## 2018年5月27日 星期日

### [ Python 常見問題 ] Understanding Python's slice notation

Source From Here
Question
I need a good explanation (references are a pluson Python's slice notation. To me, this notation needs a bit of picking up.
It looks extremely powerful, but I haven't quite got my head around it.

How-To
It's pretty simple really:
1. a[start:end] # items start through end-1
2. a[start:]    # items start through the rest of the array
3. a[:end]      # items from the beginning through end-1
4. a[:]         # a copy of the whole array
There is also the step value, which can be used with any of the above:
1. 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 between end and start is the number of elements selected (if step is 1, the default). The other feature is that startor end may be a negative number, which means it counts from the end of the array instead of the beginning. So:
1. a[-1]    # last item in the array
2. a[-2:]   # last two items in the array
3. a[:-2]   # everything except the last two items
Similarly, step may be a negative number:
1. a[::-1]    # all items in the array, reversed
2. a[1::-1]   # the first two items, reversed
3. a[:-3:-1]  # the last two items, reversed
4. a[-3::-1]  # everything except the last two items, reversed
Let's take a look at real example:
>>> alist = list(range(10))
>>> alist
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> alist[3:] # Skip first 3 items
[3, 4, 5, 6, 7, 8, 9]
>>> alist[:-3] # Skip last 3 items
[0, 1, 2, 3, 4, 5, 6]
>>> alist[::-1] # Reverse the list
[9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> alist[:-3:-1] # Inverse the list and then select element until meet '7'
[9, 8]
>>> alist[-3::-1] # Inverse the list and start from '7'
[7, 6, 5, 4, 3, 2, 1, 0]

Supplement
What does list[x::y] do? [duplicate]

### [ Python 文章收集 ] Monitoring memory usage of a running Python program

Source From  Here Preface At  Survata , we do a lot of data processing using Python and its suite of data processing libraries like  pandas ...