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10.3: Traversing a list

  • Page ID
    40782
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    The most common way to traverse the elements of a list is with a for loop. The syntax is the same as for strings:

    for cheese in cheeses:
        print(cheese)
    

    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 built-in 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 runs the body:

    for x in []:
        print('This never happens.')
    

    Although a list can contain another list, the nested list still counts as a single element. The length of this list is four:

    ['spam', 1, ['Brie', 'Roquefort', 'Pol le Veq'], [1, 2, 3]]

    This page titled 10.3: Traversing a list is shared under a CC BY-NC 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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