 to 32,767
unsigned short	unsigned small integers	16	0 to 65,535
long	integers	32	-2,147,483,648 to +2,147,483,647
unsigned long	unsigned integers	32	0 to 4,294,467,295
float	floating point #s	32
double	floating point #s (greater precision)	64
long double	floating point #s (even greater precision)	96

Variables

• The format of declaring a variable is: data_type variable_name; (i.e. int one;)
  
• The first character of a variable name MUST be a letter but the rest can be a letters, numbers or underscores(_) .
• Variables are case sensitive.
• The maximum length of the name will depend on the compiler. Although, there is a C standard that states that the initial 63 characters are significant for an internal identifier (name used for variables, types, or labels). The standard for an external identifier is that the first 31 characters are significant.
• It is a good practice to initialize (assign it a base value) a variable when you first declare it. It is known as defining a variable when you do both the initialization and declaration. The main small difference between the two is that defining it means that memory space is taken up. In a declaration, no memory is used and only the data type is mentioned.
  
• Ex. Declaration: int one; | Definition: int one = 1;
• Variables are considered local, global, or external depending on where they are defined and used. This will be discussed more in a later section.
• It is possible to declare all variables on one line (i.e. int one, two, three;). Generally, it is a better practice to declare or define them on separate lines.
  

Constants

• Constants are data that will not be modified.
• Constants are generally defined near the very top of the code and its name is all capitalized.
• They do not take up any variable storage space(memory).
• The format for defining a constant is: #define CONSTANT_NAME constant_value
• Constants can be assigned decimal, octal(put a 0 before the value; i.e. 0742), or hexadecimal(put a 0x before the value; i.e. 0xA2) values.
• To assign a data type(long or float) to a constant, put a L or F after the constant value.
• Ex. #define PI 3.14F

Special Constants

• Another type of constant is an enumeration constant. It is created in this format: enum enumeration_type {constant_name1, constant_name2,...}
• The first constant in a enumeration will be given the value 0 unless otherwise specified. The second constant will have the value of the previous constant plus one. Thus the second constant will be given the value 1 unless it or the constant before was assigned a specific value. This procedure continues on for the rest of the constants in the enumeration.
  
• Ex. enum boolean {FALSE, TRUE, MAYBE=5, NO}; FALSE will have a value of 0 and TRUE will have a value of 1. MAYBE will have a value of 5 and NO will have a value of 6.
  
• Constants can also be a character(using ''), or a string("").
• Some character constants can act as escape sequences(used to represent action or other characters that can't be represented normally).

Escape Sequences

Sequence	Description
\a	Alert
\b	Backspace
\f	Formfeed
\n	Newline
\r	Carriage return
\t	Horizontal tab
\v	Vertical tab
\\	Backslash
\?	Question mark
\'	Single quote
\"	Double quote
\0	Null (automatically at end of string)
\ooo	ASCII char. in octal
\xhh	ASCII char. in hex