Lists 01024111 Computers and Programming. Agenda What is a list? How to access elements in the list?...

ListsLists

01024111 Computers and Programming

AgendaAgenda

• What is a list?• How to access elements in the list?• The for statement• Operations on lists• Looping with for statement• Examples

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WHAT IS A LIST?WHAT IS A LIST?

3

Test scores for 5 Test scores for 5 studentsstudents

• We can use 5 variables for storing 5 numbers.

4

s1 = 10s2 = 16s3 = 15s4 = 9s5 = 23

s1 = 10s2 = 16s3 = 15s4 = 9s5 = 23

101615

239

s1s2s3

s5s4

program

101615

239

s1s2s3

s5s4

Computing statisticsComputing statistics

5

t = 0t += s1t += s2t += s3t += s4t += s5

t = 0t += s1t += s2t += s3t += s4t += s5

avg = t / 5avg = t / 5

m = 0if s1 > m: m = s1if s2 > m: m = s2if s3 > m: m = s3if s4 > m: m = s4if s5 > m: m = s5

m = 0if s1 > m: m = s1if s2 > m: m = s2if s3 > m: m = s3if s4 > m: m = s4if s5 > m: m = s5

What is the goalof this program?What is the goalof this program?

The summationThe summation

How about this?How about this?

The averageThe average

And this?And this?The maximumThe maximum

Side notesSide notes

• In the last slide, we use a new type of operators: +=.

6

a = a + 1a = a + 1 a += 1a += 1

a = a * 2a = a * 2 a *= 2a *= 2

Test scores for 50 Test scores for 50 studentsstudents

• Suppose that now you have 50 students in a class.

• How can you store all the scores?

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50 variables50 variables

• We can use 50 variables to store 50 numbers

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s1 = 10s2 = 16s3 = 15s4 = 9s5 = 23………

s1 = 10s2 = 16s3 = 15s4 = 9s5 = 23………

Extremelydifficult

to deal with

ListsLists

• In Python, we have data types that can keep many items in one place. One of these is a list.

• An example of a list:

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[10, 16, 15, 9, 23][10, 16, 15, 9, 23]

101016161515992323

A list constantA list constant

• We write a list by enclosing all items in a pair of brackets [ ], with a comma (,) separating each pair of items.

• It is OK to have extra comma after the last item.

10

,

ExamplesExamples

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• A list of numbers of days for each month

[31,28,31,30,31,30,

31,31,30,31,30,31]

[31,28,31,30,31,30,

31,31,30,31,30,31]

• A list of names of days in a week

['sun','mon','tue','wed', 'thu','fri','sat'] ['sun','mon','tue','wed', 'thu','fri','sat']

ExampleExample

• A list of names

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["somying", "somsak", "somchai"]["somying", "somsak", "somchai"]

"somying""somying""somsak""somsak""somchai""somchai"

A list is just another dataA list is just another data

• We can assign a list to a variable. That variable will refer to that list.

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scores = [10, 16, 15, 9, 23]scores = [10, 16, 15, 9, 23]

101016161515992323

scoresscores

ACCESSING ITEMS IN A ACCESSING ITEMS IN A LISTLIST

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IndexingIndexing

• We can refer to an item in a list by specifying its index in the list.

• The first item in the list has index 0; the next one has index 1, and so on.

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101016161515992323

scoresscores scores[0]scores[0]scores[1]scores[1]scores[2]scores[2]scores[3]scores[3]scores[4]scores[4]

Accessing items through Accessing items through their indicestheir indices

• Recall that the first item has index 0.

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scores = [10, 16, 15, 9, 23]scores = [10, 16, 15, 9, 23]

print(scores[0])print(scores[0])

>>> 10>>> 10

print(scores[3])print(scores[3])

>>> 9>>> 9

print(scores[1] + scores[4])print(scores[1] + scores[4])

>>> 39>>> 39

ExampleExample

0

1

2

3

4

5

6

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>>> print(days[3])>>> print(days[3])

wed'sun'

'mon'

'tue'

'wed'

'thu'

'fri'

'sat'

daysdays

Thinking CornerThinking Corner

• Write a program that print all items in the following list a. You are not allowed to write "print" more than once.

a = [1,2,1,4,3,2,4,2,5,6,3,7]

• Note: there are 12 items in the list.• Hint: try using the while statement.

