# Retrieve a single item from a list with a given index elements = ["H", "He", "Li", "B", "C", "N", "O", "F", "Ne"] print(elements) # Prints 'Li' # Use a colon inside the index brackets to get a "slice" of the list, returned # as a list. element_slice = elements[1:5] # from index 0 up to (not including) index 5 print(element_slice) # Prints ['He', 'Li', 'B', 'C'] # If a number is not specified at the beginning of the range, it's assumed to # be 0 (or the beginning of the list). element_slice = elements[:3] # get the first three elements print(element_slice) # Prints ['H', 'He', 'Li'] # If a number is not specified at the end of the range, the slice will extend # to the end of the list. element_slice = elements[6:] # from index 6 to the end of the list print(element_slice) # Prints ['O', 'F', 'Ne'] # Without either a first number or a second number, the slice extends from # the beginning of the list to the end of the list, effectively making a copy. element_slice = elements[:] print(element_slice) # Prints ['H', 'He', 'Li', 'B', 'C', 'N', 'O', 'F', 'Ne'] # Negative indexes can be used for either element of the range. element_slice = elements[-2:] # last two elements of the list print(element_slice) # Prints ['F', 'Ne'] element_slice = elements[-5:-2] # 5 elements from end to 2 elements from end print(element_slice) # Prints ['C', 'N', 'O']
# You can use slice syntax with strings as well: message = "Now is the winter of our discontent" print(message[:10]) # Prints 'Now is the' print(message[11:20]) # Prints 'winter of' print(message[-7:]) # Prints 'content'
Python's index bracket syntax allows you to easily write expressions that
evaluate to a section, or "slice," of a list. The syntax for getting a slice
of a list looks very similar to the syntax for getting a single item, except
instead of putting a single number inside the square brackets, you put two
numbers, separated by a colon, indicating where the slice should start and
For example, the expression x[n:m] evaluates to a list that contains items from index n up to (but not including) index m from a list x. If n is omitted, it's assumed to be 0, and the slice will begin at the beginning of the list. If m is omitted, the slice will end at the end of the list.
When slice indexes are negative, they follow the same rules for negative indexes in general (see index brackets for more information).
Slice syntax works with strings as well, and is a convenient way to extract substrings by numerical index.
Updated on Mon Sep 21 15:53:25 2020.
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