computer science 111 fundamentals of programming i sequences: lists

Computer Science 111

Fundamentals of Programming I

Sequences: Lists

What Is a List?

• A list is a sequence of 0 or more data values (of any types) called elements

• The programmer can access or replace the element at any position in a list

• An element can be inserted at any position or removed from any position

E1 E2 E3

0 1 2

Real-World Examples

• A shopping list

• A schedule of athletic contests

• A team roster

• An algorithm (a list of instructions)

Literals, Assignment, Comparisons, Concatenation, for Loop

Similar to the behavior of strings so far

a = [1, 2, 3]

b = list(range(1, 4)) # b now refers to [1, 2, 3]

a == b # returns Truea < [2, 3, 4] # returns True

print(a + b) # displays [1, 2, 3, 1, 2, 3]

print(len(a)) # displays 3

for element in [1, 2, 3]: print(element)

Indexing and Slicing


a = [1, 2, 3]

print(a[2]) # Displays 3

print(a[len(a) - 1]) # Displays 3

print(a[-1]) # Displays 3

print(a[0:2]) # Displays [1, 2]

Replacing an Element

To replace an element at a given position, use the subscript operator with the appropriate index

Unlike strings, lists are mutable!

<a list>[<an int>] = <an expression>

a = [1, 2, 3]

a[1] = 5 # The list is now [1, 5, 3]

print(a[1]) # Displays 5

1 2 3[1, 2, 3]0 1 2

Replacing a Subsequence

a = [1, 2, 3, 4]

a[0:2] = [5, 6]

print(a) # Displays [5, 6, 3, 4]

1 2 3[1, 2, 3, 4]0 1 2 3

4

Splittingsplit builds a list of tokens (words) from a string using the space or newline as the default separator

s = 'Python is way cool!'

lyst = s.split()

print(lyst) # Displays ['Python', 'is', 'way', 'cool!']

<a string>.split(<optional separator string>)

Pattern Matchinglyst = ['Ken', 100]

[name, grade] = lyst

print(name) # Displays Ken

print(grade) # Displays 100

Application: Find the Highest Grade

fileName = input('Enter the file name: ')inputFile = open(fileName, 'r')highestGrade = 0topStudent = 'Nobody'for line in inputFile: [name, grade] = line.split() grade = int(grade) if grade > highestGrade: highestGrade = grade topStudent = nameprint(topStudent, 'has the highest grade', highestGrade)

Assumes that each line of text in the file contains two words, a name and a grade (represented as an integer)

Joiningjoin builds a string from a list of tokens (words)

s = 'Python is way cool!'

lyst = s.split()

print(lyst) # Displays ['Python', 'is', 'way', 'cool!']

print(' '.join(lyst)) # Displays Python is way cool!

<a separator string>.join(<a list of strings>)

Application: Sentence LengthShort sentences are an index of good writing style. Word processing programs allow you to do word counts.

sentence = input('Enter a sentence: ')

words = sentence.split()

count = len(words)

print('There are', count, 'words in your sentence.')

Application: Generate Sentences

• Given a vocabulary and grammar rules, one can generate some random and perhaps rather silly sentences

• Vocabulary - the set of words belonging to the parts of speech (nouns, verbs, articles, prepositions)

• Grammar - the set of rules for building phrases in a sentence (noun phrase, verb phrase, prepositional phrase)

The Structure of a Sentence sentence

noun phrase verb phrase

A sentence is a noun phrase followed by a verb phrase

The Structure of a Sentence sentence


article noun

A noun phrase is an article followed by a noun

The Structure of a Sentence


sentence


article noun

the girl

Pick actual words for those parts of speech at random



sentence


article noun verb noun phrase prepositional phrase

the girl

A verb phrase is a verb followed by a noun phrase and a prepositional phrase



sentence



the girl hit

Pick a verb at random



sentence



article noun

the girl hit

Expand a noun phrase again



sentence



article noun

the girl hit the boy

Pick an article and a noun at random



sentence



article noun preposition noun phrase

the girl hit the boy

A prepositional phrase is a preposition followed by a noun phrase



sentence




the girl hit the boy with

Pick a preposition at random



sentence




article noun

the girl hit the boy with

Expand another noun phrase



sentence




article noun

the girl hit the boy with a bat

More random words from the parts of speech

Representing the Vocabularynouns = ['bat', 'boy', 'girl', 'dog', 'cat', 'chair', 'fence', 'table', 'computer', 'cake', 'field']

verbs = ['hit', 'threw', 'pushed', 'ate', 'dragged', 'jumped']

prepositions = ['with', 'to', 'from', 'on', 'below', 'above', 'beside']

articles = ['a', 'the']

Use a list of words for each part of speech (lexical category)

Picking a Word at Randomnouns = ['bat', 'boy', 'girl', 'dog', 'cat', 'chair', 'fence', 'table', 'computer', 'cake', 'field']

verbs = ['hit', 'threw', 'pushed', 'ate', 'dragged', 'jumped']

prepositions = ['with', 'to', 'from', 'on', 'below', 'above', 'beside']

articles = ['a', 'the']

import random

print(random.choice(verbs)) # Prints a randomly chosen verb

The random module includes functions to select numbers, sequence elements, etc., at random

Grammar Rulessentence = nounphrase verbphrase

nounphrase = article noun

verbphrase = verb nounphrase prepositionalphrase

preopositonalphrase = preposition nounphrase

A sentence is a noun phrase followed by a verb phrase

Etc., etc.

Define a Function for Each Rule# sentence = nounphrase verbphrasedef sentence(): return nounphrase() + ' ' + verbphrase()

Each function builds and returns a string that is an instance of the phrase

Separate phrases and words with a space

Define a Function for Each Rule# sentence = nounphrase verbphrasedef sentence(): return nounphrase() + verbphrase()

# nounphrase = article noundef nounphrase(): return random.choice(articles) + ' ' + random.choice(nouns)

When a part of speech is reached, select an instance at random from the relevant list

Call sentence() to Try It Out # sentence = nounphrase verbphrasedef sentence(): return nounphrase() + verbphrase()

# nounphrase = article noundef nounphrase(): return random.choice(articles) + ' ' + random.choice(nouns)

…

for x in range(10): print(sentence()) # Display 10 sentences

You can also generate examples of the other phrases by calling their functions

Just the Tip of the Iceberg

• The list is a very powerful data structure

• There are many list processing methods

• A Python method is like a function, but uses a slightly different syntax

The append Method lyst = [1, 2, 3]

lyst.append(4)

print(lyst) # Displays [1, 2, 3, 4]

<a list>.append(<an element>) # Puts the element at the end of the list # Actually modifies the list!!!

Adds an element to the end of the list

Syntax for calling the append method:

Functions vs Methods lyst = [1, 2, 3]

lyst.append(4) # A method call

print(len(lyst)) # Two function calls

file = open('testfile.txt', 'r') # A function call

wordList = file.read().split() # Two method calls

<a data object>.<method name>(<arguments>)

<function name>(<arguments>)

Syntax of method calls and function calls:

Some List MethodsExample Call What It Does

lyst.count(3) Returns the number of 3s in the list

lyst.insert('dog', 2) Inserts 'dog' at position 2, after shifting the elements at positions 2 through N - 1 to the right

lyst.pop(0) Removes the element at the first position and then shifts the remaining elements to the left by one position

lyst.remove('dog') Removes the first instance of 'dog' in the list

lyst.reverse() Reverses the elements

lyst.sort() Sorts the elements in ascending order

For Wednesday

Continue reading Chapter 5

on dictionaries

computer science 111 fundamentals of programming i sequences: lists

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