> > Hello All, > > > > I am bit confuse how the slice works in case a negative step is supplied > > a = 'abcde' > > a[::-1] gives me edcba > > > > but [4:0:-1] gives me edcb > > > > while searching net I came accross something which said the following: > > > > If a negative stride is specified and the starting or stopping indices > are > > omitted, they default to ``end of axis'' and ``beginning of axis'' > > respectively. > > In my case 4 is end of axis and 0 is begining of the axis so a[::-1] > should > > be equivalent to [4:0:-1] but result which I get in both the cases does > not > > validate. > > Your interpretation is wrong. In your case, 4 is start index, 0 is end > index (as obviously shown by the result you get). Step -1 lets you step > backwards, but this does not change the fact that the end-index is the > second argument of the slice, right? Now, python work with (right-side) > half-open intervals, meaning that the second border is excluded (s[0:4] > would return "abcd"). Which means in your case you won't get a[0] in the > result.
Hope I'm clear... It can be a bit misleading at start but, as it's > consistent, once you get used to it all works fine ;-) > Thanks for your response but I am still not clear; can you show me the equivalent of [::-1] in terms actually specifying start-index, end-index which gives me reverse of the given arrary. Dennis, I hope this clears up your question. a[begin index: end index: step] a = ['a','b','c','d','e'] there are 5 indexes 0 through 4 if you a[4:0:-1] a = ['e','d','c','b'] because you stopped at index 0 but did not include it a[::-1] uses the last index going back to and including index 0 as the following example shows >>> a =[1,2,3,4,5,6] >>> b = a[5:0:-1] >>> b [6, 5, 4, 3, 2] >>> b = a[5:-1:-1] >>> b [] >>> a [1, 2, 3, 4, 5, 6] >>> b = a[5::-1] >>> b [6, 5, 4, 3, 2, 1] >>> Like I said I hope that clears it up a little Michael _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
