Start to CodeInstructor: Xiao Liu

Recipe● Sequence of simple steps● Flow of control process that specifies when each

step is executed● A means of determining when to stop

Recipe● Sequence of simple steps● Flow of control process that specifies when each

step is executed● A means of determining when to stop

Algorithm

Basic Algorithm in Real life● Rank the top 5 football players

○ Sorting with their current performance and potentials

● Looking up a name in an alphabetically sorted list○ Linear: start at the top○ Binary search: start in the middle

● Standing in line at a bank, supermarket, customs & immigration○ Performance analysis of task scheduling

● Cooking a gourmet meal○ Parallel processing: You don’t want the meat to get cold while you’re cooking the

vegetables.

List# AssignmentA = [1, 5, 7, 2, 4, 6]# Accessfor i in range(6):

print A[i]# Swap A[1], A[2]A[1], A[2] = A[2], A[1]

Lab 4: Task 1Try to print the number with odd index in a list [1,2,3,4,5,6,7,8,9]

SortIf you want to sort the poker...

How will you do?

Random SortIf you want to sort the poker...

Randomly throw them in the air;

Pick them up;

Are they sorted?

Repeat if not sorted.

Random SortIf you want to sort the poker...

Randomly throw them in the air;

Pick them up;

Are they sorted?

Repeat if not sorted.

Monkey Sort

Random SortIf you want to sort the poker...

Randomly throw them in the air;

Pick them up;

Are they sorted?

Repeat if not sorted.

Can we do in a wise way?

Bubble Sort● Lighter goes Higher● Compare consecutive pairs ● Swap the elements, smaller first● When reach end of list, start over● Stop when no more swaps

Bubble Sort See a demo!

https://visualgo.net/en/sorting

Lab 4: Task 2How many times does the bubble sort scan from the beginning to end?

For [1,3,5,7,2,6,25,18,13]？For [1,9,4,6,8]?

Answer it by print out the sorted list in each round.

Selection Sort● Extract minimum element● Swap it with element at index 0● In remaining sublist, extract minimum element● Swap it with the element at index 1

Selection Sort● Extract minimum element● Swap it with element at index 0● In remaining sublist, extract minimum element● Swap it with the element at index 1

See a demo!

https://visualgo.net/en/sorting

Lab 4: Task 3How many times does the selection sort scan from the beginning to end?

For [1,3,5,7,2,6,25,18,13]？For [1,9,4,6,8]?

Answer it by print out the sorted list in each round.

Insertion Sort● Select the number at index N● Insert it to the right position before index N● Do it for every number

Insertion Sort● Select the number at index N● Insert it to the right position before index N● Do it for every number

See a demo!

https://visualgo.net/en/sorting

Lab 4: Task 4def insertionSort(L): for index in range(1,len(L)): currentvalue = L[index] position = index while position>0 and ??: L[position]=L[position-1] position = position-1

L[position]=currentvalue

Understand EfficiencyCompare the three sorting algorithm

Let’s time the execution by -

import timedef bubble_sort(L):

...start_time = time.time()bubble_sort(testList)print("bubble_sort: " + str(time.time() - start_time))

Lab 4: Task 5Time the execution of bubble sort on the list -testList = [1,3,5,7,2,6,25,18,13]

testList = [9,8,7,6,5,4,3,2,1]

testList = [1,2,3,4,5,6,7,8,9]

Lab 4: Task 6Time the execution of all sort algorithms on the list -testList = [1,3,5,7,2,6,25,18,13]

testList = [9,8,7,6,5,4,3,2,1]

testList = [1,2,3,4,5,6,7,8,9]

Search● search algorithm–method for finding an item or group of items with

specific properties within a collection of items.

Search● search algorithm–method for finding an item or group of items with

specific properties within a collection of items.● Collection could be explicit

○ Is a student record in a stored collection of data?

● Collection could be implicit○ Find the square root of a given number

Search Algorithm● Linear Search

○ Brute force search○ List does not have to be sorted

● Bisection search○ List must be sorted to give a correct answer○ See two implementations of the algorithm

Linear Searchdef linear_search(L, e): found = False for i in range(len(L)): if e == L[i]: found = True return found

testList = [1, 3, 4, 5, 9, 18, 27]

1 3 4 5 9 18 27

Lab 4: Task 7Return the index of the found number in the list.

Linear Search

Find 27 in this list:

Using Linear search : compare one by one

1 3 4 5 9 18 27

Linear Search

Find 27 in this list:

Using Linear search : 1 == 27 ?

1 3 4 5 9 18 27

Linear Search

Find 27 in this list:

Using Linear search : 3 == 27 ?

1 3 4 5 9 18 27

Linear Search

Find 27 in this list:

Using Linear search : 4 == 27 ?

1 3 4 5 9 18 27

Linear Search

Find 27 in this list:

Using Linear search : 18 == 27 ?

1 3 4 5 9 18 27

Linear Search

Find 27 in this list:

Using Linear search : 27 == 27 In total compare 7 times

1 3 4 5 9 18 27

Bisection Search

Find 27 in this list:

Any better solutions?

1 3 4 5 9 18 27

Bisection Search

Find 27 in this list:

1 3 4 5 9 18 27

5 < 27, 27 must be right of 5

Bisection Search

Find 27 in this list:

1 3 4 5 9 18 27

9 < 27, 27 must be right of 9

Bisection Search

Find 27 in this list:

1 3 4 5 9 18 27

18 < 27, 27 must be right of 18

Bisection Search

Find 27 in this list:

1 3 4 5 9 18 27

27 == 27 We find it

Bisection Search

Find 27 in this list:

1 3 4 5 9 18 27

In total 4 comparisons

Lab 4: Task 8def binary_search(x, search_list): left = 0 right = len(search_list)-1 mid = (right + left)/2 while search_list[mid] != x: if ??: left = mid + 1 else: right = mid - 1 mid = (right + left)/2 return mid