9.3. Traversing a listΒΆ

The most common way to traverse the elements of a list is with a for loop. The syntax is the same as for strings:

This works well if you only need to read the elements of the list. But if you want to write or update the elements, you need the indices. A common way to do that is to combine the functions range and len:

for i in range(len(numbers)):
    numbers[i] = numbers[i] * 2

This loop traverses the list and updates each element. len returns the number of elements in the list. range returns a list of indices from 0 to n-1, where n is the length of the list. Each time through the loop, i gets the index of the next element. The assignment statement in the body uses i to read the old value of the element and to assign the new value.

A for loop over an empty list never executes the body:

Although a list can contain another list, the nested list still counts as a single element. Check out the length of this list:

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