Re: simple newbee questions
- To: mathgroup at smc.vnet.net
- Subject: [mg29546] Re: [mg29518] simple newbee questions
- From: BobHanlon at aol.com
- Date: Sun, 24 Jun 2001 02:00:59 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
In a message dated 2001/6/23 2:07:47 AM, wzn at jongnederland.nl writes:
>How can I calculate the mean of a matrix (n*m), an array works fine...
>
>How can I select an area in a matix?
>
m = Array[a, {2, 3}];
The means by row
(Plus@@#)/Length[#]& /@ m
{(1/3)*(a[1, 1] + a[1, 2] + a[1, 3]),
(1/3)*(a[2, 1] + a[2, 2] + a[2, 3])}
The means by column
(Plus@@#)/Length[#]& /@ Transpose[m]
{(1/2)*(a[1, 1] + a[2, 1]),
(1/2)*(a[1, 2] + a[2, 2]),
(1/2)*(a[1, 3] + a[2, 3])}
The mean of all elements
(Plus@@#)/Length[#]&[Flatten[m]]
(1/6)*(a[1, 1] + a[1, 2] + a[1, 3] + a[2, 1] +
a[2, 2] + a[2, 3])
m = Array[a, {8, 10}];
If you want the intersection of rows (4 - 6) with columns (2 - 5)
m[[Range[4, 6], Range[2, 5]]]
Bob Hanlon
Chantilly, VA USA