Guide to Programming with Python

Guide to Programming with Python

Chapter FourStrings, and Tuples; for Loops: The Word

Jumble Game

Objectives Data type: sequences (containers)

– Strings (a sequence of letters)– Tuples (a sequence of elements of any type; immutable)– Lists (a sequence of elements of any type; mutable)

Create sequences– Assignment statement s, t = "sword", ("sword", "armor”)

– Use the range() function to create a sequence of integers

Use for loops to move through a sequence, in order Index and slice sequences (any sequence) Use sequence functions and operatorsGuide to Programming with Python 2

Understanding Sequence

Sequence: An ordered list of elements (compared to dictionary)

Element: A single item in a sequence Iterate: To move through a sequence, in order

(using for loops) List of your top-ten movies

– A sequence– Each element is a movie title– To iterate over would be to go through each title, in

order

Guide to Programming with Python 3

Using for Loop to Iterate Over a Sequence

for loop iterates over a sequence; performs loop body for each element

During each iteration, loop variable gets next element

In loop body, something usually done with loop variable


A string is a sequence of characters; So loop iterates over letters in the string (e.g., word below)

for letter in word:

print letter

loop variablesequence

Combining for and range()

for letter in word:

print letter

Can be transformed into:

for idx in range(len(word)):

letter = word[idx]

print letter

Combine for loop with range() function, and len() function


The range() Function

>>> range(5)

[0, 1, 2, 3, 4]

>>> range(0, 50, 5)

[0, 5, 10, 15, 20, 25, 30, 35, 40, 45]

Returns a sequence of integers in range range(i) returns sequence 0 through i – 1 range(i, j) returns sequence i through j – 1 range(i, j, k) returns sequence i to j - 1, step k


Counting Forward, By Fives, and Backwards

# counting forward

for i in range(10):

print i,

# counting by fives

for i in range(0, 50, 5):

print i,

# counting backwards

for i in range(10, 0, -1):

print i,


Comparing for and while Loops

for loop – Like while loop, repeats a loop body– Unlike while loop, doesn’t repeat based on condition– Repeats loop body for each element in a sequence– Ends when it reaches end of the sequence– e.g., go through sequence of game titles and print

each


Indexing Sequence

Sequential access: Access in order (see for loops)

Indexing: Process used to access a specific element of a sequence– Random access: Direct access to any element

Element/Member: An element of a sequence Python allows for random access to sequences

(such as strings) via indexing


Working with Positive and Negative Position Numbers


Use brackets and position number to index "index"[3] Positive position numbers: starts at 0; ends with the

length of a sequence - 1

Negative position numbers: starts at the end of sequence with position number: –1

Attempt to access beyond last position results in error

>>> word = "pizza"

>>> print word[0:5]

pizza

>>> print word[1:3]

iz

>>> print word[-4:3]

iz

Slice: Copy of continuous section of a sequence Can give start and end position Slicing shorthand (by omitting the start, end, or

both points) word[:4] word[2:] word[:]

Slice is a brand-new sequenceGuide to Programming with Python 11

Slicing

indexing_nd_slicing.py

import random

word = "pizza"

wlen = len(word)

print word[1]

print word[-1]

rd = random.randrange(wlen)

print word[rd]

print word[0:wlen]

print word[:]

print word[:3]

print word[-wlen]

print word[wlen]


?

Using Sequence Operators and Functions

Python has functions and operators that work with sequences

Can tell you things such as– Length of sequence e.g., len(“abcd”)– If contains specific element e.g., “a” in “abcd”


Using the len() function

>>> len("Game Over!")

10

Takes a sequence (e.g., string – a sequence of letters)

Returns the number of elements In strings, every character counts – spaces and

punctuation


Using the in Operator>>> "e" in "Game Over"

True

Tests for element membership– Returns True if element is in sequence– Returns False otherwise

Using in with index (see index_demo.py)alphabet = "abcdefghijklmnopqrstuvwxyz"

letter = "a"

if letter in alphabet:

idx = alphabet.index(letter)


Immutable vs Mutable Sequences

>>> word = "game"

>>> word[0] = "l"

TypeError: object does not support item assignment

Mutable: Changeable Immutable: Unchangeable String immutability -- Strings are immutable

sequences; can’t be changed– Tuples are immutable too; Lists are mutable!

