Kevin, I have been taking the same plunge (and prbably for the same reason). It's not clear whether knowing C++ well was a help or a hindrance. I have just read the book "Learning Python" by Lutz, published by O'Reilly, and found it pretty good. At least, I knew immediately why your progam did what it did and how to avoid it as there's an almost identical example in the book!
Now after "Learning Python" at 700pp I can move on to "Programming Python" -- same author and publisher but 1500 pages. Or maybe not.... John On 15/01/2008, William Stein <[EMAIL PROTECTED]> wrote: > > Kevin Buzzard wrote: > > I finally took the plunge and started learning python. I think it's the > > first language I've ever learnt where "everything is a pointer" and it was > > a big psychological shock! After my first day of messing around with it, I > > was utterly confused :-) I had a vector which I wanted to modify several > > times, and I wanted to keep track of what the vector looked like at > > various stages, so I initiated a list and then wrote a loop which, on > > every iteration, modified the vector and then appended the modified vector > > to the end of the list. Of course, when the loop had ended, the list > > contained 100 copies of the final state of the vector. I had no idea what > > to do! I asked David Loeffler and he told me about loading various modules > > and using functions like deepcopy() and it all left me feeling wholly > > bewildered! On the other hand I am kind of coming around to the idea now. > > Here's the sort of thing you might want to do: > > sage: v = vector(ZZ, 5) > sage: w = [v] > sage: for i in range(10): > ....: z = copy(w[-1]); z[0] += 1; w.append(z) > ....: > sage: w > [(0, 0, 0, 0, 0), > (1, 0, 0, 0, 0), > (2, 0, 0, 0, 0), > (3, 0, 0, 0, 0), > (4, 0, 0, 0, 0), > (5, 0, 0, 0, 0), > (6, 0, 0, 0, 0), > (7, 0, 0, 0, 0), > (8, 0, 0, 0, 0), > (9, 0, 0, 0, 0), > (10, 0, 0, 0, 0)] > > > This is indeed totally different than Magma which uses call by value > semantics. Python is much closer semantically to C/C++, where most serious > programs pass around pointers. > > In Magma: > > > v := VectorSpace(RationalField(), 5)!0; > > w := [v,v,v]; > > v[1] := 5; > > w; > [ > (0 0 0 0 0), > (0 0 0 0 0), > (0 0 0 0 0) > ] > > In Sage/Python: > > sage: v = vector(ZZ, 5) > sage: w = [v,v,v] > sage: v[1] = 5 > sage: w > [(0, 5, 0, 0, 0), (0, 5, 0, 0, 0), (0, 5, 0, 0, 0)] > > In Python w = [v,v,v] makes a list with 3 references to a single object. > In Magma it *tends* to make a list of 3 copies, if copying of objects > happens to be defined -- otherwise magma *still* makes a list of > 3 references !! > > > v := ModularSymbols(389); > > w := [v,v,v]; > > v`dimension := 10; > > w[2]`dimension; > 10 > > So Magma is just plain inconsistent, which in the long run is > more confusing than Python. This is especially relevant when > it comes to complicated data structures (common in mathematics) > where copying is just not defined or is very expensive. > > > By the way the copy Python function is usually enough to copy > most Sage objects sufficiently for most purposes. Deep copy > is needed only in certain special cases, e.g., a list of lists... > > -- > William Stein > Associate Professor of Mathematics > University of Washington > http://wstein.org > > > > -- John Cremona --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to sage-support@googlegroups.com To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/sage-support URLs: http://sage.math.washington.edu/sage/ and http://sage.scipy.org/sage/ -~----------~----~----~----~------~----~------~--~---
