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Engineering LibreTexts

8.3: Traversing a List

  • Page ID
    3134
  • The most common way to traverse the elements of a list is with a for loop. The syntax is the same as for strings:

    Code \(\PageIndex{1}\) (Python):

    %%python3
    
    cheeses = ['feta','gorgonzola','brie','mozzarella']
    
    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 functions range and len:

     

    Code \(\PageIndex{1}\) (Python):

    %%python3
    numbers = [96, 23, 40, 76, 7]
    
    for i in range(len(numbers)):
        numbers[i] = numbers[i] * 2
        print(numbers[i])
    

    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.

     

    for loop over an empty list never executes the body:

    for x in empty:
        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]]