Wow, that's the best explanation I've seen so far, now it's gonna stick with me!
Thank you! On Sun, May 28, 2017 at 10:00 PM, Steven D'Aprano <st...@pearwood.info> wrote: > On Sun, May 28, 2017 at 01:58:22PM -0400, C W wrote: > > Dear Python list, > > > > I am having trouble understanding the following. > [...] > > > The way to think about string indexing and slicing is that the index > positions mark *between* the characters. Take your string: > > Machine learning is awesome! > > For brevity, I'll just use the first word: > > Machine > > Imagine slicing it between the characters. I'll mark the cuts with a > vertical bar: > > |M|a|c|h|i|n|e| > > and add indexes. The indexes will only line correctly if you use a > monspaced or fixed width font like Courier, otherwise things may not > line up correctly. > > |M|a|c|h|i|n|e| > 0 1 2 3 4 5 6 7 > > Here they are again starting from the right, I've spread things out a > bit to fit in the minus signs: > > |M |a |c |h |i |n |e | > -7 -6 -5 -4 -3 -2 -1 0 > > Notice that 0 gets used twice. Of course, that's impossible, because it > would be ambiguous. If you give 0 as an index, how does Python know > whether you mean 0 at the start or 0 or the end? So the simple rule > Python uses is that 0 *always* means the start. > > When you give a single index, Python always uses the character > immediately to the right of the cut: > > s = "Machine" > s[0] > => returns "M" > > s[-1] > => returns "e" > > s[7] > => raises an exception, because there is no character to the right > > When you give two indexes, using slice notation, Python returns the > characters BETWEEN those cuts: > > s[0:7] > => returns "Machine" > > s[1:-1] > => returns "achin" > > Because 0 always means the start of the string, how do you slice to the > end? You can use the length of the string (in this case, 7) or you can > leave the ending position blank, and it defaults to the length of the > string: > > s[1:] # means the same as [1:len(s)] > > You can leave the starting position blank too, it defaults to 0: > > s[:] # means the same as [0:len(s)] > > So remember that slices always cut *between* the index positions. > > > Things get complicated when you include a step (or stride), especially > when the step is negative. For step sizes other than 1, it is > probably best to think of looping over the string: > > py> s = "Nobody expects the Spanish Inquisition!" > py> s[-1:1:-2] > '!otsun snp h tex db' > > is somewhat like: > > for i in range(len(s)-1, 1, -2): > print s[i] > > > > -- > Steve > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor > _______________________________________________ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
