Strings

Definition

• A string is a sequence of characters
• String literals can be created using single or double quotes

Valid Strings

"Hello, world"
'Hello, world'
""
''
"a"
'b'

Accessing Items

• A string is a sequence
• Items in the sequence can be accessed using square brackets []

Example

>>> name = "Jordan"
>>> name[0]
'J'
>>> name[2]
'r'

Immutability

• Strings in Python are immutable
• This means they cannot be changed

Example

>>> word = "cat"
>>> word[0]
'c'
>>> word[0] = "b"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

Creating a New String

• If we need to change a string we must create a new modified version
• This can be accomplished using string expressions

Looping

• Strings are iterable
• We can easily loop over the characters in a string using for

Example

dna = "GGTGTACGGC"

guanine_count = 0

for base in dna:
    if base == 'G':
        guanine_count += 1

print("Guanine bases:", guanine_count)

in

• The in operator can be used with strings
• An in expression will return true if the value on the left is contained within the value on the right

Example

>>> "a" in "cat"
True
>>> "b" in "cat"
False
>>> "bat" in "batman"
True
>>> "ant" in "batman"
False

String comparison

• Comparison operators are defined for strings
• == checks for equality as expected
• < and > are a little more complicated

Example

>>> "cat" == "cat"
True
>>> "cat" == "bat"
False
>>> "bat" == "batman"
False

Alphabetical order

• Generally, comparison operators can be used to confirm whether strings are in alphabetical order
• All uppercase letters come before all lowercase letter

Example

>>> "bat" < "cat"
True
>>> "cat" > "bat"
True
>>> "bat" < "Cat"
False
>>> "1" < "2"
True
>>> "12" < "102"
False

Create a program to print every other letter in a string

Create a program to find a word in a string and print the position of the word

Solution

sentence = "the quick brown fox jumped over the lazy dog"
word = "fox"

for sent_idx in range(0, len(sentence)):
    for word_idx in range(len(word)):
        if sentence[sent_idx + word_idx] != word[word_idx]:
            break
    else:
        print(sent_idx)