Slicing
Slicing
Python supports a powerful means for extracting data from a sequence (string, list or tuple) called slicing.
Basic slicing
We can take a slice through some sequence by specifying a range of indices.
>>> un_security_council = ['China', 'France', 'Russia', 'UK',
... 'USA', 'Albania', 'Brazil', 'Gabon',
... 'Ghana', 'UAE', 'India', 'Ireland',
... 'Kenya', 'Mexico', 'Norway']Let’s say we just wanted the permanent members of the UN Security Council (these are the first five in the list). Instead of providing a single index within brackets, we provide a range of indices, in the form <sequence>[<start>:<end>].
>>> un_security_council[0:5]
['China', 'France', 'Russia', 'UK', 'USA']“Hey! Wait a minute!” you say, “We provided a range of six indices! Why doesn’t this include ‘Albania’ too?”
Reasonable question. Python treats the ending index as its stopping point, so it slices from index 0 to index 5 but not including the element at index 5! This is the Python way, as you’ll see with other examples soon. It does take a little getting used to, but when you see this kind of indexing at work elsewhere, you’ll understand the rationale.
What if we wanted the non-permanent members whose term ends in 2023? That’s Albania, Brazil, Gabon, Ghana, and UAE.
To get that slice we’d use
>>> un_security_council[5:10]
['Albania', 'Brazil', 'Gabon', 'Ghana', 'UAE']Again, Python doesn’t return the item at index 10; it just goes up to index 10 and stops.
Some shortcuts
Python allows a few shortcuts. For example, we can leave out the starting index, and Python reads from the start of the list (or tuple).
>>> un_security_council[:10]
['China', 'France', 'Russia', 'UK', 'USA',
'Albania', 'Brazil', 'Gabon', 'Ghana', 'UAE']By the same token, if we leave out the ending index, then Python will read to the end of the list (or tuple).
>>> un_security_council[10:]
['India', 'Ireland', 'Kenya', 'Mexico', 'Norway']Now you should be able to guess what happens if we leave out both start and end indices.
>>> un_security_council[:]
['China', 'France', 'Russia', 'UK', 'USA',
'Albania', 'Brazil', 'Gabon', 'Ghana', 'UAE',
'India', 'Ireland', 'Kenya', 'Mexico', 'Norway']We get a copy of the entire list (or tuple)!
Guess what these do:
un_security_council[-1:]un_security_council[:-1]un_security_council[5:0]un_security_council[5:-1]
Specifying the stride
Imagine you’re on a stepping-stone path through a garden. You might be able to step one stone at a time. You might be able to step two stones at a time—skipping over every other stone. If you have long legs, or the stones are very close together, you might be able to step three stones at a time! We call this step size or stride.
In Python, when specifying slices we can specify the stride as a third parameter. This comes in handy if we only want values at odd indices or at even indices.
The syntax is <sequence>[<start>:<stop>:<stride>].
Here are some examples:
>>> un_security_council[::2] # only even indices
['China', 'Russia', 'USA', 'Brazil', 'Ghana',
'India', 'Kenya', 'Norway']
>>> un_security_council[1::2] # only odd indices
['France', 'UK', 'Albania', 'Gabon', 'UAE',
'Ireland', 'Mexico']What happens if the stride is greater than the number of elements in the sequence?
>>> un_security_council[::1000]
['China']Can we step backward? Sure!
>>> un_security_council[-1::-1]
['Norway', 'Mexico', 'Kenya', 'Ireland', 'India',
'UAE', 'Ghana', 'Gabon', 'Brazil', 'Albania',
'USA', 'UK', 'Russia', 'France', 'China']Now you know one way to get the reverse of a sequence. Can you think of some use cases for changing the stride?
Original author: Clayton Cafiero < [given name] DOT [surname] AT uvm DOT edu >
No generative AI was used in producing this material. This was written the old-fashioned way.
This material is for free use under either the GNU Free Documentation License or the Creative Commons Attribution-ShareAlike 3.0 United States License (take your pick).