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Strings and Lists

Strings and lists are two of the most common types of data you work with in Python. Strings enable you to represent text, and lists enable you to manipulate collections of data, be it lists of numbers, lists of strings, or even lists of lists! While they serve very different purposes, these two types also have a lot in common. Specifically, they are both sequences. Indeed, we can think of strings as a sequence of individual letters.

To help you learn both the common and unique features of strings and lists, we give examples here of the most common ways you’ll use strings and lists in Lab 3 and beyond.

Creating Strings and Lists

StringsLists 
message = 'Hello!'
message

'Hello!'

numbers = [ 1, 2, 3 ]
numbers

[1, 2, 3]

wordle = 'happy'
wordle

'happy'

words = [ 'moo', 'cow', 'woof' ]
words

['moo', 'cow', 'woof']

nothing = ''   # The empty string
nothing

''

noElems = [ ]    # The empty list
noElems

[]

Lengths of Strings and Lists

StringsLists 
wordle = 'happy'
len(wordle)

5

numbers = [ 1, 2, 3 ]
len(numbers)

3

Combining Strings and Lists

StringsLists 
message = 'Hel' + 'lo!'
message

'Hello!'

numbers = [ 1, 2, 3 ] + [ 4, 5, 6 ]
numbers

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

wordle = 'happy'
doubleWordle = wordle + ' and ' + wordle
doubleWordle

'happy and happy'

words = [ 'moo', 'cow', 'woof' ]
manyWords = words + words
manyWords

['moo', 'cow', 'woof', 'moo', 'cow', 'woof']

Appending to a String or List

StringsLists 
message = 'Hello'
message += '!'
message

'Hello!'

numbers = [ 1, 2, 3 ]
numbers += [ 4 ]
numbers

[1, 2, 3, 4]

numbers = [ 1, 2, 3 ]
numbers.append(4)  # alternate way to append
numbers

[1, 2, 3, 4]

What’s in a String or List?

StringsLists 
message = 'Hello'
'l' in message

True

numbers = [ 1, 2, 3 ]
3 in numbers

True

message = 'Hello'
'x' in message

False

numbers = [ 1, 2, 3 ]
33 in numbers

False

message = 'Hello'
'y' not in message

True

numbers = [ 1, 2, 3 ]
3 not in numbers

False

Iterating over Strings and Lists

StringsLists 
message = 'Hello'
for letter in message:
  print(letter)

H
e
l
l
o

numbers = [ 1, 2, 3 ]
for number in numbers:
  print(number)

1
2
3

# Build a new list containing all the vowels 
# from `message`.

message = 'Hello'
vowels = ''

for letter in message:
  if letter in [ 'a', 'e', 'i', 'o', 'u']:
    vowels += letter

vowels

'eo'

# Build a new list containing the square 
# of all odd numbers in `numbers`.

numbers = [ 1, 2, 3, 4, 5, 6 ]
squares = [ ]

for number in numbers:
  if number % 2 == 0:
    square = number * number
    squares += [ square ]

squares

[4, 16, 36]

Accessing Elements with []

StringsLists 
message = 'Hello'
message[0]

'H'

numbers = [ 5, 6, 7 ]
numbers[0]

5

message = 'Hello'
message[4]

'o'

numbers = [ 5, 6, 7 ]
numbers[2]

7

message = 'Hello'
message[-1]

'o'

numbers = [ 5, 6, 7 ]
numbers[-1]

7

Slicing Strings and Lists

StringsLists 
message = 'Hello'
message[1:3]

'el'

numbers = [ 5, 6, 7, 8 ]
numbers[0:2]

[5, 6]

message = 'Hello'
message[1:]

'ello'

numbers = [ 5, 6, 7, 8 ]
numbers[:2]

[5, 6]

message = 'abcdefghij'
message[0::2]

'acegi'

numbers = [ 5, 6, 7, 8, 9, 10 ]
numbers[1:4:2]  

[6, 8]

Handy String Operations

str.lower(): Convert to Lowercase 
message = 'Hello'
message.lower()

'hello'
str.upper(): Convert to Uppercase 
message = 'Hello'
message.upper()

'HELLO'
str.islower(): Test if a String is All Lowercase 
message = 'hello'
message.islower()

True

message = 'Hello'
message.islower()

False
str.isupper(): Test if a String is All Uppercase 
message = 'Hello'
message.isupper()

False

message = 'HELLO'
message.isupper()

True
==: Test if Two Strings Are the Same 
message = 'Hello'
message == 'Hello'

True

message = 'Hello'
other = 'Bye'
message == other

False
!=: Test if Two Strings Are Different 
message = 'Hello'
message != 'Hello'

False

message = 'Hello'
other = 'Bye'
message != other

True
str.isalpha(): Test if a String Only Contains Letters from Alphabet 
message = 'Hello'
message.isalpha()

True

message = 'Hello22'
message.isalpha()

False