For-loops in Python

This notebook introduces the concept of for-loops in Python, a fundamental tool for iterating over sequences and performing repetitive tasks efficiently.

Let’s start with a simple for-loop that iterates over a list of numbers. This demonstrates the basic syntax of a for-loop in Python.

numbers = [1, 2, 3, 4, 5]
for num in numbers:
    print(num)

1
2
3
4
5

We can also use the range() function to generate a sequence of numbers to iterate over. This is useful when you want to repeat an action a specific number of times.

for i in range(5):
    print(f"Iteration {i+1}")

Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5

For-loops can be used with strings to iterate over each character. This is helpful for tasks like counting specific characters or performing operations on each character.

word = "Python"
for char in word:
    print(char.upper())

P
Y
T
H
O
N

We can use the enumerate() function with a for-loop to get both the index and the value of each item in a sequence. This is particularly useful when you need to track the position of items.

fruits = ["apple", "banana", "cherry"]
for index, fruit in enumerate(fruits):
    print(f"Index {index}: {fruit}")

Index 0: apple
Index 1: banana
Index 2: cherry

For-loops can also be used with dictionaries. By default, looping over a dictionary iterates over its keys. We can use the .items() method to iterate over both keys and values.

person = {"name": "Alice", "age": 30, "city": "New York"}
for key, value in person.items():
    print(f"{key}: {value}")

name: Alice
age: 30
city: New York

Finally, let’s look at a practical example where we use a for-loop to calculate the sum of numbers in a list. This demonstrates how loops can be used for data processing tasks.

numbers = [10, 20, 30, 40, 50]
total = 0
for num in numbers:
    total += num
print(f"The sum of the numbers is: {total}")

The sum of the numbers is: 150