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Python: Variables

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Variables

  • Storage containers for data (numbers, text, etc.).

What is a variable

  • A variable is a named storage location in a computer’s memory that is used to hold data or values. It allows programmers to store and manipulate data within a program.

Purpose: Variables provide a way to store and manage data that can be used and manipulated throughout a program. They make programs more flexible and allow for dynamic data storage.

Assignment statement: in Python is used to assign a value to a variable. Its primary purpose is to store and manage data within a program.

Imagine variables as labeled boxes:

  • You have boxes for storing different things (numbers, words, etc.).
  • Each box has a name (label) to identify what’s inside.
  • You can put things in, take them out, and change what’s inside.

Variable names

In Python, valid variable names must adhere to the following rules:

  • Begin with a letter or an underscore: The first character of a variable name must be a letter (a-z, A-Z) or an underscore (_).

  • Followed by letters, digits, or underscores: After the first character, the variable name can contain letters, digits (0-9), or underscores.

  • Case-sensitive: Variable names are case-sensitive. For example, myVariable and myvariable would be considered different variables.

  • No reserved keywords: Variable names cannot be Python reserved keywords (e.g., if, for, while, class, etc.).

video:Function and Variable Naming |Python Best Practices video:Python Variables and Assignment video: Meaningful Variable Names | Python Best Practices video: Asterisk (*) in Variable Assignment

Example #1: Storing a name

name = "Muhammad Hamza"
print(name)

Example #2: Tracking a score:

score = 0
score = score + 10 # adds 10 to the score
print(score)

Example #3: Remembering a favorite color

favorite_color = "blue" #stores "blue"  in variable
print(favorite_color)

Example #4: Calculating the area of a rectangle

length = 10
width = 5

# calculates the area
area = length * width
print(area)

Example 5: How to assign multiple values to multiple variables? Example 6: How to Swap Variables Without a Third Variable in Python Example 7: Calculate the Area of a Circle with Radius Example 8: Python Variable Names: Case-Sensitive? Avoid This Coding Mistake!

Key Points:

  • Choose meaningful names: Use names that describe what the variable stores (e.g., pizza_slices instead of x).
  • Assign values using =: The equals sign is used to put a value into a variable.
  • Change values: You can update a variable’s value later in your code.
  • Use variables in calculations and operations: Variables can be used just like regular numbers or text in expressions.
  • Think of variables as placeholders: They hold information that can change as your program runs.

Key Terms

Fix the Errors!

Using an undefined variable

name = "Ahmad"
print(f"Hello, {lastname}")  # lastname not defined

True/False (Mark T for True and F for False)

  1. Variable names in Python are case-sensitive.
  2. In Python, variables must be declared with a specific data type before they can be used.
  3. The statement x = 5 both creates the variable x and assigns it the value 5.

Answer Key (True/False):

  1. True
  2. False
  3. True

Multiple Choice (Select the best answer)

  1. What is a variable in Python?
    • A) A reserved word in Python
    • B) A placeholder for storing data values
    • C) A function that prints data
    • D) A built-in library in Python
  2. Which statement best describes a variable in Python?
    • A) A variable can hold multiple values at once.
    • B) A variable must be declared with a data type.
    • C) A variable is a name that refers to a value.
    • D) A variable is used only in loops.

  3. What is the output of the following code?

    x = 10
print(x)
    • A) 10
    • B) x
    • C) Error
    • D) None
  4. Which of the following is not true about variables in Python?
    • A) Variables can be reassigned to different data types.
    • B) Variables must start with a letter or an underscore.
    • C) Variables are case-sensitive.
    • D) Variables must be declared before use.

  5. What will be the output of the following code? [Python Quiz #76]

    x = 5
y = x
x = 7
print(y)
    • A) 7
    • B) 5
    • C) 0
    • D) None
  6. Why is it important to use meaningful variable names?
    • A) It is required by the Python interpreter.
    • B) It helps make the code more readable and maintainable.
    • C) It increases the execution speed of the program.
    • D) It is necessary for the code to run without errors.

