mean of unique values [Matlab]

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From: enviro on 31 May 2010 07:24

How to mean columns(2 to end )of a matrix based on the unique values of column 1. Any idea?

input=[
2 10 50
1 20 100
2 30 150
1 40 200
1 60 300];

output=[
1 40 100
2 20 200
];

From: us on 31 May 2010 11:40

From: Roger Stafford on 31 May 2010 17:13

enviro <farhadnejadkoorki(a)yahoo.co.uk> wrote in message <2037152987.255198.1275319502015.JavaMail.root(a)gallium.mathforum.org>...
> How to mean columns(2 to end )of a matrix based on the unique values of column 1. Any idea?
>
> input=[
> 2 10 50
> 1 20 100
> 2 30 150
> 1 40 200
> 1 60 300];
>
> output=[
> 1 40 100
> 2 20 200
> ];
- - - - - - - - -
Call your input matrix A and the desired output B. Then do this:

B = sortrows(A,1); % Sort according to col. 1
p = find([true;diff(B(:,1))~=0;true]); % Point to steps in col. 1
n = size(A,2);
C = cumsum([zeros(1,n-1);B(:,2:n)]); % cumsums in other cols.
B = [B(p(1:end-1),1),diff(C(p,:))./repmat(diff(p),1,n-1)]; % Get means

Comments: In the last line, diff(C(p,:)) gets the needed sums by taking differences of the appropriate cumulative sum values. Dividing by diff(p) counts gives the corresponding mean values.

Note: Presumably you meant the column 3 values of your output to be the reversal of what you wrote.

Roger Stafford

From: Steve Amphlett on 31 May 2010 18:59

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