Strings

Objectives

• Understand the nature and importance of strings as a fundamental data type in Python.
• Know how to create, manipulate, and format strings using various techniques.
• Be aware of common string methods and their applications.
• Know how to effectively index, slice, and find the length of strings.
• Understand the concept of string immutability.
• Gain practical experience through programming tasks related to strings.

Earlier we saw how Python works with variables and the data stored in the variable will be of a particular type of data i.e. integers (whole numbers), reals (numbers with a fractional part), characters (e.g. 'a', '5') and boolean (either true or false). In this section we look at strings.

A string is a list of characters, a character being anything that you type on the keyboard in one keystroke e.g. a letter, a number or a symbol, including Space.

Example 3:

Read the following program, and predict what the program will do.

greeting = "Hello, "
name = "Alice"
full_greeting = greeting + name
print("Full Greeting:", full_greeting)

shout = full_greeting.upper()
print("Shout:", shout)

whisper = full_greeting.lower()
print("Whisper:", whisper)

length = len(full_greeting)
print("Length of Full Greeting:", length)

first_letter = name[0]
print("First Letter of Name:", first_letter)

sliced_name = name[1:4]
print("Sliced Name:", sliced_name)

Pay particular attention to the highlighted lines. What will be displayed on the screen when these lines are executed?

Answers

• Full Greeting: Hello Alice
• Shout: HELLO, ALICE
• Whisper: hello, alice
• Length of Full Greeting: 12
• First Letter of Name:
• A Sliced Name: lic

Copy the code and paste into a code editor and run the code to compare your predictions with the actual results.

What are Strings?

• In Python, a string is a data type used to represent text.
• Text can be anything from a single character to a whole paragraph.
• Strings are a fundamental data type in programming and are used extensively in real-world applications.
• A string is a list of characters

Creating Strings

• Strings are created by enclosing text in either single quotes (') or double quotes (").
• For example:
• 'Hello, World!'
• "Python is fun"

Warning

The type of quotation mark used must be consistent e.g. 'Hello, World!" is incorrect, the quotes do not match

• to include a quotation mark within a string, use the other one! For example:

message = "I don't like Mondays!"

String Operations

Two of the operators we saw being used as mathematical operators are repurposed when applied to string operands:

Concatenation

• You can combine two or more strings using the concatenation operator (+).

• Example:

• 'Hello' + ' ' + 'World' results in 'Hello World'.

Repetition

• You can repeat a string multiple times using the repetition operator (*).
• Example:
• 'Python' * 3 results in 'PythonPythonPython'.

String Methods

• Python provides many built-in string methods that allow you to manipulate and work with strings effectively.
• Some common string methods include upper(), lower(), len(), strip(), split(), and replace().
• Example:
• "Python".upper() results in 'PYTHON'.
• When using these do not forget the brackets after the method name, these are function calls and must have the brackets
• To find characters in a string, use the find() method. This returns the index of the first occurrence of that character.
• For example:

course = "Python for beginners"
course.find('P')                    # returns 0
course.find('for')                  # returns 7
course.find('Beg')                  # returns -1, it is case sensitive

- To replace a character, or a sequence of characters use replace() e.g. course.replace('Python', 'Java') - To check for the existence of characters in a string, use print('Python' in course). This method will returns a boolean value and line find() is case-sensitive.

String Indexing

• Each character in a string has a unique index starting from 0.
• You can access individual characters in a string using square brackets and the index.
• Example:
• "Python"[0] returns 'P'.
• "Python"[2] returns 't'.

String Slicing

• String slicing allows you to extract a portion of a string.
• It is done by specifying a range of indices inside square brackets.
• For example:

course = "Python"
print(course[0])            # 'P'
print(course[-1])           # 'n'
print(course[0:3])          # 'Pyt' nb. does not print course[3] the 4th char
print(course[0:])           # all the characters!
P
n
Pyt
Python

String Length

• You can find the length of a string using the len() function.
• Example:
• len("Hello, World!") returns 13.

Escape Characters

• Some characters have special meanings in strings, such as newline (\n) and tab (\t).
• To include these characters in a string, you need to use escape sequences.
• Example:
• "Hello\nWorld" creates a string with a newline character.

