An intro to programming concepts with Scratch

1
An intro to programming concepts with
Scratch

• Session 8
• of 10 sessions
• Working with lists sorting a list

2
Session 8 goals

• Learn how to represent a list
• Learn how to sort a list of numbers
• Learn how to do it in Scratch
• Free time to work on your own project

3
Make a list of real students we will sort the
list (line) by height

• Half the class lines up other half observes
• Who is L1? L2? L8?
• If L1 gt L2, swap positions else dont.
• If L2 gt L3, swap positions else dont.
• Continue
• If L7 gt L8, swap positions else dont.
• What is the result?

4
Start over at the front of the list and repeat
the process

• If L1 gt L2, swap positions else dont.
• If L2 gt L3, swap positions else dont.
• Continue
• If L6 gt L7, swap positions else dont.
• Why dont we consider L8?
• What is the result of these steps?

5
Start over again at the front and get the 3rd
tallest in L6

• If L1 gt L2, swap positions else dont.
• If L2 gt L3, swap positions else dont.
• Continue
• If L5 gt L6, swap positions else dont.

6
Switch the kids in line with the kids who are
observing.

• Carefully go over the sorting by height.
• Always compare only two adjacent kids.
• Count the total number of kid compares.

7
Exercise sorting 8 kids by height

• How many kid compares to get the tallest kid in
  list position 8?
• How many kid compares to get the next tallest kid
  in list position 7?
• How many compares to get all 8 kids in total
  height order?

8
You have discovered BUBBLE SORT (or SINKING SORT)

• A smaller kid bubbles up to the front of the
  list, one position each pass.
• The tallest kid sinks all the way to the end of
  the list in a single pass.

9
Exercise sorting numbers

• Sort the list of numbers using the bubble sort
• 34, 17, 23, 19, 12

10
Sorting using selection sortoptional time
permitting

• At each pass, find the shortest kid and swap to
  the front of the list.

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Start with L1, L2, , L8

• L1 walks down list comparing height to L2,
  L3,
• When Lk lt L1, put kid L1 in position k and
  then kid Lk keeps going.
• Repeat until the end of the list is reached.
• The shortest kid now moves to L1

12
Selection sort pass 2

• Pass one puts the shortest kid in position L1
• We need to sort the remaining 7 kids in positions
  L2, L3, , L8
• Use the same procedure as done in the first pass,
  but never consider L1
• The result is that the 2nd shortest kid will be
  position at L2.

13
What about selection sort?

• How many kid compares to position the shortest
  kid at the 1st position L1?
• How many kid compares to position the 2nd
  shortest kid at L2?
• How many total kid compares for the entire sort
  algorithm?
• Is this algorithm better than bubble sort?

14
Algorithms take computer effort

• Bubble sort and selection sort are good for
  sorting small lists, but there are better sorts
  for large lists.

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Sorting a list in Scratch

• Use a list variable
• Use an algorithm with nested loops.
• Difficult content time is needed.

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first pass of bubble sort

• Lets first input a list of numbers
• (Then well sink the max of the list to the end
  of the list.)

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Make list L and also a variable for its Length
Our list
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Ask the user for list length after the user hits
key L
19
Repeat Length times, ask the user for item k of
the List
The user has given 3 items and is being asked for
item k4.
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After the input loop is completed, state is shown
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The sinking pass

• Repeat 4 times
• If kid Lk gt kid Lk1, swap them
• Result is that tallest kid will be at the end L5

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Largest number sinks to L5
The swap
23
Swapping cars in 2-car garage

• Move car A to street (the copy)
• Move car B to where car A was
• Move car A from street to where car B was.
• We have to swap computer memory contents in the
  same way.

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Exercise for later (hard)

• Change the sinking loop to be repeated variable M
  times.
• Wrap the sinking loop in an outer repeat loop.
• The outer loop is repeated for k 1, 2, 3, ,
  Length-1
• The inner loop is repeated for M Length k
  times

25
Exercise test sorting lists of different lengths

• User inputs the Length
• Programs asks for Length items from the user
• When s pressed, the sort algorithm sinks the
  max to the end in Length-1 passes.

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Work on your own project!

• Work on your storyline first, then do your
  coding. Work on your game idea first, then the
  coding.