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 syntax is: a # items start through stop-1Ī # items start through the rest of the arrayĪ # items from the beginning through stop-1
0 kommentar(er)