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Thinking Corner: solutionThinking Corner: solution

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a = [1,2,1,4,3,2,4,2,5,6,3,7]i = 0while i <= 11: print(a[i]) i = i + 1

a = [1,2,1,4,3,2,4,2,5,6,3,7]i = 0while i <= 11: print(a[i]) i = i + 1

PracticePractice

• What is the output?

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ps = [2,3,5,7,11]i = 0while i<5: print(ps[i]) i = i + 2

ps = [2,3,5,7,11]i = 0while i<5: print(ps[i]) i = i + 2

2511

2511

More practiceMore practice


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ps = [2,3,5,7,11,13]nn = [1,2,1,1, 2, 1]i = 0while i<6: print(ps[i]) i = i + nn[i]

ps = [2,3,5,7,11,13]nn = [1,2,1,1, 2, 1]i = 0while i<6: print(ps[i]) i = i + nn[i]

23711

23711

Modifying data in a listModifying data in a list

• As we can refer to items in the list, we can assign new values to them.

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>>> s = [10,20,30,40,50]>>> s[2] = 5>>> s[10,20,5,40,50]>>> s[4] += 7>>> s[10,20,5,40,57]

>>> s = [10,20,30,40,50]>>> s[2] = 5>>> s[10,20,5,40,50]>>> s[4] += 7>>> s[10,20,5,40,57]

PracticePractice


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ps = [1,2,1,3,1,4]t = 0j = 0while j < 6: if ps[j] > t: t = ps[j] else: ps[j] = t print(ps[j]) j += 1

ps = [1,2,1,3,1,4]t = 0j = 0while j < 6: if ps[j] > t: t = ps[j] else: ps[j] = t print(ps[j]) j += 1

122334

122334

Anything goes…Anything goes…

• A list can hold items with different types.

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stinfo = ["dang", 20, 167, 78.5]stinfo = ["dang", 20, 167, 78.5]

"dang"dang202016716778.578.5

stinfostinfo stinfo[0]stinfo[0]stinfo[1]stinfo[1]stinfo[2]stinfo[2]stinfo[3]stinfo[3]

An Empty ListAn Empty List

• An empty list is also a list

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>>> mylist = []>>> a = mylist[0]Traceback (most recent call last): builtins.IndexError: list index out of range

>>> mylist = []>>> a = mylist[0]Traceback (most recent call last): builtins.IndexError: list index out of range

We get an error because we try to access an item which is not there.

THE FOR-STATEMENTTHE FOR-STATEMENT

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The The for for statementstatement

• We can use the for statement to iterate through all items in a list

• Syntax:

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for var in list: statement statement

for var in list: statement statement

For statement controls a block, so make sure you have the same indent.

How does the for How does the for statement work?statement work?

• The block inside the for statement is executed as the variable iteratively refers to each item in the list.

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for x in [2,3,5,7,11]: print(x)for x in [2,3,5,7,11]: print(x)

223355771111

xx

235711

xxxxxxxx


• Write a program that computes the summation of all items in the following list.

a = [1,2,1,4,3,2,4,2,5,6,3,7]

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a = [1,2,1,4,3,2,4,2,5,6,3,7]total = 0for x in a: total = total + xprint(total)

a = [1,2,1,4,3,2,4,2,5,6,3,7]total = 0for x in a: total = total + xprint(total)

This helps when This helps when processing lists…processing lists…

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s = [10,16,15,9,13]t = 0for x in s: t = t + x

s = [10,16,15,9,13]t = 0for x in s: t = t + x

m = 0for x in s: if x > m: x = m


Computingthe

summation

Computingthe

summation

Computethe

maximum

Computethe

maximum

...... even when they even when they contain lots of datacontain lots of data

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s = [10,16,15,9,13,20,12,11,2,14, 6,7,13,4,6,7,14,18,9,12]t = 0for x in s: t = t + x

s = [10,16,15,9,13,20,12,11,2,14, 6,7,13,4,6,7,14,18,9,12]t = 0for x in s: t = t + x



Computing the sumComputing the sum

Computing the maximumComputing the maximum

Nothing changesNothing changes

A quick summary on how A quick summary on how to access items in a listto access items in a list

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Referring to each item

a[5]

Iterate through a list

for x in a: c += xa a

OPERATIONS ON LISTSOPERATIONS ON LISTS

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Working with listsWorking with lists

• Operators on lists• List functions• Adding items to lists

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Operators on lists (1)Operators on lists (1)

• We can add two lists. The result is the concatenation of the two lists.

