Sequence Types - cool.ntu.edu.tw

Sequence Types

Computer Programming (EE3031), Fall 2021

Jiun-Lang Huang, GIEE, NTU

EE3031, F21 Jiun-Lang Huang, NTUEE

Sequence Types

• In Python, sequence is the generic term for an ordered collection of values.

• Basic sequence types: list, tuple, and range.

• Text sequence type: str

• Binary sequence types: bytes, bytearray, memoryview

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Creating a List

• A list is a sequence of arbitrary values.

• sequence ordered

• Ways to create lists.

• Enclosing comma-separated items with square brackets.

• Use the list() function.

• ...

⇒

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>>> members = ['John','Mary','Tom'] >>> record = [['Bond','James'],26,'EE'] >>> empty_list = [] >>> >>> empty_list = list() >>> empty_list [] >>> list_from_range = list(range(3)) >>> list_from_range [0, 1, 2] >>> list_from_string = list('hello') >>> list_from_string ['h', 'e', 'l', 'l', 'o'] >>>


Basic List Operations

• Use + to concatenate lists.

• Creating a new list without modifying the original ones.

• Use * to repeat lists: k * a_list or a_list * k.

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>>> boys = ['John','Tom','Peter'] >>> girls = ['Helen','Mary'] >>> all = boys + girls >>> all ['John', 'Tom', 'Peter', 'Helen', 'Mary'] >>>

>>> one = [1] >>> some_ones = one * 3 >>> some_ones [1, 1, 1] >>> many_ones = 3 * some_ones >>> many_ones [1, 1, 1, 1, 1, 1, 1, 1, 1] >>>

Jiun-Lang Huang, NTUEEEE3031, F21

• Use the len() function to get the number of items in a list.

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>>> boys = ['John','Tom','Peter'] >>> girls = ['Helen','Mary'] >>> len(boys) 3 >>> len(girls) 2 >>> print('There are %d boys and %d girls in the class' ... % (len(boys),len(girls))) There are 3 boys and 2 girls in the class >>> len(list()) 0 >>>


• Membership test:

• True if an item of a_list is equal to x, else False.

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x in a_list

>>> members = ['John','Mary','Tom','Helen']

>>> visitor = 'Tom'

>>> if visitor in members:

... print('welcome!')

...

welcome!

>>> visitor = 'Joe'

>>> if visitor not in members:

... print('please register first.')

...

please register first.


• min(s) / max(s)

• Return the smallest / biggest value in s.

• s.index(x[,i[,j]])

• Return the index of the first occurrence of x in s (at or after index i and before index j).

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>>> scores = [70, 43, 50, 90, 65, 89]

>>> max(scores)

90

>>> min(scores)

43

>>>

>>> numbers = [20,30,14,20,19,20]

>>> numbers.index(20)

0

>>> numbers.index(20,4)

5

>>> numbers.index(20,2,5)

3

>>> numbers.index(0)

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ValueError: 0 is not in list

>>>


• s.count(x)

• Return the total number of occurrences of x in s.

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>>> names = ['Joe','John','Mary','Joe','Tom']

>>> names.count('Joe')

2

>>> names.count('Mary')

1

>>> names.count('Peter')

0

>>>


List Indexing

• Each item in a list can be accessed by its index with the bracket operator.

• Indices are integers, from 0 to len()-1.

• An out-of-range index causes IndexError.

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>>> work_days = ['Mon','Tue','Wed','Thu','Fri'] >>> work_days[0] 'Mon' >>> work_days[3] 'Thu' >>> work_days[5] Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: list index out of range >>>


• Negative indices

• -1 yields the last item, -len() the first item.

• -0 is regarded as 0.

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>>> digits = [1,2,3,4,5]

>>> digits[-1]

5

>>> digits[-5]

1

>>> digits[-6]



IndexError: list index out of range

>>>

* In fact, Python does the conversion for you — the positive index is obtained by adding len() to a negative index.* Examples:

-1 + 5 4-5 + 5 0

→→


List Slicing

• A pair of indices, separated by a colon, identifies a contiguous section of a list, i.e., a sublist.

• left_ind:

• The lower bound, inclusive.

• Default to 0 if omitted.

• right_ind:

• The upper bound, non-inclusive.

• Default to len() if omitted.

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>>> odds = [1,3,5,7,9]

>>> odds[0:3]

[1, 3, 5]

>>> odds[2:5]

[5, 7, 9]

>>> odds[:3]

[1, 3, 5]

>>> odds[3:]

[7, 9]

>>>

a_list[left_ind,right_ind]


• Out of range slice indices are handled gracefully.

• If out of range, replaced by either 0 or len().

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>>> odds = [1,3,5,7,9]

>>> odds[1:100]

[3, 5, 7, 9]

>>> odds[-100:3]

[1, 3, 5]

>>> odds[-100:100]

[1, 3, 5, 7, 9]

>>> odds[300:400]

[]

>>>


Updating List Element(s)

• s[i] = x

• Replace item i of s by x.

