Kevin C. Tofel

03 - Python Control Flow and Loops

Python Control Flow and Loops

Mastering Control Flow in Python

Control flow is generally managed with loops and conditional statements. Loops help us define code that can run many times withough actually repeating the code.

Conditional Statements

  • If Statement: Checks if a condition is true and executes a specified block of code. For example, checking if a person's age is 18 or older to determine voting eligibility.
  • If-Else Statement: Used to evaluate a condition and execute one of two code blocks; ideal for either-or scenarios, such as determining pass/fail status based on an exam score threshold.
  • If-Elif-Else Statement: Evaluates multiple conditions in order, executing the block for the first true condition, with a default outcome set by the else. This is useful for assigning grades based on exam scores:
    • 90 or Above: Grade A
    • 80-89: Grade B
    • Below 80: Grade F

Multiple Conditional Statements

By using conditional operators (Not, And, Or) we can reduce redundancy when creating conditional statements.

Logical Operators:

  • AND: Combines multiple conditions; both must be true.
  • OR: One or more conditions must be true.
  • NOT: Inverts the Boolean value.

Practical Example - Weather Check

  • Temperature above 77°F:
    • If sunny: "Go to the beach."
    • Otherwise: "Stay inside."
  • Temperature between 50°F and 77°F:
    • If sunny: "Go for a walk."
    • Otherwise: "Stay inside."
  • Temperature 50°F or below:
    • "Stay inside."

Nesting Conditional Statements

Adds layers to decision making processes. Breaks down complex logic into manageable parts.

Creating structured code through logical operators and nested conditions simplifies complex decision trees and enhances human readability.

Example:

temperature = 51
is_sunny = True

if temperature > 77:
    if is_sunny:
        print("Go to the Beach")
    else:
        print("Stay inside")
elif temperature > 50:
    if is_sunny:
        print("Go for a walk")
    else:
        print("Stay inside")
else:
    print("Stay inside")

#Output: Go for a walk

Loops in Python

Purpose of Loops: Enable code to run multiple times seamlessly. Avoid repetitive coding, enhancing efficiency and scalability.

Types of Loops: While Loops: Continue executing as long as a specified condition is true. For Loops: Iterate over items in a sequence such as lists or strings.

Benefits: Useful when dealing with large datasets or numerous user interactions. Saves time and reduces human error with scalable code.

While loops are good for situations with an unknown number of iterations.

While loops

While loops are essential for executing code repeatedly until a specific condition becomes false. They are part of control structures in programming and are commonly used when the number of iterations isn't known in advance.

NOTE: Video showed how to read each line in a file until the end by using a while loop. It did not cover the details of read_line. the .strip method and with open() as input_file

While loops provide flexibility for scenarios where the loop run count isn't predetermined, such as waiting for user inputs or processing data until a file's end.

While loop pitfalls

  • Infinite loops
  • Off by one errors
  • Ensuring termination

Infinite loop example:

desired_temperature = 68
current_temperature = 66

while current_temperature < desired_temperature:
    print(f"Heating... Current temperature: {current_temperature}")
    current_temperature += 1  # Increase temperature by 1 degree each cycle

print("Desired temperature reached. Heating system turned off.")

Off by one example:

# Incorrect version: counts 0 through 5 (6 numbers instead of 5)
count = 0
while count <= 5:
    print(f"Count: {count}")
    count += 1

To ensure a while loop termination, there has to be some variable updated correctly to have a false condition.

For Loops

Good for when you know the number of iterations, particularly over a data structure; for each item in a collection or sequence.

Where do for loops make sense?

  • Sequences or ranges of numbers
  • Strings
  • Collections (lists (or arrays), tuples, and sets)

Example:

cart = ['apple', 'banana', 'cherry', 'grapefruit']
for item in cart:
    print(item)

Tuples: data is stored with parenthesis and is immutible.

Sets: A collection of unique, unordered and unindexed mutaable items in curly braces. However, any items added to a set are immutable.

Dictionaries: Key / value pairs of data. The key is an identifier and the values are data of any type.

  • For Loop with Range:
    • Creates a sequence of numbers, allowing iteration across a specified range.
    • Example: for i in range(5) prints numbers 0 through 4.
  • For Loop with Strings:
    • Iterates over each character in a string.
    • Example: for letter in "Python" prints each character of the word.
  • For Loop with Lists:
    • Iterates over items in a list.
    • Example: Lists of fruits allow each item to be printed: apple, banana, etc.
  • For Loop with Tuples:
    • Fine for handling immutable sequences of data.
    • Example: Prints coordinates stored in a tuple.
  • For Loop with Sets:
    • Iterates over unordered, unique items.
    • Ensures duplicates are ignored, printing only unique entries.
  • For Loop with Dictionaries:
    • Iterates over key-value pairs.
    • Example: Retrieves each piece of data within a dictionary structured like a person's profile.

Common challenges with For Loops

  • One by one errors where the loop runs one too many or one two few times
  • Editing loop variables
  • Modifying mutable sequences rather than iterating over a copy a of the sequence.