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>>> [1,2] + [5,10,15][1,2,5,10,15]>>> [1,2] + [5,10,15][1,2,5,10,15]

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51015

Operators on lists (2)Operators on lists (2)

• We can multiply a list with an integer. The result is the concatenation of copies of the original list.

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>>> [1,2] * 3[1,2,1,2,1,2]>>> [1,2] * 3[1,2,1,2,1,2]

12 Very useful

for initializing lists.

Very useful for initializing lists.

Example for multiplying Example for multiplying with an integerwith an integer

• If we want to create a list with 10 items all of them are zero.

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>>> [0] * 10[0,0,0,0,0,0,0,0,0,0]>>> [0] * 10[0,0,0,0,0,0,0,0,0,0]

List functionsList functions

• Important functions are

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names objectives examples results

len Returns the number of items

len([1,2,5]) 3

sum Returns the summation

sum([1,2,5]) 8

max Returns the maximum

max([1,2,5]) 5

min Returns the minimum

min([1,2,5]) 1

Examples of usageExamples of usage

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>>> len([])0>>> s = [1,2] + [3,5,10]>>> sum(s)21>>> max(s)10>>> len(s)5>>> len(s + [1,6]) #=>[1,2,3,5,10,1,6]7

>>> len([])0>>> s = [1,2] + [3,5,10]>>> sum(s)21>>> max(s)10>>> len(s)5>>> len(s + [1,6]) #=>[1,2,3,5,10,1,6]7

Appending to listsAppending to lists

• We can add items to lists by using method append as shown below.

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>>> s = [1,2,5]>>> s.append(10)>>> s[1,2,5,10]

>>> s = [1,2,5]>>> s.append(10)>>> s[1,2,5,10]

EXAMPLE 1EXAMPLE 1

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StatisticsStatistics

• We shall write a program that computes data statistics The program reads data items from the

user until the user enter -1 Then the program reports the

summation, the average, the maximum, and the minimum.

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Main programMain program

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data = read_input() # read data as a list

total = _____________________

average = ___________________

max = _______________________

min = _______________________

# display output

data = read_input() # read data as a list

total = _____________________

average = ___________________

max = _______________________

min = _______________________

# display output

sum(data)sum(data)

total / len(data)total / len(data)

max(data)max(data)

min(data)min(data)


• Write function read_list that reads integers from the user until the user enters -1, and returns a list of the integers.

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def read_list(): print("Enter list (-1 to end):") data = []

return data

def read_list(): print("Enter list (-1 to end):") data = []

return data

x = int(input()) while x!=-1: data.append(x) x = int(input())

x = int(input()) while x!=-1: data.append(x) x = int(input())

FOR LOOPSFOR LOOPS

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for Loopsfor Loops

• We can use the for statement to simplify many of the loops written with the while statement.

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t = 1for i in [1,2,3,4,5]: t *= iprint(t)

t = 1for i in [1,2,3,4,5]: t *= iprint(t)

What is this program doing?

PrintingPrinting

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months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dev']days = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

for i in [0,1,2,3,4,5,6,7,8,9,10,11]: print(months[i],days[i])


for i in [0,1,2,3,4,5,6,7,8,9,10,11]: print(months[i],days[i])

Function Function rangerange

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for i in range(12): print(months[i],days[i])


for i in range(12): print(months[i],days[i])

Function Function rangerange

• Function range returns a result that acts like a list. Its usages are shown below.

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usages resultrange(r) a list from 0 to r-1

range(7) => [0,1,2,3,4,5,6]

range(a,b) a list from a to b-1range(3,7) => [3,4,5,6]

Quick practiceQuick practice

• What does this program compute?

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t = 0for i in range(100): t += i*iprint(t)

t = 0for i in range(100): t += i*iprint(t)

More quick practiceMore quick practice

• What does this program compute?

