Arrays and Array Visualization

1
Arrays and Array Visualization

• Alice

2
What is an array?

• An array is a collection of objects or
  information that is organized in a specific
  order.
• The individual components (elements) of an array
  are of the same type (object, number, color, ).

3
Analogy

• An analogy of an array is a music CD.
• A CD contains a collection of songs, listed in
  the order recorded.
• A CD player allows you to
• play any specific song by selecting its number,
  or
• play all the songs in sequence.

4
Arrays in Alice

• In Alice, an array is a data structure used to
  organize objects or information of the same type
  (object, number, color, etc.)
• Just as you cannot actually "see" the digital
  bits of the songs recorded on a CD, you cannot
  actually "see" the digital information in an
  array.
• In other words, an array is not visible.

5
Array Visualization

• We will use a built-in 3D model that has been
  specifically designed to help you "see" how an
  array works.
• The elements of this special array visualization
  are visual Alice objects

6
Initial World

• As an example, we will create a visualization of
  an array of bugs.
• First, we add five bugs to a new world.
• This is not yet an array just five bug objects!

7
Initialize array

• Now add an arrayVisualization object to the
  world.
• A dialog box pops up to allow you to add each bug
  to the array.

8
Completed array

• Each bug added to the array is automatically
  moved to its location in the array.
• The locations are numbered 0, 1, 2, 3, 4 (see the
  digits in the blue squares).
• The location number is the index.

9
elements

• An arrayVisualization object has a built-in
  class-level variable, named elements, that keeps
  track of which objects are in the array and where
  each is located.
• The order of the objects in the elements variable
  matches the order of the objects in the array.

10
Demo Access

• Ch10Lec2Array-V1
• Concepts illustrated in this example
• An index is used to order the elements in an
  array.
• To access (get at) an individual element in the
  array, create an instruction that uses the index
  to select the element.
• An element can be randomly selected from the
  array.
• The maximum value should be the number of
  elements in the array.
• Be sure to select the integer only option for
  the random number function.

11
Permutation

• Changing the order of elements in an ordered
  group is a math operation known as creating a
  permutation.
• To demonstrate how to perform a permutation,
  let's write a swap method that exchanges the
  positions of two elements in an array.

12
Storyboard

• Two number parameters are needed, indexOne and
  indexTwo, to specify the indexes of the two
  objects to be swapped.
• A temporary location will be used to hold one
  object while the other object changes location.

swap Parameters indexOne, indexTwo (of type
number) Do in order relocate element at
indexOne to a temporary location relocate
element at indexOne to indexTwo relocate
element from temporary location to indexOne
13
Temporary location

• An objectVisualization object is added to the
  world to be used as a temporary holding pen.

14
Demo swap

• Ch10Lec2Array-V2
• Concepts illustrated in this example
• In Alice, a let instruction is used to relocate
  an element in an array.
• A temporary holding space, as can be seen in
  this visualization, is not part of the array.

15
Walking through an array

• In addition to the ability to access an
  individual element of an array, it is also
  possible to "walk through" an array, accessing
  each element of the array in order.

16
Demo Index variable

• Ch10Lec2Array-V3
• Concepts illustrated in this example
• To walk through an array, use the complicated
  form of the Loop instruction.
• An index variable is a counter for the Loop.
• When a Loop is used with an array, the Loop's
  index variable can be used to access successive
  elements in the array.

17
Assignment

• Read Chapter 10 Section 2, Arrays

18
Lab

• Chapter 10 Lecture 2 Lab