Chapter 8 - Working with Sequences: Strings & Lists

sequence

an object that holds multiple items of data, stored one after the other

What are the 2 ways to access the individual characters in a string?

• for loops

• indexing

How can you use for loops to access individual characters in a string?

each time the loop iterates, the variable will reference a copy of a character in string, beginning with the first character

What is the output for this code?

name = "Juliet"
for ch in name:
     print ch

J

u

l

i

e

t

How can you use indexing to access individual characters in a string?

each character in a string has an index that specifies its position in the string; indexing starts at 0, and the index of the last character in a string is 1 less than the number of characters in the string; index -1 is the last character

What is the output for this code?

my_string = "Roses are red"
ch = my_string[6]
print ch

a

What is the output for this code?

my_string = "Roses are red"
print (my_string[0], my_string[6], my_string[10])

R a r

When does an Index Error exception occur?

when you try to use an index that is out of range for a particular string

• most likely to happen when a loop incorrectly iterates beyond the end of a string

len()

returns the length of a sequence

• useful in preventing loops from iterating beyond the end of a string

What does it mean for a string to be immutable?

it cannot be changed

What is a consequence of strings being immutable?

you cannot use an expression in the form string[index] on the left side of an assignment operator

slice

a span of items that are taken from a sequence

What do you get when you take a slice from a string?

a span of characters from the string

What are string slices called?

substrings

string[start:end]

gives a slice of a string

• start is the index of the 1st character in the slice, and end is the index marking the end of the slice

• the expression will return a string containing a copy of the characters from start up to but not including the end

What is the output for this code?

text = "Four score and seven years ago"
if "seven" in text:
     print("The string "seven" was found")
else:
     print("The string "seven" was not found")

The string “seven” was found

string testing methods

test a string for specific characters

isalnum()

true if the string contains only numeric digits and is at least one character long

isalpha()

true if the string contains only alphabetic letters and is at least one character long

islower()

true if the all characters in the string are lower case

isspace()

true if the string contains only whitespace characters (spaces, newlines, and tabs) and is at least on character long

isupper()

true if all the alphabetic letters are uppercase and it is at least on character long

string modification methods

return a modified version of strings

lower()

converts all alphabetic letters to lowercase

lstrip()

removes all leading whitespace characters (spaces, newlines, tabs at the beginning)

lstrip(char)

removes all instances of char that appear the beginning of the string

rstrip()

removes all trailing whitespace characters (spaces, newlines, tabs at the end)

rstrip(char)

removes all instances of char that appear at the end of the string

strip()

removes all leading and trailing whitespace characters

strip(char)

removes all instances of char that appear at the beginning and end of the string

upper()

converts all alphabetic letters to uppercase

search and replace methods

search for substrings and replace the occurrences of a substring with another string

endswith(substring)

true if the string ends with substring

find(substring)

returns the lowest index in the string where the substring is found; if substring is not found, returns -1

replace(old, new)

replaces all instances of old with new

startswith(substring)

true if the string starts with substring

repetition operator

when the operand on the left side of the * symbol is a string and the operand on the right side is an integer

string_to_copy * n

What is the output for this code?

print("Hello"*5)

HelloHelloHelloHelloHello

list

an object that contains multiple data items

• mutable - contents can be changed during a program’s execution

• dynamic data structures - items may be added to or removed from them

• indexing, slicing and various methods can be used on them

What is the output for this code?

numbers = [5, 10, 15, 20]
print numbers

[5, 10, 15, 20]

What is the output for this code?

my_list = [10, 20, 30, 40]
print(my_list[0], my_list[1], my_list[2], my_list[3])

10 20 30 40

What is the output for this code?

days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]
mid_days = days[2:5]
print(mid_days]

[Tuesday, Wednesday, Thursday]

True or false: an expression in the form list[index] can appear on the left side of an assignment operator

true; lists are mutable

What is the output for this code?

numbers = [1, 2, 3, 4, 5]
print numbers
numbers[0] = 99
print numbers

[1, 2, 3, 4, 5]

[99, 2, 3, 4, 5]

append(item)

adds item to the end of the list

index(item)

returns index of the first element whose value is equal to item

insert(index, item)

inserts item into the list at the specific index, shifting every item after it by one position toward the end of the list

• if the specified index is beyond the list’s end, the item is added to the end of it

• if the specified index is negative, the item is inserted at the beginning of list

sort()

sorts the items into ascending order

remove(item)

removes the first occurrence of item from the list; a ValueError exception is raised if item is not found in the list

del statement

removes an element from a specific index, regardless of the item that is stored at that index

What is the output for this code?

my_list = [1, 2, 3, 4, 5]
print("Before deletion:", my_list)
del my_list[2]
print("After deletion:", my_list)

Before deletion: [1, 2, 3, 4, 5]
After deletion: [1, 2, 4, 5]

min()

accepts a sequence as an argument and returns the item that has the lowest value in the sequence

What is the output for this code?

my_list = [5, 4, 3, 2, 50, 40, 30]
print("The lowest value is", min(my_list))

The lowest value is 2

max()

accepts a sequence as an argument and returns the item that has the highest value in the sequence

What is the output for this code?

my_list = [5, 4, 3, 2, 50, 40, 30]
print("The highest value is", max(my_list))

The highest value is 50

What is the output for this code?

list1 = [1, 2, 3, 4]
list2 = [5, 6, 7, 8]
list3 = list1 + list2
print(list3)

[1, 2, 3, 4, 5, 6, 7, 8]

What is the output for this code?

def main():
     numbers = [2, 4, 6, 8, 10]
     total = 0
     for value in numbers:
        total += value
     print("The total of the elements is", total)
main()

The total of the elements is 30

What is the output for this code?

def main():
     my_string = "One two three four"
     word_list = my_string.split()
     print(word_list)
main()

[“One”, “two”, “three”, “four”]

What is the output for this code?

date_string = "3/6/2008"
date_list = date_string.split("/")
print(date_list)

[“3”, “6”, “2008”]

two dimensional lists

contain other lists as their elements

• a.k.a. nested lists

• useful for working with multiple data sets

• two indexes are needed to process data

• typically use nested loops to process

tuples

immutable sequences that are very similar to lists

• format: tuple_name = (item1, item2)

• support operations as lists

• do not support append, remove, sort, reverse, or insert

How are tuples advantageous over lists?

they are safe and faster in processing

list()

converts tuple to list

tuple()

converts list to tuple

What is the output for this code?

data = (1, 2, 3)
a, b, c = data
print(b)

2

What is the output for this code?

numbers = [1, 2, 3, 4, 5]
a, *middle, c = numbers
print(a, middle, b)

1 [2, 3, 4] 5

What is the output for this code?

nums = (3, 5)
print(add(*nums))

8

list comprehensions

a concise way to create lists, offering a shorter syntax than loops

syntax: [expression for item in iterable]

What is the output for this code?

myString = "Hello"
myList = [c for c in myString]
print(myList)

[‘H’, ‘e’, ‘l’, ‘l’, ‘o’]

What is the output for this code?

squares = [x*x for x in range (0,10)]
print(squares)

[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]