![]() Using heuristic 1 and 2 it's easy to get your head around indexing an empty slice: > p = Īnd then once you've seen that, slice assignment to the empty slice makes sense too: > p = The trickiest part to get used to is assignment to empty slices. The third rule of slice assignment is, the assigned list (iterable) doesn't have to have the same length the indexed slice is simply sliced out and replaced en masse by whatever is being assigned: > p = # Start over The second rule of slice assignment, which you can also see above, is that whatever portion of the list is returned by slice indexing, that's the same portion that is changed by slice assignment: > p ![]() The first rule of slice assignment is that since slicing returns a list, slice assignment requires a list (or other iterable): > p > p # does NOT include the last item!Īnother heuristic is, "for any slice, replace the start by zero, apply the previous heuristic to get the end of the list, then count the first number back up to chop items off the beginning" > p # Start at the beginning and count out 4 items > p # the last of six items, indexed from zero One heuristic is, for a slice from zero to n, think: "zero is the beginning, start at the beginning and take n items in a list". To understand slice assignment, it's helpful to add another concept to the ASCII art: +-+-+-+-+-+-+ The answers above don't discuss slice assignment. , x ]Įxtended slicing (with commas and ellipses) are mostly used only by special data structures (like NumPy) the basic sequences don't support them. If stride is negative, the ordering is changed a bit since we're counting down: > x #, x. Of course, if (high-low)%stride != 0, then the end point will be a little lower than high-1. While the :-based notation is very helpful for simple slicing, the explicit use of slice() objects simplifies the programmatic generation of slicing.Įnumerating the possibilities allowed by the grammar for the sequence x: > x #, x. a is equivalent to a or a is equivalent to a. To skip specifying a given argument, one might use None, so that e.g. both slice(stop) and slice(start, stop) are supported. Slice objects also behave slightly differently depending on the number of arguments, similarly to range(), i.e. Relationship with the slice objectĪ slice object can represent a slicing operation, i.e.: a Sometimes you would prefer the error, so you have to be aware that this may happen. For example, if you ask for a and a only contains one element, you get an empty list instead of an error. Python is kind to the programmer if there are fewer items than you ask for. Similarly, step may be a negative number: a # all items in the array, reversedĪ # everything except the last two items, reversed So: a # last item in the arrayĪ # everything except the last two items The other feature is that start or stop may be a negative number, which means it counts from the end of the array instead of the beginning. So, the difference between stop and start is the number of elements selected (if step is 1, the default). The key point to remember is that the :stop value represents the first value that is not in the selected slice. There is also the step value, which can be used with any of the above: a # start through not past stop, by step The slice notation can be used to asign new values to elements of a certain sublist.The syntax is: a # items start through stop-1Ī # items start through the rest of the arrayĪ # items from the beginning through stop-1 Replacing Elements of a Sublist with Slice Notation Instead of stepping forward, we're stepping backwards, from the end of the list to the start and including these elements: a_list = If we set it to a negative value, the resulting list will be reversed, with the step value. ![]() Let's go ahead and separate a tail and head of a list: # The length of the tailĪ_list = Įven the step parameter can be negative. The head of a list is a sublist that contains the first n elements of a list, and the tail is a sublist that contains the last n elements. ![]() This can be used to obtain the head and tail of a list of a given length. Negative indexing works the same way as regular indexing, except for the fact that it starts indexing from the last element which has the index -1. ![]() The slice notation can be used with negative indexing as well. No two ways to slice it code#That snippet of code prints out: įinding the Head and Tail of List with Slice Notation Likewise, if end isn't listed, the sublist will end at the ending of the original list: a_list = If step isn't listed, the sublist will start from the beginning. ![]()
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