I had a conversation about this issue in the mailing list several months ago: in short, if the spacings are regular you can do what you want. either:
a[1:4:2,1:4:2] += 100
or:
ind = slice(1,4,2)
a[ind, ind] += 100
Nadav
-----הודעה מקורית-----
מאת: [EMAIL PROTECTED] בשם Brian Blais
נשלח: ש 26-יולי-08 16:35
אל: Discussion of Numerical Python
נושא: [Numpy-discussion] indexing (compared to matlab)
Hello,
I wanted to do the following thing that I do in Matlab (on a bigger
problem), setting the values of a part of a matrix with indexing:
>> a=floor(rand(5,5)*10) % make an example matrix to work on
a =
2 4 7 9 8
6 9 2 5 2
6 3 5 1 8
1 5 6 1 2
1 2 8 2 9
>> ind=[2,4]
ind =
2 4
>> a(ind,ind)=a(ind,ind)+100
a =
2 4 7 9 8
6 109 2 105 2
6 3 5 1 8
1 105 6 101 2
1 2 8 2 9
===========================
In numpy, the same gives:
In [11]:a=floor(random.rand(5,5)*10)
In [14]:a
Out[14]:
array([[ 7., 7., 8., 1., 9.],
[ 0., 4., 9., 0., 5.],
[ 4., 3., 7., 8., 3.],
[ 2., 0., 4., 2., 4.],
[ 9., 5., 0., 9., 9.]])
In [15]:ind=[1,3]
In [20]:a[ind,ind]+=100
In [21]:a
Out[21]:
array([[ 7., 7., 8., 1., 9.],
[ 0., 104., 9., 0., 5.],
[ 4., 3., 7., 8., 3.],
[ 2., 0., 4., 102., 4.],
[ 9., 5., 0., 9., 9.]])
which only replaces 2 values, not all the values in the row,col
combinations of [1,1],[1,2],etc...[3,3] like matlab. Is there a
preferred way to do this, which I think should be fairly common. If
I know that the indices are regular (like a slice) is there a way to
do this?
thanks,
Brian Blais
--
Brian Blais
[EMAIL PROTECTED]
http://web.bryant.edu/~bblais
<<winmail.dat>>
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