[Tutor] array

[Alex Cobb]

Hi there,

         I'm hardly a Numeric wiz but I've been using this:

 >>> import Numeric
 >>> M = Numeric.arange(0, 25)           ## creates a 1-D example array of 
values 0 to 24
 >>> M = Numeric.reshape(M, (5,5))               ## makes this into a 
square 2-D matrix
 >>> M
array([[ 0,  1,  2,  3,  4],
        [ 5,  6,  7,  8,  9],
        [10, 11, 12, 13, 14],
        [15, 16, 17, 18, 19],
        [20, 21, 22, 23, 24]])
 >>> max = Numeric.maximum.reduce(M.flat)
 >>> max
24

I don't know how this compares in terms of speed to the solution
below (" max([max(row) for row in M]) "), but it's a pretty
convenient if you're already using NumPy.

Hope this helps

Alex

At 08:58 PM 3/17/2002 -0700, Paul Sidorsky wrote:
>Karshi wrote:
>
> >  How do find the maximum number in an array: say A[i,j] ?
> > I've got the following error when I tried to use "max":
> > --------------------------------------------------------------------
> > >>> M=array([[1,2,3,],[3,4,56],[4,6,8]], Float32)
> > >>> M
> > array([[  1.,   2.,   3.],
> >        [  3.,   4.,  56.],
> >        [  4.,   6.,   8.]],'f')
> > >>> max(M)
> > array([ 4.,  6.,  8.],'f')
> > ---------------------------------------------------------------------
> > which is not true.
>
>Actually it is true.  Python does a memberwise comparison of sequence
>types, and 4>3>1 so the last row is the biggest according to Python's
>comparison logic.
>
>Anyhow, try this:
>
> >>> max([max(row) for row in M])
>56.0
>
>This pulls the maximum out of each row and then takes the maximum of
>those maximums.
>
>I don't know if there's a faster solution using NumPy itself, but at
>least this is something that seems to work.
>
>--
>======================================================================
>Paul Sidorsky                                          Calgary, Canada
>paulsid@shaw.ca                        http://members.shaw.ca/paulsid/
>
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