Help doing it the "python way"

[Jan Kuiken]

On 5/24/12 22:22 , Scott Siegler wrote:

> I am an experienced programmer but a beginner to python.  As such, I can figure out a way to code most algorithms using more "C" style syntax.
>
> I am doing something now that I am sure is a more python way but i can't quite get it right.  I was hoping someone might help.
>
> So I have a list of grid coordinates (x, y).  From that list, I want to create a new list that for each coordinate, I add the coordinate just above and just below (x,y+1) and (x,y-1)
>
> right now I am using a for loop to go through all the coordinates and then separate append statements to add the top and bottom.
>
> is there a way to do something like: [(x,y-1), (x,y+1) for zzz in coord_list] or something along those lines?

If you have lot's of numerical data you can use the NumPy module 
(http://numpy.scipy.org/), your problem would reduce to something like 
this (copied from an IPython shell, could be shorter)

Regards,
Jan Kuiken


In [1]: first_list = np.arange(0, 10).reshape((5,2))

In [2]: above = np.array([0,-1])

In [3]: below = np.array([0,+1])

In [4]: N,d = first_list.shape

In [5]: second_list = np.empty((N*2,d))

In [6]: second_list[0::2] = first_list + above

In [7]: second_list[1::2] = first_list + below

In [8]: second_list
Out[8]:
array([[  0.,   0.],
        [  0.,   2.],
        [  2.,   2.],
        [  2.,   4.],
        [  4.,   4.],
        [  4.,   6.],
        [  6.,   6.],
        [  6.,   8.],
        [  8.,   8.],
        [  8.,  10.]])

In [9]: first_list
Out[9]:
array([[0, 1],
        [2, 3],
        [4, 5],
        [6, 7],
        [8, 9]])