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t = 0for i in range(200): if i % 3 == 0: t += iprint(t)

t = 0for i in range(200): if i % 3 == 0: t += iprint(t)

PracticePractice

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def is_prime(n):def is_prime(n):

Prime numbers are positive integers having exactly two positive factors.

Write function is_prime that returns True if and only if n is a prime number.

Let's do it step by stepLet's do it step by step


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def count_factors(n):def count_factors(n):

Write function count_factors that returns the number of positive integers that divides n using for-statement.

def count_factors(n): count = 0 for f in range(1, n + 1): if n % f == 0: count += 1 return count


Write function count_factors that returns the number of positive integers that divides n using for-statement.


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def count_factors(n):def count_factors(n):def count_factors(n): count = 0 for f in range(1, n + 1): if n % f == 0: count += 1 return count


range(n + 1):

What would happen if we change to this?What would happen if we change to this?

ZeroDivisionError: integer division or modulo by zero

ComparisonComparison

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def count_factors(n): count = 0 for f in range(1, n+1): if n % f == 0: count = count + 1 return count

def count_factors(n): count = 0 for f in range(1, n+1): if n % f == 0: count = count + 1 return count

def count_factors(n): count = 0 f = 1 while f <= n: if n % f == 0: count = count+1 f = f + 1 return count

def count_factors(n): count = 0 f = 1 while f <= n: if n % f == 0: count = count+1 f = f + 1 return count

PracticePractice

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def is_prime(n):def is_prime(n):

Use that to write is_primeUse that to write is_prime

def is_prime(n): return count_factors(n)==2def is_prime(n): return count_factors(n)==2

Prime numbers are positive integers having exactly two positive factors.

Write function is_prime that returns True if and only if n is a prime number.

EXAMPLE 2EXAMPLE 2

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Counting votesCounting votes

• In one election for student representatives, there are 5 candidates, numbers from 1 to 5.

• Write a program that reads a list of votes and then finds out who wins the election.

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How can we keep vote How can we keep vote counts?counts?

• Since we have 5 candidates, we will use a list that can keep 5 items.

• However, because the candidate numbers start with 1, we will actually use a list with 6 items so that we can access items with indices 1 to 5.

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0 0 0 0 0 0

Does not useDoes not use

For storing vote countsFor storing vote counts

InitializationInitialization

• Everyone starts with 0 votes.

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scores = [0,0,0,0,0,0]scores = [0,0,0,0,0,0]scores = [0] * 6scores = [0] * 6 Much easier

to write when we

have more candidates.

Much easier to write when we

have more candidates.

Read and update vote Read and update vote countscounts

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scores = [0] * 6choice = int(input())while choice != -1: scores[choice] += 1 scores[choice] += 1 choice = int(input())

scores = [0] * 6choice = int(input())while choice != -1: scores[choice] += 1 scores[choice] += 1 choice = int(input())

ReportReport

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scores = [0] * 6choice = int(input())while choice != -1: scores[choice] += 1 choice = int(input())show_result(scores)show_result(scores)

scores = [0] * 6choice = int(input())while choice != -1: scores[choice] += 1 choice = int(input())show_result(scores)show_result(scores)

def show_result(s): for i in range(5): cnum = i + 1 print(cnum,"gets",s[cnum],"votes")

def show_result(s): for i in range(5): cnum = i + 1 print(cnum,"gets",s[cnum],"votes")

def show_result(s): for cnum in range(1,len(s)): print(cnum,"gets",s[cnum],"votes")

def show_result(s): for cnum in range(1,len(s)): print(cnum,"gets",s[cnum],"votes")


• Write function freq(ls,x) that accepts list ls and a item x, and returns the number of times item x appears in the list.

• Example of usage:

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a = [1,2,1,1]print(freq(a,1))print(freq(a,2))print(freq(a,3))

a = [1,2,1,1]print(freq(a,1))print(freq(a,2))print(freq(a,3))

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Thinking Corner: solutionThinking Corner: solution

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def freq(ls,x): c = 0 for y in ls: if y == x: c += 1 return c

def freq(ls,x): c = 0 for y in ls: if y == x: c += 1 return c

Lists 01024111 Computers and Programming. Agenda What is a list? How to access elements in the list?...

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Transcript of Lists 01024111 Computers and Programming. Agenda What is a list? How to access elements in the list?...