Strings

An ordered collection of characters accessed via index numbers.

Table of Contents

Type

str

Synatx

"char,..."

Bases

Sequence, Iterable

State

Immutable

Position

Ordered

Composition

Homogeneous

Diversity

Repeatable

Access

Subscriptable

Value

Hashable

Part 1: Introduction

While most of the time we can think of strings as text data, under the hood strings are a sequence of characters.

Part 1.1: Creating

Strings store text data and can be enclosed in either single or double quotes.

""
"Hello."
'Farewell.'
"It's a lovely day."
'"Well hello!", he said.'

'"Well hello!", he said.'

For a string that spans multiple lines, enclose it in triple double (""") or triple single (''') quotes.

rhyme = """Roses are red,
Violets are blue,
Sugar is sweet,
And so are you."""

print(rhyme)

Roses are red,
Violets are blue,
Sugar is sweet,
And so are you.

Backslashes (\) are used to escape quotes or to indicate special characters, like \n for a new line.

print('It\'s a nice day.')
print("line one\nline two")

It's a nice day.
line one
line two

For strings that contain backslashes use a double backslash (\\) or prefix the string with the letter r for a raw string.

print('"\\n" is used to add a newline')
print(r"C:\Documents\nodes")

"\n" is used to add a newline
C:\Documents\nodes

Part 1.2: Exercises

Exercise 58 (String Creation)

1. Use a docstring to split the lymric or lullaby of your choice into a multi-line string and assign it to the variable verse, then print it.

2. Using one double-string or single-quoted string, print the words "one", "two", and "three" on three seperate lines.

3. Print a double quoted string that includes double quotes. Print a single quoted string that includes single quotes.

4. Print a string that includes backslashes by escaping the backslash. Print a second string that includes backslashes using an r-string.

Solution to Exercise 58 (String Creation)

#1: Verse Docstring

Listing 185 String Creation Exercise #1

verse = """Jack and Jill
Went up the hill
To fetch a pail of water
Jack fell down and broke his crown
And Jill came tumbling after."""

print(verse)

#2: One, Two, Three lines, muah hah haa!

Listing 186 String Creation Exercise #2

print("one\ntwo\nthree")

#3: Quotable quotes

Listing 187 String Creation Exercise #3

print('...for he\'s a jolly good fellow, that nobody can deny!')
print("She muttered, \"Well, I for one can deny it.\"")

#4: Sometimes backslashes just want to be seen

Listing 188 String Creation Exercise #4

print('This is a backslash: "\\".')
print(r'So is this: "\".')

Part 1.2: Modifying

Strings are immutable, which means that once created, they cannot be changed. But it’s only the exact object stored in memory that can’t be changed. Most of the time, we don’t care so much about the exact object, just the end result.

What you can do is create a new string cobbled together from the parts of other strings, and you can even assign it back to the same variable name.

Strings can be concatenated (joined together) by using the + operator.

name = "coding class"
print("Welcome to " + name + ".")

Welcome to coding class.

String literals (the ones in quotes) next to each other are automatically concatenated.

text = "Today we're talking about" 'strings.'
print(text)

Today we're talking aboutstrings.

You can take advantage of this to break long text into multiple lines of code. Put strings on consecutive lines without separating commas and enclose the whole expression in parentheses ( ).

text = (
  "Lorem ipsum dolor sit amet, consectetur "
  "adipiscing elit, sed do eiusmod tempor "
  "incididunt ut labore et dolore magna "
  "aliqua. Ut enim ad minim veniam, quis "
  "nostrud exercitation ullamco laboris "
  "nisi ut aliquip ex ea commodo consequat."
)

print(text)

Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat.

To effectively append to an existing string, you can use the += operator.

text = "Unfinished "
text += "business."
print(text)

Unfinished business.

You can repeat a string multiple times using the * operator.

text = "The horror! " * 2
print(text)

The horror! The horror! 

Similarly you can use the *= operator to effectively append a value to itself a number of times.

text = "All work and no play makes Jack a dull boy.\n"
text *= 5
print(text)

All work and no play makes Jack a dull boy.
All work and no play makes Jack a dull boy.
All work and no play makes Jack a dull boy.
All work and no play makes Jack a dull boy.
All work and no play makes Jack a dull boy.

Use an f-string for string interpolation by prefixing the string with the letter f then enclose the variable or other evaluated code curly braces ({ }).

price = 1.25
count = 5

print(f"I am buying {count} apples for ${price} each.")
print(f"The total is ${price*count}.")

I am buying 5 apples for $1.25 each.
The total is $6.25.

Part 1.2: Exercise

Exercise 59 (String Modification)

1. Use the + operator to join two strings together and print the result.

2. Concatonate two string literals without using the + operator and print the result.

3. Choose a long paragraph of text. Break it up onto multiple lines in your code while still all on one line in the string itself. Assign it to the variable paragraph and print it.

4. Assign the string "Four out of five dentists agree: " to the varable text. Choose something the dentists agree on and on a new line, append it to text. Print text.

5. Use both the + and * operators to assign the string "Noooooo!" to the variable text then print it. Change the number of "o"s and print it again.

6. Pick something that Bart Simpson may have written on the blackboard and assign it to the variable admonition. Append it to itself 10 times then print it.
   Note: This should take no more than 2 lines of code. Bonus: Write a function to do this.

7. Choose a name and assign it to the variable name. Using an f-string, print:
   "Hello my name is: NAME."

8. Create a visual line by repeating the underscore character ("_") some number of times, (say, 20) and assign it to the variable line. Using an f-string, make a string with a single line of a sign-up sheet that looks something like this:

   "Name: ____________________   Email: ____________________"

   Assign it to the variable row and print it 25 times to generate a sign-up sheet.