Multiline Strings

• You can create multiline strings by enclosing them in triple quotes (''' or """).
• Useful for long text or preserving the formatting of text.
• Example:

multiline_text = '''
This is a multiline
string in Python.
'''

Printing strings

Python provides three different ways to print the contents of the string to the screen:

Compare the following:

first = "Imran"
second = "Khalisa"
print(first + second)       # concatenation operator
print(first, second)        # comma operator, inserts a space between items
print(f"{first} {second}")  # f-string

It makes little different which you choose to use but notice the concatenation operator does not insert a space between items being printed.

String Immutability

• Strings in Python are immutable, which means their values cannot be changed after creation.
• Any operation that seems to modify a string actually creates a new string.
• Example:
• text = "Hello"
• new_text = text + ", World!"

Activity

Modify the given example by:

• Changing the variable values and observe the output.
• Adding more string manipulations using different methods.
• Experimenting with slicing to extract different parts of the string.

Climate Quest Project

Throughout this topic we'll be working on a large scale project: Climate Quest. In this project a player embarks on a journey to combat the effects of climate change by making decisions that impact the environment. Each choice affects the outcome of the game, emphasizing the importance of individual actions in addressing climate change.

Go to task 3 - String Handling

Questions

1. What is the primary purpose of strings in Python?

To represent mathematical calculations

To store and manipulate textual data

To create graphical user interfaces

To perform logical operations

Strings in Python are used to store and manipulate sequences of characters, such as words or sentences. They are fundamental for handling text in programs.

Submit

2. How do you create a string in Python?

By enclosing text in single quotes

By enclosing the string in backticks

By using the str() function

By using square brackets

In Python, a string can be created by enclosing text in either single quotes (') or double quotes ("). For example, 'Hello' and "World" are both valid strings.

Submit

3. Which of the following string operations is used to combine two or more strings?

Division

Subtraction

Concatenation

Multiplication

Concatenation is the operation used to join or combine two or more strings into one. In Python, this is done using the + operator, like "Hello" + "World" resulting in "HelloWorld".

Submit

4. What is the result of Python * 3?

PythonPython

PythonPythonPython

Python3

9

In Python, multiplying a string by an integer repeats the string that many times. Therefore, "Python" * 3 results in "PythonPythonPython".

Submit

5. Which Python method is used to convert a string to uppercase?

upper()

lower()

length()

find()

The upper() method in Python converts all characters in a string to uppercase. For example, "hello".upper() results in "HELLO".

Submit

6. What does the find() method return if the specified character or substring is not found in the string?

True

False

-1

0

The find() method searches for a substring within a string and returns the index of the first occurrence. If the substring is not found, it returns -1.

Submit

7. How do you access the first character of a string named text?

text[0]

text[1]

text[-1]

text(0)

In Python, strings are indexed starting from 0. To access the first character of a string text, you use text[0].

Submit

8. What does string slicing allow you to do?

Combine two strings

Repeat a string

Extract a portion of a string

Convert a string to uppercase

String slicing in Python allows you to extract a substring from a string by specifying a start and end index. For example, text[1:4] extracts characters from index 1 to 3.

Submit

9. What is the length of the string 'Hello, World!'?

11

12

13

14

The string 'Hello, World!' has 14 characters, including the punctuation and space. The len() function in Python can be used to determine the length of a string.

Submit

10. How do you include a newline character in a string?

Using \n

Using \t

Using \

Using #

The escape sequence is used in Python to include a newline character in a string, causing the text to continue on the next line when printed.

Submit

Programming Tasks

For each of the following tasks write a program using Python.

Greeting a friend

Write a Python program that asks the user to enter their first name and last name separately. Then, print a formatted message such as 'Hello, [First Name] [Last Name]!', replacing `[First Name]` and `[Last Name]` with the data entered by the user.

Read more ...

Programming is fun

Given the string 'Python programming is fun' write a program that converts and displays to screen the string in (a) upper case, (b) lower case and (c) splits the string into individual words

Read more ...

Splitting strings

Given the string 'Python programming is fun' use string slicing to extract the following words from the string: (a) 'gram' (b) 'Python' (c) 'program'

Read more ...

Word Count

Write a Python program that takes a user's input as a sentence and counts the number of words in that sentence. Words are separated by spaces. Print the word count. (For this exercise look up the `split()` method.)

Read more ...