1 mean in python?
Possible Duplicate:
The Python Slice Notation
I'm trying to port some Python code to C, but I came across this line and I can't figure out what it means:
if message.startswith('<stream:stream'):
message = message[:-1] + ' />'
I understand that if ' message starts with <stream:stream then something needs to be appended. However I can't seem to figure out where it should be appended. I have absolutely no idea what :-1 indicates. I did several Google searches with no result.
Would somebody be so kind as to explain what this does?
It is list indexing, it returns all elements [:] except the last one -1 . Similar question here
For example,
>>> a = [1,2,3,4,5,6]
>>> a[:-1]
[1, 2, 3, 4, 5]
It works like this
a[start:end]
>>> a[1:2]
[2]
a[start:]
>>> a[1:]
[2, 3, 4, 5, 6]
a[:end]
Your case
>>> a = [1,2,3,4,5,6]
>>> a[:-1]
[1, 2, 3, 4, 5]
a[:]
>>> a[:]
[1, 2, 3, 4, 5, 6]
It's called slicing, and it returns everything of message but the last element.
Best way to understand this is with example:
In [1]: [1, 2, 3, 4][:-1]
Out[1]: [1, 2, 3]
In [2]: "Hello"[:-1]
Out[2]: "Hell"
You can always replace -1 with any number:
In [4]: "Hello World"[:2] # Indexes starting from 0
Out[4]: "He"
The last index is not included.
It's called slicing
"Return a slice object representing the set of indices specified by range(start, stop, step)."
-from this link: http://docs.python.org/2/library/functions.html#slice
You'll notice it's similar to the range arguments, and the : part returns the entire iterable, so the -1 is everything except the last index.
Here is some basic functionality of slicing:
>>> s = 'Hello, World'
>>> s[:-1]
'Hello, Worl'
>>> s[:]
'Hello, World'
>>> s[1:]
'ello, World'
>>> s[5]
','
>>>
Follows these arguments:
a[start:stop:step]
Or
a[start:stop, i]
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