Solution to Exercise 59 (String Modification)

#1: Concatonate with +

Listing 189 String Modification Exercise #1

text = "Hello " + "world!"
print(text)

#2: Concatonate without an operator

Listing 190 String Modification Exercise #2

text = "Goodbye " "cruel world!"
print(text)

#3: Paragraph concatonation

Listing 191 String Modification Exercise #3

paragraph = (
  "Quo usque tandem abutere, Catilina, "
  "patientia nostra? Quam diu etiam "
  "furor iste tuus nos eludet? Quem "
  "ad finem sese effrenata iactabit "
  "audacia?"
)
print(paragraph)

#4: Dentists conclude…

Listing 192 String Modification Exercise #4

text = "Four out of five dentists agree: "
text += "that other guy is nuts."
print(text)

#5a: Variously emphatic "No!"

Listing 193 String Modification Exercise #5a

text = "N" + ("o"*1) + "!"
print(text)

text = "N" + ("o"*3) + "!"
print(text)

text = "N" + ("o"*8) + "!"
print(text)

#5b: (Bonus) Efficient variously emphatic "No!"

Listing 194 String Modification Exercise #5b

def no(ohs):
  text = "N" + ("o"*ohs) + "!"
  print(text)

no(1)
no(3)
no(8)

#6: Chalkboard repetition

Listing 195 String Modification Exercise #6

admonition = "I will not repeat myself.\n"
admonition *= 10
print(admonition)

#7: Nametag

Listing 196 String Modification Exercise #7

name = "Inigo Montoya"
print(f"Hello my name is: {name}.")

#8: Sign-up Sheet

Listing 197 String Modification Exercise #8

line = "_" * 20
row = f"Name: {line}    Email: {line}\n"
print(row * 25)

Part 2: Splitting and Joining

We often want to split a string apart in various ways, or create a string from a list of strings. In other words, to convert a string to a list and vice versa.

Part 2.1: Converting to and from lists

The most common way to split a string is using the .split() method. With no arguments, it splits the string on any whitespace.

name = "Jean-Claude Van Damme"
name.split()

['Jean-Claude', 'Van', 'Damme']

If you pass an argument to .split(), it will use that character or substring as the delimiter, instead of whitespace.

phone = "555-867-5309"
phone.split("-")

['555', '867', '5309']

text = "one and two and three and four"
text.split(" and ")

['one', 'two', 'three', 'four']

Or if you want a list where each character is an element call list().

text = "apart"
list(text)

['a', 'p', 'a', 'r', 't']

To convert a list (or other iterable) to a string, use the str.join() method.

letters = ['t', 'o', 'g', 'e', 't', 'h', 'e', 'r']
"".join(letters)

'together'

shapes = ["triangle", "square", "circle"]
", ".join(shapes)

'triangle, square, circle'

Part 2.1: Exercises

Exercise 60 (Splitting and Joining Exercise)

1. Take the quote of your choice and assign it to the variable quote. Split it into a list of words (seperated by whitespace) and print the result.

2. Assign this string to the variable flavors:
   "banana split;hot fudge;cherry;malted;black and white"
   Split it into a list on the deliter ";".

3. Look up the word of the day and assign it to the variable word. Convert it to a list of characters and print it.

4. Make a list with every letter in the current day of the week and assign it to the variable day. Turn it into a string (join it using a blank string as the delimiter) and print it.

5. Make a list of three dinosaurs. Join it on a newline delimiter then print it.

Solution to Exercise 60 (Splitting and Joining Exercise)

#1: Quote words

Listing 198 Splitting and Joining Exercise #1

quote = "Life is what happens when you're busy making other plans."
print(quote.split())

#2: Flavors

Listing 199 Splitting and Joining Exercise #2

flavors = "banana split;hot fudge;cherry;malted;black and white"
print(flavors.split(";"))

#3: Letters of the Day

Listing 200 Splitting and Joining Exercise #3

word = "treacle"
print(list(word))

#4: Day Letters to Word

Listing 201 Splitting and Joining Exercise #4

day = ['T', 'u', 'e', 's', 'd', 'a', 'y']
print("".join(day))

#5: Dinosaurs

Listing 202 Splitting and Joining Exercise #5

dinosaurs = ["Tyrannosaurus Rex", "Stegosaurus", "Velociraptor"]
print("\n".join(dinosaurs))

Part 3: Strings as sequences

Under the hood a string is a sequence of characters. Let’s take a simple example.

word = "cat"

This could be visualized as:

Part 3.1: Sequence features

Since it is a sequence, that means you can do most of the same things you can do with a list.

You can use subscription to access characters via negative or positive index number.

print(word[0])
print(word[-1])

c
t

You can get part of a string via slice.

print(word[1:])

at

You can get the length using the len() function.

len(word)

3

You can check if a single character or a substring is part of a string using the in and not in operators.

"c" in word

True

"at" not in word

False

You can check how many times a letter or substring occurs in a string using the .count() method.

word.count("c")

1

word.count("at")

1

You can find out the index number of the first occurrence of a letter or substring.

word.index("c")

0

word.index("at")

1

You can iterate over every character.

for char in word:
    print(char)

c
a
t

However, since strings are immutable, you cannot modify a string via subscription.

word[0] = "b"

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[29], line 1
----> 1 word[0] = "b"

TypeError: 'str' object does not support item assignment

Part 3.2: Exercises

Exercise 61 (String as Sequence Exercise)

1. Make a list of words and assign it to the variable sentence. Iterate over the list and print the first letter in each word.

Solution to Exercise 61 (String as Sequence Exercise)

Listing 203 String as Sequence Exercise

sentence = ['The', 'quick', 'brown', 'fox', 'jumps', 'over', 'the', 'lazy', 'dog.']

for word in sentence:
    print(word[0])

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