Nested Control Structures

You've learned how to make your programs execute code conditionally using if, elif, and else, and how to repeat actions using while and for loops. The real power comes when you combine these structures. You can place control flow statements inside other control flow statements. This technique is called nesting, and it allows you to build more sophisticated logic into your programs.

Imagine you need to check multiple conditions before performing an action, or you need to perform a repetitive task for each item processed in another repetition. Nesting makes this possible.

Nested Conditional Statements

You can place an if, elif, or else statement inside another if, elif, or else block. This allows you to check for secondary conditions only if a primary condition is met.

Consider a scenario where you need to check if a person is eligible for a specific discount: they must be a student and have a valid ID.

is_student = True
has_valid_id = False
age = 20

print("Checking discount eligibility...")

if age < 25: # Outer condition: Age check
    print("Age requirement met.")
    if is_student: # Inner condition 1: Student status
        print("Student status confirmed.")
        if has_valid_id: # Inner condition 2: ID check
            print("Valid ID confirmed.")
            print("Congratulations! You are eligible for the discount.")
        else: # Inner else (belongs to ID check)
            print("Valid ID not found. Discount not applicable.")
    else: # Inner else (belongs to student status check)
        print("Not a student. Discount not applicable.")
else: # Outer else (belongs to age check)
    print("Age requirement not met. Discount not applicable.")

print("Eligibility check complete.")

Notice the indentation. The inner if/else blocks are indented further than the outer if/else block. Python uses this indentation to understand which statements belong to which block. The inner checks (student status, valid ID) are only performed if the preceding outer condition (age < 25) evaluates to True.

Nested Loops

Loops can also be nested. A common use case is iterating over multi-dimensional data structures (like grids or tables) or processing combinations of items from different sequences.

Nested for Loops

Let's say you want to print all possible coordinate pairs (x, y) for a small 3x2 grid (where x goes from 0 to 2, and y goes from 0 to 1).

print("Generating grid coordinates:")

# Outer loop iterates through x values
for x in range(3): # x will be 0, 1, 2
    # Inner loop iterates through y values for EACH x
    for y in range(2): # y will be 0, 1
        print(f"  ({x}, {y})") # Print the coordinate pair

print("Coordinate generation finished.")

Output:

Generating grid coordinates:
  (0, 0)
  (0, 1)
  (1, 0)
  (1, 1)
  (2, 0)
  (2, 1)
Coordinate generation finished.

Here's how it executes:

1. The outer loop starts with x = 0.
2. The inner loop runs completely for x = 0. It iterates with y = 0, then y = 1, printing (0, 0) and (0, 1).
3. The inner loop finishes for x = 0.
4. The outer loop proceeds to the next iteration, setting x = 1.
5. The inner loop runs completely again for x = 1. It iterates with y = 0, then y = 1, printing (1, 0) and (1, 1).
6. The inner loop finishes for x = 1.
7. The outer loop proceeds, setting x = 2.
8. The inner loop runs completely one last time for x = 2, printing (2, 0) and (2, 1).
9. Both loops have completed.

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

Nested while Loops

You can nest while loops too, although it requires careful management of the loop conditions to avoid infinite loops.

outer_count = 0
print("Outer loop starting...")
while outer_count < 2:
    print(f"Outer loop iteration {outer_count}")
    inner_count = 0
    # Inner loop runs while inner_count < 3
    while inner_count < 3:
        print(f"  Inner loop iteration {inner_count}")
        inner_count += 1 # Crucial: update inner loop variable
    outer_count += 1 # Crucial: update outer loop variable
print("Outer loop finished.")

Output:

Outer loop starting...
Outer loop iteration 0
  Inner loop iteration 0
  Inner loop iteration 1
  Inner loop iteration 2
Outer loop iteration 1
  Inner loop iteration 0
  Inner loop iteration 1
  Inner loop iteration 2
Outer loop finished.

Just like with nested for loops, the inner while loop runs to completion for each iteration of the outer while loop. Ensure that the conditions for both loops will eventually become False.

Combining Loops and Conditionals

One of the most frequent patterns is nesting conditional statements inside loops, or vice versa.

if Statements Inside Loops

This is extremely common. You iterate through a sequence and perform an action only if a certain condition is met for an item.

Example: Print only the even numbers from a list.

numbers = [11, 22, 37, 44, 58, 61, 76]
print("Even numbers found:")

for num in numbers:
    # Check the condition for each number in the list
    if num % 2 == 0: # Is the number divisible by 2?
        print(f"  {num}")

print("Search complete.")

Output:

Even numbers found:
  22
  44
  58
  76
Search complete.

The for loop iterates through each num in the numbers list. Inside the loop, the if statement checks if the current num is even. Only if the condition num % 2 == 0 is true does the print() statement execute.

Loops Inside if Statements

You might want to execute a loop only if a certain condition is true.

Example: If a list is not empty, print its contents.

data = ["apple", "banana", "cherry"]
# data = [] # Try uncommenting this line to see the difference

if data: # Checks if the list is not empty (non-empty lists are True-like)
    print("Processing data items:")
    for item in data:
        print(f"  - {item}")
else:
    print("No data items to process.")

Output (with the original data list):

Processing data items:
  - apple
  - banana
  - cherry

Output (if data = [] is uncommented):

No data items to process.

Here, the for loop is nested inside the if block. It only runs if the initial if data: check passes (meaning the list is not empty).

The Importance of Indentation

Remember, Python uses indentation to define code blocks. With nested structures, correct indentation is absolutely essential. Each level of nesting requires an additional level of indentation (typically 4 spaces). Incorrect indentation will lead to IndentationError or, worse, code that runs but produces incorrect results because the logic is not interpreted as you intended.

# Correct Indentation
for i in range(2):
    print(f"Outer {i}")
    if i == 0:
        print("  Inner check for i=0") # Indented under if
    for j in range(2):
        print(f"    Inner loop {j}") # Indented under inner for

# Incorrect Indentation (will likely cause errors or wrong logic)
# for i in range(2):
# print(f"Outer {i}") # Error: Expected an indented block
# if i == 0:
# print("  Inner check for i=0")
# for j in range(2):
# print(f"    Inner loop {j}") # Error or wrong scope

Keeping Nesting Readable

While nesting is powerful, deeply nested structures (e.g., a loop inside a loop inside an if inside another loop) can become very difficult to read and understand. Generally, try to limit nesting to two or three levels. If you find yourself needing deeper nesting, consider if you can break down the logic into smaller, separate functions (which you'll learn about in Chapter 5).

Nesting control structures allows you to create complex program flows by combining simple if, while, and for statements. Mastering nesting and paying close attention to indentation are significant steps in writing effective Python code.