Looping Through Sequences

1. The Need for Repetition: Introducing the for Loop

Imagine you have a list of numbers and you want to print each one on a new line. Based on what you learned, you might write this:

my_numbers = [10, 20, 30, 40]

print(my_numbers[0])
print(my_numbers[1])
print(my_numbers[2])
print(my_numbers[3])

This works, but it's repetitive and clumsy. What if the list had 100 numbers? This approach is not scalable. This is where loops come in. A for loop lets you execute a block of code once for each item in a sequence.

The article below gives a great introduction to why loops are so useful.

The basic structure, or syntax, of a for loop in Python is simple and readable:

for variable_name in iterable:



    # Code to execute for each item
    # This block must be indented

• for and in are keywords.
• iterable is the sequence you want to go through (like a list or a string).
• variable_name is a temporary variable that holds the current item from the iterable in each pass of the loop. You can name it anything you like, but it's best to choose a descriptive name.

The concept of an "iterable" is central here. For now, you can think of it as any object that contains a sequence of items. Let's get a more formal definition.

This flowchart visualizes how a for loop works. It checks if there's a next item, processes it, and repeats until the sequence is finished.

2. Iterating Over Common Sequences

The for loop works consistently across different types of sequences. Let's see it in action.

First, watch this short video for a demonstration of looping through a list of numbers.

As you saw, the loop variable num holds 1, then 2, then 3, and so on, until the list nums is exhausted. This same principle applies to other sequences you've encountered.

• Lists: You can iterate over any list, no matter what data types it contains.
• Strings: A for loop will iterate over each character in the string.
• Tuples: Tuples are ordered collections just like lists, but they are immutable (meaning they cannot be changed after creation). You create them with parentheses (). For looping purposes, they behave exactly like lists.

The following reading expands on this with clear examples for each type.

Here are some code examples to solidify your understanding. Try running them yourself!

# Iterating over a list of strings
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like to eat {fruit}s.")

print("---")




# Iterating over a string
my_name = "Alex"
for char in my_name:
    print(char)

print("---")




# Iterating over a tuple of numbers
point = (10, 20, 30) # A 3D coordinate
for coordinate in point:
    print(f"Coordinate value: {coordinate}")

3. Repeating a Specific Number of Times with range()

Sometimes you don't have a sequence to loop over, but you want to repeat an action a specific number of times. Python's built-in range() function is perfect for this. It generates a sequence of numbers that you can use in a for loop.

The range() function can be used in a few ways:

1. range(stop): Generates numbers from 0 up to (but not including) stop.
2. range(start, stop): Generates numbers from start up to stop - 1.
3. range(start, stop, step): Generates numbers from start to stop - 1, incrementing by step.

Let's watch a quick video to see how this works.

Here are examples of the three forms of range():

# Example 1: range(stop)
print("Countdown:")
for i in range(3):
    print(i) # Prints 0, 1, 2

print("---")




# Example 2: range(start, stop)
print("Numbers from 5 to 7:")
for i in range(5, 8):
    print(i) # Prints 5, 6, 7

print("---")




# Example 3: range(start, stop, step)
print("Even numbers under 10:")
for i in range(0, 10, 2):
    print(i) # Prints 0, 2, 4, 6, 8

4. Nested Loops: A Loop Inside a Loop

You can place one loop inside another. This is called a nested loop. It's useful for working with 2D data (like a grid or a matrix) or for generating combinations of items from two different sequences.

For every single iteration of the "outer loop", the "inner loop" will run completely from start to finish.

This video shows a simple example of generating pairs.

A common use case is iterating through a list of lists, like a tic-tac-toe board:

board = [
    ['X', 'O', 'X'],
    ['O', 'X', 'O'],
    ['O', 'O', 'X']
]




# The outer loop gets each row
for row in board:
    print(f"Processing row: {row}")



    # The inner loop gets each cell in the current row
    for cell in row:
        print(f"  Cell value: {cell}")

5. Practice Challenge: Data Analysis

Let's combine everything you've learned: loops, conditionals, and list methods. You are given a list of daily temperature readings for a week. Your task is to perform a simple analysis.

Given data:
temperatures = [25.2, 18.3, 29.5, 15.0, 32.1, 12.8, 20.0]

Your tasks:

1. Calculate the average temperature:

   • Initialize a variable total_temp to 0.0.
   • Use a for loop to iterate through the temperatures list and add each temperature to total_temp.
   • After the loop, calculate the average by dividing total_temp by the number of days (you can use len(temperatures) for this).
   • Print the average temperature.

2. Identify the hot days:

   • Create a new, empty list called hot_days.
   • Use another for loop to iterate through the temperatures list.
   • Inside the loop, use an if statement to check if the temperature is greater than 25.0.
   • If it is, use the .append() method to add that temperature to your hot_days list.
   • After the loop, print the hot_days list.

Write your code in a Python file and run it to see the results.

temperatures = [25.2, 18.3, 29.5, 15.0, 32.1, 12.8, 20.0]




# Task 1: Calculate the average temperature
total_temp = 0.0
for temp in temperatures:
    total_temp += temp  # This is a shorthand for total_temp = total_temp + temp

average_temp = total_temp / len(temperatures)
print(f"The average temperature is: {average_temp:.2f}") # Formats to 2 decimal places

print("---")




# Task 2: Identify the hot days
hot_days = []
for temp in temperatures:
    if temp > 25.0:
        hot_days.append(temp)

print(f"The hot days were: {hot_days}")




# Expected Output:
# The average temperature is: 23.27
# ---
# The hot days were: [25.2, 29.5, 32.1]

Conclusion

Great job! You've mastered iteration, a cornerstone of programming. You can now automatically process every item in a sequence, which is essential for nearly every algorithm you'll write.

Key Takeaways:

• for loops are used to repeat a block of code for each item in an iterable (like a list, string, or tuple).
• The syntax is for item in iterable:. The item variable holds the current element in each iteration.
• The range() function generates a sequence of numbers, allowing you to loop a specific number of times.
• Nested loops (a loop inside another loop) are used to process 2D data structures or create combinations.
• You can combine loops with if statements to selectively process items in a sequence.

The practice challenge involved creating a new list based on a condition applied to an existing list. This is a very common pattern. In the next lesson, we will explore list comprehensions, a powerful and "Pythonic" shorthand for creating new lists from existing ones, often in a single, readable line of code.