[Tutor] Best way to store and access a fixed coordinate list

[Steven D'Aprano]

David Crisp wrote:
> Hello,
> 
> I have a large grid of numbers  100 * 100
> 
> I then randomly select an X and Y to act as a "centre" point.
> 
> I have a list of numbers which are coordinate offsets which are then
> applied to the centre point as per:
> 
> X = (-2,2),(-4,2),(4,2),(2,2)  (The list is about 200 coordinate pairs long)


Two-tuples (x, y) are probably the most light-weight way of doing this. 
You then add them like this:


 >>> a = (1, 2)
 >>> b = (100, 200)
 >>> (a[0] + b[0], a[1] + b[1])
(101, 202)


A helper function for adding them together will make life easier:

def add(p1, p2):
     """Return points p1 + p2."""
     return (p1[0] + p2[0], p1[1] + p2[1])


And in action:

 >>> a = (1, 2)
 >>> b = (100, 200)
 >>> add(a, b)
(101, 202)



Slightly less light-weight would be a namedtuple, available from Python 
2.6 on up. This lets you define a light-weight Point class with named 
attributes:

 >>> from collections import namedtuple
 >>> Point = namedtuple("Point", "x y")
 >>> a = Point(23, 42)
 >>> a.x
23
 >>> a.y
42


Named tuples still behave like tuples, so you can use them like ordinary 
tuples:

 >>> a[0]
23
 >>> a == (23, 42)
True


and the add() helper function will continue to work.


A more heavy-weight solution would be a full-blown Point class, that 
defines all the arithmetic operations. Here's one way to do it:

class Point(namedtuple("Point", "x y")):
     def __add__(self, other):
         return (self[0] + other[0], self[1] + other[1])
     __radd__ = __add__
     # and similar for subtraction __sub__ and __rsub__


And in action:

 >>> a = Point(1, 2)
 >>> b = Point(100, 200)
 >>> a + b
(101, 202)


The advantage here is that you don't need the helper function, you can 
just add Points with the + operator.




-- 
Steven