Welcome to the section dedicated to loops!

In this chapter, we'll dive into how loops serve as a key tool for automating repetitive tasks and are essential for efficiently handling lists and other iterable data types.

Join Alex as he demonstrates the use of for loops to simplify operations in our grocery store:

The usage of for loops in Python allows you to execute a block of code repeatedly for each item in a collection (iterable object). Here's what you need to know about for loops:

Syntax

Initiate a for loop with the for keyword, followed by a variable name such as item to represent each element in the sequence. This is followed by the in keyword and then the iterable object such as groceryItems. The code block within the loop is executed once for each element.

In this example, item acts as the iterator variable that sequentially accesses each element in the groceryItems list and executes the print() function for each one:

Iteration refers to the process of executing a block of code repeatedly. Python supports two main types of iteration:

Definite Iteration

Where the number of iterations is known in advance. for loops are a classic example, automatically stopping when they reach the end of the sequence.

Indefinite Iteration

In which execution continues until a specific condition is met, typically handled by while loops, which will be explored in detail in the upcoming chapter.

Example Applications

for loops in Python can be used with any iterable object (lists, tuples, dictionaries, strings), allowing for data manipulation on a per-element basis.

Whether you need to access items in a list, keys or values in a dictionary, or characters in a string, for loops provide a clear syntax that simplifies code for repetitive tasks.

Here's how you can use a for loop to iterate over the elements in a string and a tuple, similar to how we saw with lists:

When using a for loop with a dictionary, iterating directly over the dictionary will loop through its keys by default.

Here's what to expect when you directly iterate over a dictionary:

Notice how the iterator variable product only accesses the dictionary keys.

To iterate over the values of a dictionary, you can use the values() method.

This is useful for operations that require access to values without needing to reference the keys:

If you need to access the keys and values simultaneously, the items() method allows you to loop through key-value pairs in a dictionary.

To use this method in a for loop, we specify two variables before the in keyword — one for the key (product) and one for the value (stock):

Note

In this example, we use f-strings (also known as formatted string literals) to embed variables directly into strings. The syntax is simple: add an f before the opening quotation mark ", and place variables or expressions inside curly braces {} within the string. This method makes combining strings and variables in print statements much more readable, so it's worth learning.

Drawing on your knowledge of if/else, boolean operators, and for loops, we can run a simple inventory check on a dictionary:

Note

You may notice that we've placed if statements inside the for loop, which added extra indentation to the if block. As mentioned earlier, each indented block of code can be treated as independent, regardless of where it's placed. The key is to ensure that the indentation is consistent and correct between them.