Class 13: Fundamental Types in C

Held: Monday, 10 February 2003

Summary: Today we consider many of the fundamental types in C and selected operations on those types.

Related Pages:

• EBoard.
• Lab.

Notes:

• I'm planning to give you a take-home exam some time in the near future. However, I'm not yet sure when. Possibly tomorrow. Possibly Friday.
• I will try to make our exploration of issues in C as much experiment-based as I can.

Overview:

• Integer types.
• Real types.
• Arithmetic operations.
• Enumerated types.
• Lab.

Fundamental Types in C

• C, like most modern programming languages, permits programmers to assign types to values.
• C also lets programmers build their own types.
• What is a type?
  • Some of you have heard this question in other classes.
  • Three perspectives: Set of values; Set of restrictions; Rules for encoding as bits.
• We'll start by focusing on the numeric (and numeric-like) types.

Integer Types

• C provides a large number of integer-like types, including
  • int
  • short int (or just short)
  • long int (or just long)
  • char
  • long long int (in some implementations).
• They differ in the number of bits used to represent them.
• The particular number of bits is implementation-dependent.
• Some programmers, interested in more portability, often define type aliases for the different sizes (e.g., four-byte-int).
• Each integer type can either be signed or unsigned.
• When writing integer constants, you may need to indicate its type by suffixing U (for unsigned) and/or L (for long) or S for short.

Real Types

• Lots of floating-point types, too.
  • float
  • double (often more bits than float)
  • long double (more more bits than double)
• The particular size and implementation depends on the architecture.

Simple Enumerated Types

• Enumerated types are types that you can easily map to and from the integers.
• From one perspective, they are simply a series of constant definitions.
• However, they can also be used to help with correctness of code (so that you don't accidentally assign a value of one type to a variables of another type).
• In C, you write

  enum enumeration-name { name1 ... namen };
  

• For example

  enum days { SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY };
  

• You can also assign particular numeric values by using name=value.

Numeric Operations

• All the ones you'd expect.
• I'd suggest that you not regularly rely on precedence (that is, parenthesize for clarity).
• The increment and decrement operators come in multiple forms
  • Pre-increment, as in ++x, increments a variable and returns its new value.
  • Post-increment, as in x++, increments a variable and returns its old value.
  • Why is the name of C++ a subtle joke?
• Many binary operations can be combined with = to update the left-hand-side using the operation and the value on the right hand side.
  • x += 5 increments x by 5, storing the result in x.
  • x *= 2 multiplies x by 2, storing the result in x.
  • x -= 3 subtracts three from x, storing the result in x.
• They can also be written with the equals sign first.
  • This notation caused some confusion in early versions of C.
  • What is the meaning of x=+2?

Promotion and Demotion

• Although splint suggests that you explicit indicate when you change the type of a value, C does have formal rules for implicit promotion and demotion of values.
• When you assign to any type, C tries to convert the value to that type.
• C also converts when operators have different types of arguments.