Control Flow: Loops

Loops allow a program to perform a repetitive task efficiently. Python has two primary loop constructs: the while loop and the for loop.

1. The while Loop

The while loop executes a block of code as long as a specified Boolean condition remains True.

• Initialization: Requires a loop control variable to be set before the loop starts.
• Condition: The loop continues as long as this condition is met.
• Update: The loop control variable must be changed inside the loop to eventually make the condition False, preventing an infinite loop.

# Initialization
count = 1

# Condition: Loop continues as long as count <= 5
while count <= 5:
    print(count)
    # Update: Increment count to move towards the exit condition
    count = count + 1 

print("Loop finished!")

2. The for Loop

The for loop is used to iterate over a sequence (like a list, string, or tuple) or any iterable object. It is Python’s most common loop type.

A. Iterating Over a Sequence

The loop automatically steps through each item in the sequence.

fruits = ["apple", "banana", "cherry"]

# The variable 'fruit' takes the value of each item in the list sequentially
for fruit in fruits:
    print(f"I like {fruit}.")

B. Iterating with range()

The built-in range() function generates a sequence of numbers, which is often used in a for loop when you need a specific number of iterations.

# Loop 5 times (0, 1, 2, 3, 4)
for i in range(5):
    print(f"Iteration {i}")

3. Loop Control Statements

These statements allow you to alter the normal execution flow of a loop.

A. The break Statement

The break statement immediately terminates the current loop entirely, regardless of the loop condition, and execution jumps to the statement following the loop.

items = ["A", "B", "STOP", "C"]

for item in items:
    if item == "STOP":
        break # Exit the loop immediately
    print(item) 
# Output: A, B (C is never reached)

B. The continue Statement

The continue statement immediately skips the rest of the code in the current iteration of the loop and moves to the next iteration.

numbers = [1, 2, 3, 4, 5]

for num in numbers:
    if num % 2 == 0: # If number is even
        continue     # Skip the print statement for this number
    print(num)
# Output: 1, 3, 5 (Only odd numbers are printed)

4. The else Clause with Loops

Python uniquely allows an else block to be attached to both for and while loops.

• The else block executes only if the loop finishes without encountering a break statement.

found = False
for i in range(5):
    if i == 10:
        found = True
        break
else:
    # This block executes because the loop completed naturally (i != 10)
    print("Item not found in the loop range.")