On Sat, 25 Sep 2010 06:54:36 -0700, Yingjie Lan wrote: > For the rule above, how about the > case to reverse and multiply: > >>>> L*-3 #L reversed and repeated three times > > v.s. > >>>> L[::-1]*3 #L reversed and repeated three times > > The first one is simpler (4 chars v.s. 9 chars). I thought it was also > intuitive because if you multiply a vector by -1, you should get a > vector in the reversed direction.
Reversing the order of elements in a vector does NOT reverse the vector's direction. Vector multiplication by a scalar does elementwise multiplication. In mathematics and physics: [1, 2, 3]*5 => [5, 10, 15] [1, 2, 3]*-1 => [-1, -2, -3] is a reflection of the vector. It certainly doesn't reverse the order of elements! [1, 2, 3] => [3, 2, 1] is a rotation, not a reflection. Python doesn't treat sequences as vectors, since it's a programming language and sequences are more general. Instead it performs repetition. This is conceptually simple: multiplying a sequence by n *repeats* the sequence n times: L*n = L+L+L+...+L # n times where + is concatenation, not elementwise addition. If n is zero, you get the empty sequence: L*0 = an empty sequence of the same type as L just like you get nothing if you multiply a number by n. If you ask for zero copies of a sequence, you get nothing. The only tricky parts are what to do for fractional values of n, and negative values. For practical reasons, the current behaviour is the most useful thing to do. Once you see sequence multiplication as repetition, then the natural interpretation of multiplication by a negative value *cannot* be reversal, or any other re-ordering or permutation. How many repetitions of [1, 2, 3, 4] does it take to give [1, 4, 2, 3]? Answer -- you can't get any other permutation by repeating the original. Singling out reversal as a privileged permutation is just "tacking on" extra functionality, for no real benefit. It would be like making: sorted(my_list) => sequence in sorted order sorted(my_list, reversed=True) => sequence in reverse sorted order sorted(my_list, reversed=3) => sequence with every element multiplied by 3, then sorted Yes, you could do it, and yes, it might save somebody a line of code somewhere, but WTF??? But even if it wasn't, for reasons of backward compatibility, to say nothing of the moratorium, it isn't going to change. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
