Guide to Programming with Python
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Transcript of 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
Guide to Programming with Python 4
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
Guide to Programming with Python 5
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
Guide to Programming with Python 6
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,
Guide to Programming with Python 7
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
Guide to Programming with Python 8
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
Guide to Programming with Python 9
Working with Positive and Negative Position Numbers
Guide to Programming with Python 10
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]
Guide to Programming with Python 12
?
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”
Guide to Programming with Python 13
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
Guide to Programming with Python 14
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)
Guide to Programming with Python 15
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)
Guide to Programming with Python 16
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"
Guide to Programming with Python 17
The No Vowels Program
Guide to Programming with Python 18
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)
Guide to Programming with Python 19
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
Guide to Programming with Python 20
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
Guide to Programming with Python 21
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."
Guide to Programming with Python 22
Indexing & Slicing Tuples
inventory = ("sword", "armor", "shield",
"healing potion")
inventory[0] ?
inventory[0][1]?
inventory[0:2]?
Guide to Programming with Python 23
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
Guide to Programming with Python 24
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)
Guide to Programming with Python 25
The Word Jumble Game
Figure 4.1: Sample run of the Word Jumble game
This jumble looks “difficult.”
Guide to Programming with Python 26
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
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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
Guide to Programming with Python 28
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
Guide to Programming with Python 29
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 30