But can create new strings from existing ones (like through concatenation)


String Immutability

Cannot modify an existing string But can "build" (create) a new string with

concatenation operator

The following do NOT workword = "game"

word[0] = "G” The following work

word = "game"

word = "Game"


The No Vowels Program


VOWELS = "aeiou"

Constant: Name associated with value not meant to be changed Convention is to use all uppercase variable names Can make programs clearer No true constants in Python

new_message += letter

Concatenation creates brand-new string– Remember, strings are immutable– So, new_message becomes the newly created string

resulting from concatenation

Tuples

Tuple: Immutable sequence of values of any type Could have tuple of integers for a high score list,

for example Tuples elements don't need to all be of same type

a = ("Monday", 3, 4.5)


Tuple Basics Creating an Empty Tuple

inventory = ()

Treating a Tuple as a Conditionif not inventory:

print "You are empty-handed."

Creating a Tuple with Elementsinventory = ("sword", "armor", "shield",

"healing potion")

Printing a tupleprint "\nThe tuple inventory is:\n", inventory

Looping through a tuple’s elementsfor item in inventory:

print item


Using Tuples

Tuples are a kind of sequence (like strings) so can:– Get length with len()– Iterate through elements with for loop– Test for element membership with in – Index, slice, and concatenate


Using len() and in with Tuples

The len() function with tuples– Just as with strings, returns number of elementsprint "You have", len(inventory), "items."

The in operator with tuples– Just as with strings, tests for element membershipif "healing potion" in inventory:

print "You will live to fight another day."


Indexing & Slicing Tuples

inventory = ("sword", "armor", "shield",

"healing potion")

inventory[0] ?

inventory[0][1]?

inventory[0:2]?


Tuple Immutability

>>> inventory = ("sword", "armor", "shield",

"healing potion")

>>> inventory[0] = "battleax"

TypeError: object doesn't support item assignment

Tuples are immutable But can create new tuples from existing ones


Concatenating Tuples>>> inventory = ("sword", "armor", "shield",

"healing potion")

>>> chest = ("gold", "gems")

>>> inventory += chest

>>> print inventory

('sword', 'armor', 'shield', 'healing potion', 'gold', 'gems')

Concatenation operator, +, works with tuples just like with strings

The Hero’s Inventory Game (to demo creating, indexing, slicing, and concatenating tuples)


The Word Jumble Game

Figure 4.1: Sample run of the Word Jumble game

This jumble looks “difficult.”


Planning the Word Jumble Game

The jumble creation section was planned first in pseudocode

Create an empty jumble word

While the chosen word has letters in it

extract a random letter from the chosen word

add the random letter to the jumble word


Summary An ordered list of elements is called what?

– A sequence To move through a sequence, in order, is called what?

– Iterate When a for loop iterates over a sequence, how many

times does it perform its loop body?– As many times as there are elements in the sequence

What would range(20,10,-2) return?– [20, 18, 16, 14, 12]

What would len(range(20,10,-2)) return?– 5


Summary (continued)

If I use the in operator to test for element membership in a tuple, what does it return if the element is there?– True

What is the name of the technique used to access a specific element of a sequence?– Indexing

Match the following pairs of words:– mutable unchangeable– immutable changeable

Strings are immutable sequences, true or false?– True

Constants are values that are meant to change, true or false?– False


Summary (continued) String concatenation adds onto an existing string, true or false?

– False, it creates brand-new strings What does None evaluate to when treated as a condition?

– False

Slicing creates a copy of a discontinuous collection of elements from a sequence, true or false?– False, it only copies a continuous segment of elements from a

sequence A tuple is an immutable sequence of elements of what variable

type?– Any!

The concatenation operator, +, works with tuples just like with strings, true or false?– True


Guide to Programming with Python

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