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# Why are slice indices the way they are in python?

 P: n/a A couple of off the wall questions. It seems to me that there is usually a solid *reason* for things in Python and I'm wondering about the rationale for the way slicing works: my_string[2:5] gets you the 3rd through the 3rd through the 5th character of the string because indexing starts at 0 and you get everything up to, but not including the second index. Why? It doesn't seem intuitive to me. To me, it makes it harder, not easier, to work with slices than if indexing started at 1 and the above expression got you the 2nd throught the 5th character. Another thing that I've been puzzling over is the pow() function. pow(x,y) gives x**y. Fine. But pow(x,y,z) gives (x**y) % c I'm curious to know what the pressing reason for such a feature was. Nov 30 '06 #1
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 P: n/a "Steve Bergman" A couple of off the wall questions. It seems to me that there is usually a solid *reason* for things in Python and I'm wondering about the rationale for the way slicing works Yes, see below. my_string[2:5] gets you the 3rd through the 3rd through the 5th character of the string because indexing starts at 0 and you get everything up to, but not including the second index. Why? len(s[2:5])== 5-2 s[2:5] + s[5:7] == s[2:7] Another thing that I've been puzzling over is the pow() function. pow(x,y) gives x**y. Fine. But pow(x,y,z) gives (x**y) % c you obvious meant (x**y) % z > I'm curious to know what the pressing reason for such a feature was. Such exponential remainders are used in cryptography work, for instance, with fairly large values, and can be efficiently computed, especially in C, *without* computing a humongous x**y intermediate value. Since this is an int function, it does not really belong in the floating point math module. Terry Jan Reedy Nov 30 '06 #2

 P: n/a Steve Bergman wrote: .... Why? It doesn't seem intuitive to me. To me, it makes it harder, not easier, to work with slices than if indexing started at 1 and the above expression got you the 2nd throught the 5th character. Dijkstra has an article about this: http://www.cs.utexas.edu/users/EWD/t...xx/EWD831.html In practice it probably wouldn't make a great deal of difference. Here at my current job, we have a situation where a bunch of Progress 4GL programmers have been taught Java and let loose on the code base. we now have a situation where there are a plethora of methods written designed to be 1-based, and you have to trawl through the code to work out if the method was written by a Java programmer or a 4GL programmer to save on array index exceptions... The infuriating thing is that if you are iterating over an array in Java, and have to use one of the functions based on an array index, you have to use function(i + 1). Drilling back through the code, and invariably you have the function written: def function(i): return array[i-1] !!! (at least you would if the Java was written in Python by 4GL programmers, but you get the idea ;-) ) Nov 30 '06 #3

 P: n/a Why? It doesn't seem intuitive to me. To me, it makes it harder, not easier, to work with slices than if indexing started at 1 and the above expression got you the 2nd throught the 5th character. Zero-based indices and excluding last index often works nicer, it is not the rule, but all my experience says that. a[i:i] is very handy to have in many algorithms, so you don't have to exclude this case explicitly. Index starting at 1 or including N often adds +-1 expressions to the code. I think that http://en.wikipedia.org/wiki/Modular_arithmetic makes it more intuitive. And if talking about dates, then I suggest NEVER use 2006-12-31 23:59:59 in data, always 2007-01-01 00:00:00 instead. If customer wants to see former on the screen just substruct 1 millisecond from the data and round it to what is needed. It makes all arithmetics, finding period collapses, time differences, etc much more painless. You sometimes don't even know if your database supports seconds, milliseconds, microseconds and what your operating system and programming language support. So if you are using inclusive end date you may find yourself in trouble very soon. Again, its not intuitive for ordinary people, but math people and programmers often find 0-based and exlusive ends are nicer to work with, at least me :) Another thing that I've been puzzling over is the pow() function. pow(x,y) gives x**y. Fine. But pow(x,y,z) gives (x**y) % c I'm curious to know what the pressing reason for such a feature was. This makes cryptography code 100 times faster. Oleg Nov 30 '06 #4

 P: n/a On Thu, 2006-11-30 at 08:58 -0800, og****@gmail.com wrote: And if talking about dates, then I suggest NEVER use 2006-12-31 23:59:59 in data, always 2007-01-01 00:00:00 instead. Forgive me for my ignorance, but isn't "2006-12-31 23:59:59" actually one entire second earlier than "2007-01-01 00:00:00"? Hence they are two different dates are they not? Michael Dec 1 '06 #5

 P: n/a Michael Torrie wrote: Forgive me for my ignorance, but isn't "2006-12-31 23:59:59" actually one entire second earlier than "2007-01-01 00:00:00"? Hence they are two different dates are they not? given the context, maybe the poster meant that he's storing date *ranges* as [start, stop) (i.e. including start but not including stop). Dec 1 '06 #6

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