For Loops

This lesson covers how to use for loops in Python to iterate over sequences and other iterable objects, which is essential for automating repetitive tasks in programming.

Iteration

The process of repeating a block of code for each item in a collection or sequence.

Example: Processing each file in a directory or calculating a sum for every number in a list.

A for loop provides a structured way to process every item within a collection, such as a list, tuple, or string. It assigns each item, one by one, to a temporary loop variable, then executes a defined block of code. This process continues until all items in the collection have been processed, making it ideal for tasks that require repeated actions on distinct pieces of data.

Looping Through Sequences: Lists, Tuples, and Strings

pythonIterating Over a List of Fruits

pythonIterating Over a String

Try It Yourself

Write a for loop to iterate over the given tuple of numbers and print each number multiplied by 2.

python

Check Your Understanding

What happens if a for loop attempts to iterate over an empty list?

Generating Number Sequences with `range()`

The built-in range() function is commonly used with for loops to generate sequences of numbers. It's particularly useful when you need to repeat an action a specific number of times or iterate through indices. range() can be used in three main ways:

range(stop): Generates numbers from 0 up to, but not including, stop.
range(start, stop): Generates numbers from start up to, but not including, stop.
range(start, stop, step): Generates numbers from start up to, but not including, stop, incrementing by step each time.

pythonUsing `range(stop)`

pythonUsing `range(start, stop, step)`

Try It Yourself

Use a for loop and the range() function to print all multiples of 3 between 10 and 20, including 10 and 20 if they are multiples.

python

Iterating Over Dictionaries

Dictionaries store data as key-value pairs, and for loops offer flexible ways to iterate through their contents. By default, when you iterate directly over a dictionary, the loop processes its keys. To access values or both keys and values, Python provides specific dictionary methods. The .keys() method returns an iterable of all keys, .values() returns an iterable of all values, and .items() returns an iterable of key-value pairs as tuples.

pythonDictionary Iteration Methods

Check Your Understanding

If you need to access both the key and the value when iterating through a dictionary, which method should you use?

Key Takeaways

For loops automate repetitive tasks by processing each item in a collection.
They iterate over various sequences like lists, tuples, and strings, executing a code block for each element.
The range() function is crucial for generating numerical sequences, useful for fixed-count loops or index-based iteration.
Dictionaries can be iterated over their keys (default), values (using .values()), or key-value pairs (using .items()).
Mastering for loops is essential for efficiently processing data and controlling program flow.

← All lessons in Control Flow

For Loops

Looping Through Sequences: Lists, Tuples, and Strings

Generating Number Sequences with range()

Iterating Over Dictionaries

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Generating Number Sequences with `range()`