Python Control Structures

Introduction

In this lab, you will explore fundamental Python control structures: conditional statements and loops. Building upon your knowledge from previous labs, you will learn how to control the flow of your programs using if-else statements, for loops, and while loops. This hands-on experience will deepen your understanding of Python's core concepts and prepare you for writing more complex and dynamic programs.

Skills Graph

Understanding Conditional Statements

In this step, you will learn about conditional statements in Python, specifically the if-else structure.

1. Open the Python interpreter by typing the following command in your terminal:

   python

   You should see the Python prompt (>>>), indicating that you're now in the Python interactive shell.

   

2. Let's start with a simple if statement. In the Python interpreter, type the following:

   >>> age = 20
   >>> if age >= 18:
   ...     print("You are an adult.")
   ...
   You are an adult.

   Tips 1: Becareful with the indentation, it's important in Python. Using four spaces for indentation is recommended.

   Tips 2: The >>> prompt indicates the start of a new line, and the ... prompt indicates a continuation of the previous line. You should not type these prompts; they are automatically displayed by the Python interpreter.

   Tips 3: You can press the Enter key to move to the next line or continue typing the code on the same line. Double Enter to execute the code.

   The if statement checks if the condition age >= 18 is true. If it is, the indented code block is executed.

3. Now, let's add an else clause:

   >>> age = 15
   >>> if age >= 18:
   ...     print("You are an adult.")
   ... else:
   ...     print("You are a minor.")
   ...
   You are a minor.

   The else clause provides an alternative action when the condition is false.

4. For more complex conditions, we can use elif (else if) clauses:

   >>> age = 65
   >>> if age < 13:
   ...     print("You are a child.")
   ... elif age < 20:
   ...     print("You are a teenager.")
   ... elif age < 65:
   ...     print("You are an adult.")
   ... else:
   ...     print("You are a senior citizen.")
   ...
   You are a senior citizen.

   The elif clauses allow you to check multiple conditions in sequence.

Remember, indentation is crucial in Python. It defines the code blocks associated with each condition.

Exploring For Loops

In this step, you will learn about for loops, which are used to iterate over sequences (like lists, strings, or ranges) in Python.

1. Let's start with a simple for loop using a range. In the Python interpreter, type:

   >>> for i in range(5):
   ...     print(i)
   ...
   0
   1
   2
   3
   4

   The range(5) function generates a sequence of numbers from 0 to 4, and the loop iterates over each number.

2. range() can take multiple arguments to specify the start, end, and step values. Let's try a different range:

   >>> for i in range(1, 10, 2):
   ...     print(i)
   ...
   1
   3
   5
   7
   9

   • The range(1, 10, 2) function generates a sequence of numbers starting from 1, up to (but not including) 10, with a step of 2.

3. Now, let's iterate over a list:

   >>> fruits = ["apple", "banana", "cherry"]
   >>> for fruit in fruits:
   ...     print(f"I like {fruit}")
   ...
   I like apple
   I like banana
   I like cherry

   Here, the loop iterates over each item in the fruits list.

4. You can also use for loops with strings:

   >>> for char in "Python":
   ...     print(char.upper())
   ...
   P
   Y
   T
   H
   O
   N

   This loop iterates over each character in the string "Python".

5. Let's combine a for loop with conditional statements:

   >>> numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
   >>> for num in numbers:
   ...     if num % 2 == 0:
   ...         print(f"{num} is even")
   ...     else:
   ...         print(f"{num} is odd")
   ...
   1 is odd
   2 is even
   3 is odd
   4 is even
   5 is odd
   6 is even
   7 is odd
   8 is even
   9 is odd
   10 is even

   This loop checks each number in the list and prints whether it's even or odd.

Understanding While Loops

In this step, you will learn about while loops, which are used to repeat a block of code as long as a condition is true.

1. Let's start with a simple while loop. In the Python interpreter, type:

   >>> count = 0
   >>> while count < 5:
   ...     print(count)
   ...     count += 1
   ...
   0
   1
   2
   3
   4

   This loop continues to execute as long as count is less than 5.

2. While loops are often used when you don't know in advance how many times you need to iterate. Here's an example:

   >>> import random
   >>> number = random.randint(1, 10)
   >>> guess = 0
   >>> while guess != number:
   ...     guess = int(input("Guess a number between 1 and 10: "))
   ...     if guess < number:
   ...         print("Too low!")
   ...     elif guess > number:
   ...         print("Too high!")
   ... print(f"Congratulations! You guessed the number {number}!")

   This creates a simple guessing game where the user keeps guessing until they get the correct number.

3. Be careful with while loops - if the condition never becomes false, you'll create an infinite loop. Let's see an example (but don't actually run this):

   >>> while True:
   ...     print("This will print forever!")

