Re: [Tutor] Philosophical question about string slicing from end of a string
On Mon, Nov 24, 2014 at 12:32 PM, boB Stepp robertvst...@gmail.com wrote: Python 2.7.8 Win7Pro str = 0123456789 str[-1] '9' str[-3:-1] '78' str[-3:] '789' I understand that the above is the way it is in Python, but I am puzzled why the designers did not choose that str[-3:-1] returns '789', especially since str[-1] returns '9'. What is the reason for choosing Python's actual behavior? For the same reason as this behavior with positive indices: s = '0123456789' s[1] '1' s[3] '3' s[1:3] '12' Here's what you can do with it: s[-5:-3] + s[-3:-1] == s[-5:-1] True len(s[-5:-3]) == -3 - -5 True Think of slicing as everything from this index (-3) up to but not including (:) this index (-1). Have I clarified or muddied it for you? :) Regards, -- Zach ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Philosophical question about string slicing from end of a string
On Mon, Nov 24, 2014 at 12:57 PM, Zachary Ware zachary.ware+py...@gmail.com wrote: [...] Have I clarified or muddied it for you? :) Clarified, I believe, if my following statements are correct: I did not consider that the behavior was symmetric with positive indices. So, index 0 is the center relative to which positive and negative indices create identical behaviors. Thanks! -- boB ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Philosophical question about string slicing from end of a string
On Mon, Nov 24, 2014 at 1:06 PM, boB Stepp robertvst...@gmail.com wrote: On Mon, Nov 24, 2014 at 12:57 PM, Zachary Ware zachary.ware+py...@gmail.com wrote: [...] Have I clarified or muddied it for you? :) Clarified, I believe, if my following statements are correct: I did not consider that the behavior was symmetric with positive indices. So, index 0 is the center relative to which positive and negative indices create identical behaviors. Hmm. There is a flaw in my observed symmetry in that str[0:3] and str[-3:0] do not behave the same as the latter returns an empty string. -- boB ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Philosophical question about string slicing from end of a string
On Mon, Nov 24, 2014 at 1:06 PM, boB Stepp robertvst...@gmail.com wrote: On Mon, Nov 24, 2014 at 12:57 PM, Zachary Ware zachary.ware+py...@gmail.com wrote: [...] Have I clarified or muddied it for you? :) Clarified, I believe, if my following statements are correct: I did not consider that the behavior was symmetric with positive indices. So, index 0 is the center relative to which positive and negative indices create identical behaviors. Hmm, either I'm not quite following you, or that's not quite right. For the purposes of slicing, 0 is positive and slices that have either all positive or all negative indices behave the same way. Mixing positive and negative indices is of course possible, but takes a little bit more thought since you're counting from opposite ends. Also note that there's no way to get the last member with a negative second index. [Received while writing the above] On Mon, Nov 24, 2014 at 1:16 PM, boB Stepp robertvst...@gmail.com wrote: Hmm. There is a flaw in my observed symmetry in that str[0:3] and str[-3:0] do not behave the same as the latter returns an empty string. Correct. Here's another way to think about negative indices, particularly for converting between positive and negative indices: s[-5:-3] '56' len(s)-5 5 len(s)-3 7 s[len(s)-5:len(s)-3] '56' s[5:7] '56' So, when you try to do s[-3:0], you're doing s[len(s)-3:0], which equates to s[7:0]. Building from my example with the length of the slice in my previous message, the length of that slice would be 0 - 7, or -7, which obviously won't work very well! Instead of bailing out with an error there, Python just gives you the shortest substring it can, ''. [*] Btw, I've been using 's' instead of 'str' just because it's good practice not to shadow builtin names like 'str'. -- Zach ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Philosophical question about string slicing from end of a string
On Mon, Nov 24, 2014 at 12:32:27PM -0600, boB Stepp wrote: Python 2.7.8 Win7Pro str = 0123456789 str[-1] '9' str[-3:-1] '78' str[-3:] '789' I understand that the above is the way it is in Python, but I am puzzled why the designers did not choose that str[-3:-1] returns '789', especially since str[-1] returns '9'. What is the reason for choosing Python's actual behavior? The slice indexes fall *between* characters, not on them. So the string abcdef will have indexes: |a|b|c|d|e|f| 0 1 2 3 4 5 6 (Things may not line up perfectly unless you read this will a monospaced font like Courier.) Think of slicing as cutting on the lines, so the slice [1:4] cuts just before b and just after d. Negative indexes work the same way, just in the opposite direction: |a|b|c|d|e|f| -6-5-4-3-2-1 ? Note that the final line, marked with a question mark, would have been 0 except that 0 is already used for positive slicing. So the slice [-3:-1] cuts before the d and just after e. -- Steven ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Philosophical question about string slicing from end of a string
On Mon, Nov 24, 2014 at 1:33 PM, Zachary Ware zachary.ware+py...@gmail.com wrote: Also note that there's no way to get the last member with a negative second index. Also note that, given a -1 step, there's no way to get the first member with a non-negative second index. s[-1:0:-1] '987654321' It requires a negative index that's at least one less than the negative index of the first member: s[-1:-len(s)-1:-1] '9876543210' If possible, leave this implicit in the slice: s[::-1] '9876543210' ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
