Class 06: Types: Typedefs, Enums, Structs, Unions, and Arrays

Held Friday, February 4, 2000

Overview

Today, we consider types in C.

Notes

• Are there questions on the remainder of Wednesday's lab?
• Are there questions on assignment 1?
  • A few of you have noted that in some circumstances, you cannot re-bind the socket for a few minutes. That's fine.
• Some of you have asked where to find the standard .h files. You can look in /usr/include. For example,
  • <stdio.h> is in /usr/include/stdio.h
  • <netinet/in.h> is in /usr/include/netinet/in.h

Contents

• Types in C
  • Primitive Types
  • Naming Types
  • Enumerated Types
  • Structs
  • Arrays
• Type Exercises

Summary

• Type basics
• Compound types: structs and arrays
• Exercise: enumerated types

Types in C

• Most substantial programs in any language require programmers to define their own types.
• As you may have realized, there are different philosophies on what a type is.
  • To some, a type is simply a collection of values.
  • To others, a type is essentially a collection of capabilities.
• C follows the first model.
• C provides:
  • Basic primitive data types
  • Mechanisms for building new types
  • A mechanism for naming types and creating type aliases

Primitive Types

• C comes with some number of built-in types, including:
  • int: integers
  • float: floating-point values
  • double: floating-point values, with potentially greater precision than floats
  • char: characters (which you can also treat as integers)
• Unlike Java, which precisely specifies the number of bits in each primitive type and the representation used, C leaves it up to the implementation.
  • The same program will often run differently on different platforms.
  • Good programming technique suggests that you define platform-independent names for your types (e.g., int16 for sixteen-bit integers) and hope that someone will rename appropriately on each platform
• Strings are part of C, but arguably not ``primitive''.

Naming Types

• If you are going to use such types, you need an easy way to name them.
• One interesting and useful feature of C is that it allows you to create new names for existing types with the typedef operation.
• The general form is

  typedef type name;
  

• For example, I can say that ``grade is an alias for integer'' with

  typedef int grade;
  

• Clearly, we can use this technique to write more readable programs by defining type names for common uses.

Enumerated Types

• At times, we want types to contain a set of values.
• Typical examples include colors, days of the week, genders.
• While it is possible to define these with a lot of constants, it is easier and more appropriate to define them as an explicit group.
• C provides enumerated types for just this situation.
• You define an enumerated type with

  enum { val1, val2, ,..., valn }
  

• Wait! Where's the type name?
  • There is none.
• It is legal to write

  enum { alpha, beta, gamma } stuff;
  

  • This declares a variable named stuff whose legal values are (more or less), alpha, beta, and gamma.
• Fortunately, we can name them with the typedef operation.

  typedef enum { male, female, neuter } gender;
  gender x = male;
  

• Note that, behind the scenes, C treats every enumerated type as int and numbers the values of that type from 0 to whatever.

Structs

• In C, you group fields together in structs.
• structs are a lot like classes except that they don't have methods.
• For example,

  struct date {
    int year;
    int month;
    int day;
  }; /* struct date */
  

• The name of this struct is struct date.
• For example,

  struct date today;
  today.year = 2000;
  today.month = 2;
  today.day = 4;
  

• Once again, it is good practice to name your structs with typedefs.
• C also includes a variant of structs called unions. The primary difference is that all the fields of a union share the same memory.
  • It's a way to save space
  • It's a way to provide a form of polymorphism
  • It's a nice trip for novice programmers

Arrays

• In C, when you declare an array variable, you typically also give the size of the array.

  int grades[34];
  

• There is no way to get the length of an array in C! You must keep track of it separately.
  • The following is perfectly legal C, although it has unexpected results:

    int stuff[2];
    int i;
    for (i = 0; i < 10; i++)
      printf("%d\n", stuff[i]);
    

• When you write a function that takes an array as a parameter, you can use type[] as the parameter.

Type Exercises

• Write a program that defines an enumerated type for days of the week.
• Print out each of the days using a for loop.
• Add a toString(day) method that converts an enumerated day to the corresponding string.
• Add a fromString(str) method that turns a string in to the corresponding enumerated day of the week.