   If you accidentally run an infinite loop, you can stop it by pressing Ctrl+C.

4. You can use a break statement to exit a loop prematurely:

   >>> count = 0
   >>> while True:
   ...     print(count)
   ...     count += 1
   ...     if count >= 5:
   ...         break
   ...
   0
   1
   2
   3
   4

   This loop would normally run forever, but the break statement exits it when count reaches 5.

Nested Loops and Loop Control Statements

In this step, you will learn about nested loops and additional loop control statements.

1. Nested loops are loops inside other loops. Here's an example of nested for loops:

   >>> for i in range(3):
   ...     for j in range(2):
   ...         print(f"i: {i}, j: {j}")
   ...
   i: 0, j: 0
   i: 0, j: 1
   i: 1, j: 0
   i: 1, j: 1
   i: 2, j: 0
   i: 2, j: 1

   The inner loop completes all its iterations for each iteration of the outer loop.

2. In addition to break, Python provides the continue statement, which skips the rest of the current iteration and moves to the next one:

   >>> for num in range(10):
   ...     if num % 2 == 0:
   ...         continue
   ...     print(num)
   ...
   1
   3
   5
   7
   9

   This loop only prints odd numbers, skipping the even ones.

3. You can use else clauses with loops. The else block is executed if the loop completes normally (without encountering a break):

   >>> for num in range(2, 10):
   ...     for i in range(2, num):
   ...         if num % i == 0:
   ...             print(f"{num} is not prime")
   ...             break
   ...     else:
   ...         print(f"{num} is prime")
   ...
   2 is prime
   3 is prime
   4 is not prime
   5 is prime
   6 is not prime
   7 is prime
   8 is not prime
   9 is not prime

   This nested loop checks for prime numbers. The else clause is executed when a number is prime.

Putting It All Together

In this final step, you will create a simple program that utilizes the control structures you've learned in this lab.

1. Exit the Python interpreter by typing exit() or pressing Ctrl+D.

2. Open the WebIDE in the LabEx VM environment.

   

3. Create a new file named number_analyzer.py in the ~/project directory using the following command:

   touch ~/project/number_analyzer.py

4. Open the newly created file in the WebIDE editor.

5. Copy and paste the following code into the file:

   def analyze_numbers():
       numbers = []
       while True:
           user_input = input("Enter a number (or 'done' to finish): ")
           if user_input.lower() == 'done':
               break
           try:
               number = float(user_input)
               numbers.append(number)
           except ValueError:
               print("Invalid input. Please enter a number or 'done'.")
   
       if not numbers:
           print("No numbers entered.")
           return
   
       total = sum(numbers)
       average = total / len(numbers)
       maximum = max(numbers)
       minimum = min(numbers)
   
       print(f"\nAnalysis of {len(numbers)} numbers:")
       print(f"Total: {total}")
       print(f"Average: {average:.2f}")
       print(f"Maximum: {maximum}")
       print(f"Minimum: {minimum}")
   
       print("\nNumber distribution:")
       for num in numbers:
           if num < average:
               print(f"{num} is below average")
           elif num > average:
               print(f"{num} is above average")
           else:
               print(f"{num} is equal to average")
   
   if __name__ == "__main__":
       analyze_numbers()

   This program demonstrates the use of while loops, for loops, conditional statements, and exception handling.

6. Save the file (auto-save is enabled) and run it using the following command in the terminal:

   python ~/project/number_analyzer.py

7. Enter some numbers when prompted, then type 'done' to see the analysis. You should see output similar to this:

   Enter a number (or 'done' to finish): 10
   Enter a number (or 'done' to finish): 20
   Enter a number (or 'done' to finish): 30
   Enter a number (or 'done' to finish): 40
   Enter a number (or 'done' to finish): done
   
   Analysis of 4 numbers:
   Total: 100.0
   Average: 25.00
   Maximum: 40.0
   Minimum: 10.0
   
   Number distribution:
   10.0 is below average
   20.0 is below average
   30.0 is above average
   40.0 is above average

If you make a mistake while entering data, you can run the program again. This is a good opportunity to practice running Python scripts multiple times with different inputs.

Summary

In this lab, you have explored fundamental Python control structures: conditional statements (if-else), for loops, and while loops. You have learned how to control the flow of your programs, make decisions based on conditions, and iterate over sequences of data. You have also practiced using nested loops and loop control statements like break and continue.

These control structures form the backbone of Python programming, allowing you to create more complex and dynamic programs. You've seen how these concepts can be combined to create a useful program that analyzes a set of numbers input by the user.

As you continue your Python journey, you'll find these control structures essential in solving a wide variety of programming problems. Remember to practice these concepts regularly, experimenting with different combinations and use cases to reinforce your understanding.