       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

```

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