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Python Loops for Beginners

Loops are fundamental programming constructs that allow you to repeat a block of code multiple times. These help you accomplish repetitive tasks, like searching for a letter in a word or printing "Hello" 100 times.

In Python, there are two main types of loops: for loops and while loops. Let's explore each of them in detail.

1. For Loops - In-Depth​

Iterables in For Loops​

In Python, an iterable is any object that can be looped over. Common iterables include:

  • Lists
  • Tuples
  • Strings
  • Dictionaries
  • Sets
  • Files

Let's look at examples of using different iterables in for loops:

Lists​

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")
Output:
I like apple
I like banana
I like cherry

Tuples​

coordinates = (4, 5)
for coord in coordinates:
    print(f"Coordinate: {coord}")
Output:
Coordinate: 4
Coordinate: 5

Strings​

word = "Python"
for char in word:
    print(f"Letter: {char}")
Output:
Letter: P
Letter: y
Letter: t
Letter: h
Letter: o
Letter: n

Dictionaries​

When looping through dictionaries, you can access keys, values, or both:

person = {"name": "Alice", "age": 30, "city": "New York"}

# Looping through keys
for key in person:
    print(f"Key: {key}")

# Looping through values
for value in person.values():
    print(f"Value: {value}")

# Looping through both keys and values
for key, value in person.items():
    print(f"{key}: {value}")
Output:
Key: name
Key: age
Key: city
Value: Alice
Value: 30
Value: New York
name: Alice
age: 30
city: New York

Sets​

unique_numbers = {1, 3, 5, 7, 9}
for num in unique_numbers:
    print(f"Number: {num}")
Output:
Number: 1
Number: 3
Number: 5
Number: 7
Number: 9

Using range() in For Loops​

The range() function is a versatile tool for creating sequences of numbers. It can take one, two, or three arguments:

  • range(stop): Generates numbers from 0 to stop-1
  • range(start, stop): Generates numbers from start to stop-1
  • range(start, stop, step): Generates numbers from start to stop-1, incrementing by step

Examples:

# range with one argument
for i in range(5):
    print(i)  # Prints 0, 1, 2, 3, 4

# range with two arguments
for i in range(2, 6):
    print(i)  # Prints 2, 3, 4, 5

# range with three arguments
for i in range(1, 10, 2):
    print(i)  # Prints 1, 3, 5, 7, 9

# Counting backwards
for i in range(5, 0, -1):
    print(i)  # Prints 5, 4, 3, 2, 1

Enumerate Function​

When you need both the index and the value in a loop, use the enumerate() function:

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

Output:

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

2. While Loops - In-Depth​

While loops are more flexible than for loops and can be used in situations where the number of iterations is not known in advance.

Using While Loops with Counters​

A common pattern is to use a counter variable to control the loop:

counter = 0
while counter < 5:
    print(f"Counter is {counter}")
    counter += 1

While Loops with Complex Conditions​

You can use more complex conditions in while loops:

import random

total = 0
while total < 21:
    roll = random.randint(1, 6)
    total += roll
    print(f"Rolled a {roll}. Total is now {total}")

print("Game over!")

Infinite Loops and Break Statements​

Sometimes, you might want to create an infinite loop and use a break statement to exit when a certain condition is met:

while True:
    user_input = input("Enter a number (or 'q' to quit): ")
    if user_input.lower() == 'q':
        break
    number = float(user_input)
    print(f"The square of {number} is {number**2}")

print("Thanks for playing!")

Continue Statement​

The continue statement skips the rest of the current iteration and moves to the next one:

for num in range(10):
    if num % 2 == 0:
        continue
    print(f"Odd number: {num}")

This will only print odd numbers.

Real-Life Use Cases​

For Loops: Data Processing​

Imagine you have a list of temperatures in Celsius and want to convert them to Fahrenheit:

celsius_temps = [0, 10, 20, 30, 40]
fahrenheit_temps = []

for temp in celsius_temps:
    fahrenheit = (temp * 9/5) + 32
    fahrenheit_temps.append(fahrenheit)

print(f"Celsius: {celsius_temps}")
print(f"Fahrenheit: {fahrenheit_temps}")

While Loops: User Input Validation​

Here's an example of using a while loop for input validation:

while True:
    age = input("Please enter your age: ")
    if age.isdigit() and 0 < int(age) < 120:
        age = int(age)
        break
    print("Invalid input. Please enter a number between 1 and 119.")

print(f"Your age is {age}")

This loop continues until the user provides a valid age.

Both for loops and while loops are powerful tools in Python. The choice between them often depends on the specific requirements of your task and the nature of the data you're working with.