An intro to programming concepts with Scratch Session

An intro to programming concepts with Scratch Session 8 of 10 sessions Working with lists; sorting a list

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

Make a list of real students: we will sort the list (line) by height • • • Half the class lines up; other half observes Who is L[1]? L[2]? … L[8]? If L[1] > L[2], swap positions; else don’t. If L[2] > L[3], swap positions; else don’t. Continue If L[7] > L[8], swap positions; else don’t. • What is the result?

Start over at the front of the list and repeat the process • • • If L[1] > L[2], swap positions; else don’t. If L[2] > L[3], swap positions; else don’t. Continue If L[6] > L[7], swap positions; else don’t. Why don’t we consider L[8]? • What is the result of these steps?

Start over again at the front and get the 3 rd tallest in L[6] • • If L[1] > L[2], swap positions; else don’t. If L[2] > L[3], swap positions; else don’t. Continue If L[5] > L[6], swap positions; else don’t.

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.

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?

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.

Exercise: sorting numbers Sort the list of numbers using the bubble sort: {34, 17, 23, 19, 12}

Sorting using selection sort [optional: time permitting] At each pass, find the shortest kid and swap to the front of the list.

Start with L[1], L[2], …, L[8] • L[1] walks down list comparing height to L[2], L[3], … • When L[k] < L[1], put kid L[1] in position k and then kid L[k] keeps going. • Repeat until the end of the list is reached. • The shortest kid now moves to L[1]

Selection sort pass 2 • Pass one puts the shortest kid in position L[1] • We need to sort the remaining 7 kids in positions L[2], L[3], …, L[8] • Use the same procedure as done in the first pass, but never consider L[1] • The result is that the 2 nd shortest kid will be position at L[2].

What about selection sort? • How many kid compares to position the shortest kid at the 1 st position L[1]? • How many kid compares to position the 2 nd shortest kid at L[2]? • … • How many total kid compares for the entire sort algorithm? • Is this algorithm better than bubble sort?

Algorithms take computer effort Bubble sort and selection sort are good for sorting small lists, but there are better sorts for large lists.

Sorting a list in Scratch Use a list variable; Use an algorithm with nested loops. Difficult content: time is needed.

first pass of bubble sort Let’s first input a list of numbers; (Then we’ll sink the max of the list to the end of the list. )

Make list L and also a variable for its Length Our list

Ask the user for list length after the user hits key ‘L’

Repeat Length times, ask the user for item k of the List The user has given 3 items and is being asked for item k=4.

After the input loop is completed, state is shown

The sinking pass • Repeat 4 times • If kid L[k] > kid L[k+1], swap them • Result is that tallest kid will be at the end L[5]

Largest number sinks to L[5] The swap

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.

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

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.

Work on your own project! Work on your storyline first, then do your coding. Work on your game idea first, then the coding.