Python Loops

Python offers many ways to iterate over sequences (like lists, tuples, strings) or perform repetitive tasks. This guide dives into the core looping constructs: for and while loops, demonstrating their usage with clear examples.

The for Loop: Iterating Over Iterables

The for loop is ideal for iterating over a sequence, executing a block of code for each item. Its syntax is remarkably clean and readable:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
  print(fruit)

This code snippet prints each fruit in the fruits list. Notice how the fruit variable automatically takes on the value of each item during each iteration.

You can also use for loops with range() to iterate a specific number of times:

for i in range(5):  # Iterates from 0 to 4
  print(i)

range() is incredibly versatile. You can specify a start, stop, and step value:

for i in range(1, 11, 2):  # Iterates from 1 to 10, incrementing by 2
  print(i)

Iterating through dictionaries requires a slightly different approach:

student = {"name": "Alice", "age": 20, "grade": "A"}
for key, value in student.items():
  print(f"{key}: {value}")

This example uses the .items() method to iterate through both keys and values simultaneously.

The while Loop: Repeating Until a Condition is False

The while loop continues executing a block of code as long as a specified condition remains true. It’s perfect for situations where the number of iterations isn’t known beforehand.

count = 0
while count < 5:
  print(count)
  count += 1

This loop prints numbers from 0 to 4. It’s important to ensure the condition eventually becomes false; otherwise, you’ll create an infinite loop.

Here’s an example demonstrating a while loop with user input:

answer = ""
while answer.lower() != "quit":
  answer = input("Enter a command (or 'quit' to exit): ")
  print(f"You entered: {answer}")

This loop continues until the user enters “quit” (case-insensitive).

Loop Control Statements: break and continue

break and continue offer fine-grained control over loop execution:

• break: Immediately terminates the loop.
• continue: Skips the remaining code in the current iteration and proceeds to the next.

for i in range(10):
  if i == 5:
    break  # Stops the loop when i is 5
  print(i)


for i in range(10):
  if i % 2 == 0:
    continue  # Skips even numbers
  print(i)

These examples demonstrate how break and continue modify the standard loop behavior. Understanding these statements enhances your ability to create more efficient and flexible Python code.

Nested Loops

Python also supports nested loops, where one loop is placed inside another. This is commonly used for tasks like processing matrices or generating patterns.

for i in range(3):
  for j in range(3):
    print(f"({i}, {j})", end=" ")
  print() # New line after each inner loop completes

This code produces a 3x3 grid of coordinates. Nested loops are powerful but can be computationally expensive if not carefully designed.