Friday, March 15, 2013

The Python Slice Notation clear explaination

It's pretty simple really:

    a[start:end] # items start through end-1
    a[start:]    # items start through the rest of the array
    a[:end]      # items from the beginning through end-1
    a[:]         # a copy of the whole array

There is also the `step` value, which can be used with any of the above:

    a[start:end:step] # start through not past end, by step

The key point to remember is that the `:end` value represents the first value that is *not* in the selected slice. So, the difference beween `end` and `start` is the number of elements selected (if `step` is 1, the default).

The other feature is that `start` or `end` may be a *negative* number, which means it counts from the end of the array instead of the beginning. So:

    a[-1]    # last item in the array
    a[-2:]   # last two items in the array
    a[:-2]   # everything except the last two items

Python is kind to the programmer if there are fewer items than you ask for. For example, if you ask for `a[:-2]` and `a` only contains one element, you get an empty list instead of an error. Sometimes you would prefer the error, so you have to be aware that this may happen.

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