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I'm trying to understand the following piece of code:
# node list n = [] for i in xrange(1, numnodes + 1): tmp = session.newobject(); n.append(tmp) link(n[0], n[-1])Specifically, I don't understand what the index -1 refers to. If the index 0 refers to the first element, then what does -1 refer to?
Henry Ecker
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asked Jul 6, 2012 at 18:41
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Negative numbers mean that you count from the right instead of the left. So, list[-1] refers to the last element, list[-2] is the second-last, and so on.
answered Jul 6, 2012 at 18:43
ToomaiToomai
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List indexes of -x mean the xth item from the end of the list, so n[-1] means the last item in the list n. Any good Python tutorial should have told you this.
It's an unusual convention that only a few other languages besides Python have adopted, but it is extraordinarily useful; in any other language you'll spend a lot of time writing n[n.length-1] to access the last item of a list.
answered Jul 6, 2012 at 18:43
Russell BorogoveRussell Borogove
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Negative Indexing is used to in Python to begin slicing from the end of the string i.e. the last. Slicing in Python gets a sub-string from a string. The slicing range is set as parameters i.e. start, stop and step.
Syntax
Let us see the syntax −
#slicing from index start to index stop-1 arr[start:stop] # slicing from index start to the end arr[start:] # slicing from the beginning to index stop - 1 arr[:stop] # slicing from the index start to index stop, by skipping step arr[start:stop:step]If the values above are in negative, that would mean negative indexing i.e. slicing from the end of the string.
Slice a String with Negative Indexing
Example
myStr = 'Thisisit!' print("String = ", myStr) print("String after slicing (negative indexing) = ", myStr[-4:-1])
Output
String = Thisisit! String after slicing (negative indexing) = sitSlice a String with Negative Indexing and set a step
The slicing range is set as parameters i.e. start, stop and step. For negative indexing, set the start and stop as negative values i.e slice from the end −
Example
myStr = 'Thisisit. We did it!' print("String = ", myStr) print("String after slicing (negative indexing) = ", myStr[-9:-3:2])
Output
String = Thisisit. We did it! String after slicing (negative indexing) = eddReverse the order of a string with Negative Indexing
To display the 1st element to last element in steps of 1 in reverse order, we use the [::-1]. The [::-1] reverses the order
Example
Let’s see the example
myStr = 'Hello! How are you?' print("String = ", myStr) print("Reverse order of the String = ", myStr[::-1])
Output
String = Hello! How are you? Reverse order of the String = ?uoy era woH !olleH
Updated on 16-Sep-2022 12:17:41
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