Dynamic or not?

[rishiyoor at gmail.com]

On Dec 12, 11:33 pm, Steven D'Aprano <st... at REMOVE-THIS-
cybersource.com.au> wrote:
> On Wed, 12 Dec 2007 19:18:20 -0800, rishiyoor wrote:
> > I'm trying to write a program that will find the distance between two
> > groups of points in space, which have cartesian co-ordinates X,Y and Z.
>
> > I need to find the distances between each point in one group and every
> > point in the other group. So if group 1 has 6 points and group 2 had 8
> > points, I will calculate 6 x 8 = 48 distances. But I do not know the
> > number of points the user will want to use. It is typically between 50
> > and 500 in both groups combined, but may be more. Since the memory
> > required for the distances will be much larger than the points
> > themselves, I am wondering if I will have to allocate memory dynamically
> > or if there are easier ways of solving such problems in Python.
>
> Writing in Python, you never need to manually allocate memory. Sometimes,
> for especially complicated data structures, you need to take care about
> _de_allocating memory, but that's rare.
>
> In this case, you may not have enough memory to calculate all the
> combinations at once, so you should consider an iterator-based solution.
> Read up on generators and iterators.
>
> points1 = [ (1.2, 3.4), (5.7, 9.2) ]
> points2 = [ (-6.3, 0.0), (14.1, -7.8), (2.6, 12.8) ]
>
> import math
> def distance(p1, p2):
>     return math.hypot(p1[0]-p2[0], p1[1]-p2[1])
>
> def distances(list1, list2):
>     """Yield the distances from every pair of points."""
>     for pt1 in list1:
>         for pt2 in list2:
>             yield distance(pt1, pt2)
>
> Now, if you want the distances one at a time you do this:
>
> >>> D = distances(points1, points2)
> >>> for d in D:
>
> ...     print d
> ...
> 8.23468275042
> 17.0836178838
> 9.50368349641
> 15.1208465371
> 18.9620673978
> 4.75078940809
>
> and if you want them all at once, you can do this:
>
> >>> list(distances(points1, points2))
>
> [8.2346827504160718, 17.083617883809037, 9.5036834964133785,
> 15.120846537148639, 18.962067397834023, 4.7507894080878819]
>
> --
> Steven

Thanks, that helps.

When you say python automatically allocates memory, what would you do
if you don't know the size of the list of, say for example, the
nearest pairs between the two groups. I would probably iterate over
all the pairs and create a new list. I did a similar operation in
another program, but I had to initialize the list (statically) with x
= [0]*50 before I could use it in the for loop.