Composite Variables

• This semester we have encountered a number of simple data types.
  • integers, characters, floats.
  • These are data types that are represented at the basic hardware level.
  • They also contain only one value.
• While these are extremely useful, as you go along in programming it is convenient to group data together.
• These form what the book calls composite data types.
  • For now, we will focus on two of these.
• An array is a homogeneous collection of items.
  • Items in an array are called elements.
  • Items are accessed by an index or number.
• The index normally runs between 0 and size-1, when the array holds size elements.
  • This is because the index is an offset from the beginning of the array.
• We indicate the item with [].
  • Consider the array Names.

  • Index	Data
    0	Fred
    1	Sue
    2	Amanda
    3	Bob
    4	Lisa
    5	Tim
    6	Todd
    7	Scott

  • What is at Names[0]
  • What is at Names[6]
  • What is the index of Lisa?
  • How many elements are in the array?
  • What is the largest index?
• When an index is below 0 or greater than or equal to the size, an error occurs.
  • This is an array index out of bounds error.
• Why count from zero?
  • Address of data = base address + index * size_of(elements)
• When we deal with arrays, we normally have three different data items
  1. The array
  2. The capacity of the array or the maximum number of items it can hold.
  3. The size of the array, or the actual number of items in the array.
• A record, sometimes called a structure
  • This is a heterogeneous collection of data.
  • Generally the data is called a field
  • And fields are accessed by field name.
  • Frequently fields are accessed by structure.field.
  • Consider the structure Student

    • Field	Type	Value
      Name	string	Bob
      Age	int	19
      GPA	float	2.81

    • What is contained in Student.Age?
    • What is contained in Student.GPA
    • What do you think would happen if we executed Student.name=Fred
• We use arrays when we have a bunch of the same thing.
  • Think attendance roster, list of party guests, shopping list.
• We use structures when we want to group different types of data together.
  • Think driver's license, student information, parts to a piece of furniture.