  7. What will be the output of the following code? [Python Quiz #77]

    a = 1
b = a
a = a + 1
print(a, b)
    • A) 1 1
    • B) 2 1
    • C) 1 2
    • D) 2 2
  8. Which of the following is a valid variable name in Python?
    • A) 2ndValue
    • B) value#2
    • C) _value2
    • D) value-2
  9. Which of the following is a correct way to declare a variable in Python?
    • A) int x = 5
    • B) x = 5
    • C) declare x = 5
    • D) var x = 5
  10. What is the output of the following code? [Python Quiz 78] 
       x = 5
   y = "Hello"
   print(x + y)
    • A) 5Hello
    • B) Hello5
    • C) TypeError
    • D) Hello 5
  11. Which of the following is not a valid variable name in Python?
    • A) my_var
    • B) _var
    • C) 2var
    • D) var2
  12. Which of the following statements is true about variable assignment in Python?
    • A) Variables must be declared before they are assigned a value.
    • B) Variables are created when they are first assigned a value.
    • C) Variable names must begin with a number.
    • D) Python variables must be declared with a type.

38 What is the purpose of declaring a variable in Python?

a) To reserve memory space for the variable b) To give the variable a name c) To initialize the variable with a value d) All of the above Answer: d

Answer key (Mutiple Choice):

  1. B) A placeholder for storing data values
  2. C) A variable is a name that refers to a value.
  3. A) 10
  4. D) Variables must be declared before use.
  5. B) 5
  6. B) It helps make the code more readable and maintainable.
  7. B) 2 1
  8. C) _value2
  9. B) x = 5
  10. C) TypeError
    • Explanation: In Python, the + operator is used for both arithmetic addition and string concatenation. However, it cannot be used to add an integer and a string directly. The code provided attempts to add an integer (x = 5) to a string (y = “Hello”), which is not a valid operation and will result in a TypeError.
  11. C) 2var
    • Explanation: In Python, variable names must start with a letter or an underscore and cannot start with a number. Thus, my_var, _var, and var2 are valid, but 2var is not.
  12. B) Variables are created when they are first assigned a value.
  13. A) 5

Fill in the Blanks

  1. Variable names in Python must start with a letter or an ____.
  2. Variables in Python are ____, meaning they can change type when assigned a new value.
  3. The assignment operator in Python is the ____ symbol.

Answer Key (Fill in the Blanks):

  1. underscore (_)
  2. dynamic
  3. equals (=)

Exercises

Exercise 1: Basic Variable Assignment

  1. Create a variable called name and assign your name to it.
  2. Create a variable called age and assign your age to it.
  3. Create a variable called city and assign the city you live in to it.
  4. Print all three variables.

Exercise 2: Variable Reassignment

  1. Create a variable called favorite_color and assign your favorite color to it.
  2. Print the value of favorite_color.
  3. Reassign a new color to favorite_color.
  4. Print the new value of favorite_color.

Exercise 3: Variable Operations

  1. Create two variables called a and b and assign them the values 5 and 10, respectively.
  2. Create a new variable called sum and assign it the value of a plus b.
  3. Create a new variable called difference and assign it the value of a minus b.
  4. Create a new variable called product and assign it the value of a times b.
  5. Print the values of sum, difference, and product.

Exercise 4: String Concatenation

  1. Create a variable called first_name and assign your first name to it.
  2. Create a variable called last_name and assign your last name to it.
  3. Create a new variable called full_name and assign it the value of first_name concatenated with last_name (with a space in between).
  4. Print the value of full_name.

Example Solution:

first_name = "Alice"
last_name = "Johnson"

full_name = first_name + " " + last_name
print(full_name)

Exercise 5: Input and Variables

  1. Use the input() function to get the user’s name and store it in a variable called user_name.
  2. Use the input() function to get the user’s age and store it in a variable called user_age.
  3. Print a message saying “Hello [user_name], you are [user_age] years old.”

Example Solution:

user_name = input("Enter your name: ")
user_age = input("Enter your age: ")

print("Hello", user_name + ", you are", user_age, "years old.")
  1. Calculate the Area of a Circle with Radius Example Solution
  2. How to Swap Variables Without a Third Variable in Python. Example Solution
  3. How to assign multiple values to multiple variables. Example Solution

Review Questions

1. What is a variable in computer programming? Answer: A variable is a named storage location in a computer’s memory that is used to hold data or values. It allows programmers to store and manipulate data within a program.

2. What is the purpose of using variables in programming? Answer: Variables provide a way to store and manage data that can be used and manipulated throughout a program. They make programs more flexible and allow for dynamic data storage.

3. What is the difference between declaring and initializing a variable? Answer: Declaring a variable involves specifying its name and data type, while initializing a variable means giving it an initial value. Initialization usually follows declaration but is not always required.

  1. What is a variable in computer programming? Give examples of integer, string, float, and Boolean variables.

References and Bibliography