On 9/8/10 3:27 PM, Terry Reedy wrote:
On 9/8/2010 2:55 PM, Jonno wrote:
I know that I can index into a list of lists like this:
a=[[1,2,3],[4,5,6],[7,8,9]]
a[0][2]=3
a[2][0]=7
but when I try to use fancy indexing to select the first item in each
list I get:
a[0][:]=[1,2,3]
a[:][0]=[1,2,3]
Why is this and is there a way to select [1,4,7]?
You are trying to look at a list of lists as an array and have discovered where
the asymmetry of the former makes the two non-equivalent. To slice
multi-dimensional arrays any which way, you need an appropriate package, such as
numpy.
A motivating example:
[~]
|1> import numpy
[~]
|2> a = numpy.array([[1,2,3],[4,5,6],[7,8,9]])
[~]
|3> a
array([[1, 2, 3],
[4, 5, 6],
[7, 8, 9]])
[~]
|4> a[0,2]
3
[~]
|5> a[2,0]
7
[~]
|6> a[0,:]
array([1, 2, 3])
[~]
|7> a[:,0]
array([1, 4, 7])
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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