Variable types and names

• Rules of variable names
  1. Allowed characters: Alphabets (a-z), digits (0-9), and underscore (_)
  2. The first character must be an alphabet.
     Actually _ can be the first character, but not recommended.
  3. Case senstive (popsize and popSize are different variables).
  4. At least first 31 characters are significant.
  5. Some reserved words (keywords) can't be used. e.g., if, else, int, double
  6. Symbolic constant naming should follow these rules, too. Traditionally, upper case was used for symbolic constants (e.g. MAX_ITERATION), but this is not required.

  Examples: can you tell which ones are legal variable names?
      Percent    %increase    big_mac    1stPtr
      camelNotation    y?n    L____    _Height    if

• Types of variables.
  • All variables must be declared (defined) because different types of variables requires different amount of memory storage space.
        int lower;
        int i, j, k, upper;
        double probability1, probability2;
    
  • int
    Integer variables hold values without no fractional part.
        e.g. 2001, 0, -12
  • double
    Double-precision Floating-point variables can have a fractional part (i.e., real numbers)
        e.g. 3.141516, 2.0000, 4.123E+25 (i.e. $4.123 \times 10^{25}$)
  • char
    Character variables hold a single character.
        e.g, 'c', 's', '?', '9'

• Range of variables (in linux/ix86)

  keyword	type	bytes required	min	max
  char	Character	1	-128	127
  short	Short integer	2	-32768	32767
  int	Integer	4	-2,147,483,648	2,147,438,647
  long	Long Integer	4	-2,147,483,648	2,147,438,647
  long long	Long long integer	8	-9223372036854775808	9223372036854775807
  unsigned char	Unsigned character	1	0	255
  unsigned short	Unsigned short interger	2	0	65535
  unsigned int	Unsigned integer	4	0	4,294,967,295
  unsigned long	Unsigned long integer	4	0	4,294,967,295
  unsigned long long	Unsigned long long integer	8	0	18446744073709551615
  float	Single-precision floating-point	4	~1.2E-38	~3.4E+38
  double	double-precision floating-point	8	~2.2E-308	~1.8E+308

  Different architectures have different ranges. You can find this out for your computer by compiling and running a program.

• Significant digits (precision) of floating points:
      float: about 8-10 digits
  

  Exercises:
  

  • Copy /home/progClass/src/immig-emig/floatPrec.c, compile it, and test the number of significant digits.

  • Modify the code, so you can check the significant digits of double-precision floating-point numbers.