Lists and for loops¶

It is also possible to perform list traversal using iteration by item as well as iteration by index.

It almost reads like natural language: For (every) fruit in (the list of) fruits, print (the name of the) fruit.

We can also use the indices to access the items in an iterative fashion.

In this example, each time through the loop, the variable position is used as an index into the list, printing the position-eth element. Note that we used len as the upper bound on the range so that we can iterate correctly no matter how many items are in the list.

Since lists are mutable, it is often desirable to traverse a list, modifying each of its elements as you go. The following code squares all the numbers from 1 to 5 using iteration by position.

Take a moment to think about range(len(numbers)) until you understand how it works. In this case, since len(numbers) is 5, it’s the same as saying range(5). We are interested here in both the value (10, 20, 30, etc.) and its index within the list (0, 1, 2, etc.), so that we can assign a new value to the position in the list.

Later in this chapter you will see how to transform an existing list into a new list without mutating (modfiying) the existing list.