•s[i:j] = t

• Replace slice of s from i to j by contents of the iterable t.

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>>> record = ['math',90,'physics',93,'chemistry',85]

>>> record[1] = 91

>>> record

['math', 91, 'physics', 93, 'chemistry', 85]

>>> record[-2] = 'calculus'

>>> record

['math', 91, 'physics', 93, 'calculus', 85]

>>> record[2:4] = ['english',70]

>>> record

['math', 91, 'english', 70, 'calculus', 85]

>>>

* len(t) can be different from j-i.


Adding Elements to List

• s.append(x)

• Append x to the end of s.

• s.extends(t) or s += t

• Extend s with the contents of iterable t.

•s.insert(i,x)

• Insert x into s at the index given by i.

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>>> odds = [1]

>>> odds.append(3)

>>> odds

[1, 3]

>>> odds.extend([7,9])

>>> odds

[1, 3, 7, 9]

>>> odds.insert(2,5)

>>> odds

[1, 3, 5, 7, 9]

>>>


Removing List Elements

•del s[i:j]

• Remove elements of s[i:j] from s.

•s.remove(x)

• Remove the first x from s.

•s.pop([i])

• Return and remove the item at i.

• i defaults to -1.

•s.clear()

• Remove all items from s.

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>>> odds = [1,3,5,7,9,11,13]

>>> del odds[0]

>>> odds

[3, 5, 7, 9, 11, 13]

>>> del odds[1:3]

>>> odds

[3, 9, 11, 13]

>>> odds.remove(9)

>>> odds

[3, 11, 13]

>>> odd = odds.pop()

>>> odd

13

>>> odds

[3, 11]

>>> odds.clear()

>>> odds

[]

>>>


Modifying List Order

•s.sort(reverse=False)

• Sort s in place in ascending order.

• Set reverse to True for descending order.

•s.reverse()

• Reverse the items of s in place.

•sorted(s,reverse=False)

• Return a new sorted list from the items in s.

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>>> odds = [3,7,5,1,9]

>>> odds.reverse()

>>> odds

[9, 1, 5, 7, 3]

>>> odds.sort(reverse=True)

>>> odds

[9, 7, 5, 3, 1]

>>> odds.insert(1,11)

>>> odds

[9, 11, 7, 5, 3, 1]

>>> new_odds = sorted(odds)

>>> odds

[9, 11, 7, 5, 3, 1]

>>> new_odds

[1, 3, 5, 7, 9, 11]

>>>


What is a tuple?

• Roughly like a list, except that it is immutable.

• For the purpose of representing a fixed collection of values.

• Ways to construct tuples:

• A pair of parentheses denotes an empty tuple: ().

• Using a trailing comma for a singleton tuple: a, or (a,).

• Separating items with comma(s): a, b, c or (a, b, c).

• Use the tuple() function: tuple() or tuple(iterable).

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Jiun-Lang Huang, NTUEEEE3031, F21 20

>>> t = ()

>>> type(t)

<class 'tuple'>

>>> t

()

>>> colors = 'red','green','blue'

>>> colors

('red', 'green', 'blue')

>>> letters = tuple('abcde')

>>> letters

('a', 'b', 'c', 'd', 'e')

>>> days = tuple(['M','T','W','T','F'])

>>> days

('M', 'T', 'W', 'T', 'F')

>>> series = tuple(range(1,5,2))

>>> series

(1, 3)

>>>

Jiun-Lang Huang, NTUEEEE3031, F21 21

>>> (0,1,2) + (4,5)

(0, 1, 2, 4, 5)

>>> digits = (0,1,2,3,4,5,6,7,8,9)

>>> min(digits)

0

>>> max(digits)

9

>>> 10 in digits

False

>>> len(digits)

10

>>> digits[3]

3

>>> digits[2:5]

(2, 3, 4)

>>> digits.index(3)

3

>>> digits.count(0)

1

>>>


Why Tuples?

• Their immutability is the main point.

• Benefits

• Tuples are faster than lists.

• Protect data against accidental changes, e.g., days of a week.

• Tuples can be used as keys in dictionaries, while lists cannot.

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Tuple Assignment, Packing, and Unpacking

• In Python, you can assign multiple values with one single assignment statement.

• A very convenient way to swap values.

• In packing, we put values into a new tuple, while in unpacking, we extract those into variables.

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>>> records = 'John','farmer',25

>>> name,occupation,age = records

>>> print(name,occupation,age)

John farmer 25

>>>

>>> a,b = 10,20

>>> print(a,b)

10 20

>>> a,b = b,a

>>> print(a,b)

20 10

>>>

String


Python String Literals

• String is a sequence of characters.

• Single quotes and double quotes strings.

• Use double quotes if the string contains the single quote character, and vice versa.

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>>> brand = 'Ford' >>> model = "Focus" >>> print(brand,model) Ford Focus

>>> string_with_quote = "It's not true." >>> string_with_quote = 'Python has a "print" function.'


Escape Sequences

• Backslashes are used to introduce special character codings, known as escape sequences.

• Some commonly used ones:

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escape sequence meaning

\\ backslash

\' single quote

\" double quote

\n newline

\t horizontal tab


• Examples

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>>> print('\\\\\\') \\\ >>> print('It\'s true.') It's true. >>> print("The \"print\" function.") The "print" function. >>> print('Hi, there!\nWhere are you going today?') Hi, there! Where are you going today? >>> print('Monday\t20\nTuesday\t27') Monday 20 Tuesday 27 >>>


Triple Quoted String Literal

• For convenient coding of multiline text data, e.g., docstring.

• Syntax

• Begins with three quotes (of either the single or double variety),

• followed by any number of lines of text, and

• closed with the same triple-quote sequence that opened it.

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>>> msg = """line1 ... line2 ... line3""" >>> msg 'line1\nline2\nline3' >>> print(msg) line1 line2 line3 >>>


The str() Function

• Return a string version of the given object.

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>>> str(1) '1' >>> str(12.345) '12.345' >>> str(True) 'True' >>> str(range(2)) 'range(0, 2)' >>>


Strings Are Immutable!

• You cannot change an existing string.

• You can only create a new string to replace the old one (by assignment).

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>>> name = 'cindy'

>>> name[1] = 'a' # try to change name to 'candy'



TypeError: 'str' object does not support item

assignment

>>>

>>> name = 'ca' + name[2:]

>>> name

'candy'

>>>


General String Operations

• Support most list operations.

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>>> 'Hello,' + 'Peter!' 'Hello,Peter!' >>> '*' * 8 '********' >>> 5 * '=' '=====' >>>

>>> msg = 'Hello' >>> min(msg) 'H' >>> max(msg) 'o' >>> 'lo' in msg True >>> msg.index('l') 2 >>> msg.count('l') 2 >>> msg[0] 'H' >>> msg[2:] 'llo' >>>


Some String Methods

• s.join(iterable)

• Return a string which is the concatenation of the strings in iterable, separated by s.

• s.lower() / s.upper()

• Return a copy of s with all the cased characters converted to lowercase / uppercase.

• s.replace(old,new[,count])

• Return a copy of s with all occurrences of substring old replaced by new.

• If count is given, only the first count occurrences are replaced.

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Check Python standard library for more.

>>> members = ['John','Tom','Joe'] >>> ', '.join(members) 'John, Tom, Joe' >>> 'YES'.lower() 'yes' >>> 'banana'.replace('na','ba') 'bababa' >>>


• s.split()

• Return a list of the words in the string using white spaces as the delimiter string.

• Runs of consecutive whitespace are regarded as a single separator.

• The result will contain no empty strings at the start or end if the string has leading or trailing whitespace.

• One can specify the separator by setting the sep parameter; however, the splitting algorithm will be different.

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>>> scores = ' 12 8 9 \t 3 \n 10' >>> scores.split() ['12', '8', '9', '3', '10'] >>>


• s.strip()

• Return a copy of the string with the leading and trailing white space removed.

• Use s.lstrip() or s.rstrip() to remove only the leading or trailing whitespace characters.

• Can also specify the characters to strip.

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>>> ' this is a big apple.\t '.strip() 'this is a big apple.' >>> ' this is a big apple.\t '.rstrip() ' this is a big apple.' >>> ' this is a big apple.\t '.lstrip() 'this is a big apple.\t ' >>>

>>> 'mississippi'.strip('mip') 'ssiss' >>> 'mississippi'.rstrip('mip') 'mississ' >>> 'mississippi'.lstrip('mip') 'ssissippi' >>>

Traversing a Sequence


Traversing with for Statement

• The items in seq is assigned to var one by one.

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for var in seq: for_blk

>>> msg = 'Good morning!' >>> o_count = 0 >>> for c in msg: ... if c == 'o': ... o_count += 1 ... >>> print(o_count) 3 >>>

>>> msg = input('enter some numbers: ') enter some numbers: 3 10 90 20 78 >>> tokens = msg.split() >>> tokens ['3', '10', '90', '20', '78'] >>> total = 0 >>> for token in tokens: ... total += int(token) ... >>> print('average = %.2f' % (total/len(tokens))) average = 40.20 >>>


Traversing with Indices

• Sometimes we need to know the item index while traversing a sequence.

• Use range() to produce the indices.

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>>> winners = ['John','Mary','Peter','Joe'] >>> for index in range(len(winners)): ... print('%d: %s' % (index+1,winners[index])) ... 1: John 2: Mary 3: Peter 4: Joe >>>


• Use the enumerate() function to obtain indices and values at the same time.

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>>> names = ['John','Mary','Tom','Helen'] >>> for index,name in enumerate(names): ... print('rank %d: %s' % (index+1,name)) ... rank 1: John rank 2: Mary rank 3: Tom rank 4: Helen >>>

Sequence Types - cool.ntu.edu.tw

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