finding average of a percentile of rows in a matrix?
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Hi All,
In the below given matrix, I wish to calculate the mean/average of first 20% rows and subtract it from the mean of the last 20% rows.
x =
1 2 3 4 5
2 4 6 8 10
3 6 9 12 15
4 8 12 16 20
5 10 15 20 25
1 2 3 4 5
2 4 6 8 10
3 6 9 12 15
4 8 12 16 20
5 10 15 20 25
this is a sample matrix, so there can be 100 rows and 100 columns but I need to average the values in the first 20% and subtract it from the average of last 20%. No sorting is needed in my matrix. therefore, in the above matrix, I would need to average the first two rows and subtract the resultant from the average of the last two rows.
any help will be appreciated.
Regards,
AMD.
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Accepted Answer
Azzi Abdelmalek
on 26 Jul 2012
Edited: Azzi Abdelmalek
on 26 Jul 2012
n=size(x,1);n1=round(n/5);row_mean=mean(x(n-n1+1:end,:))-mean(x(1:n1,:))
result_mean=mean(row_mean)
% you can use the result: row_mean wich is a line % or the result: result_mean which is the mean of the line row_mean
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More Answers (2)
Wayne King
on 26 Jul 2012
Edited: Wayne King
on 26 Jul 2012
Assume A is your matrix.
numrows = round(0.2*size(A,1));
lastrowstart = size(A,1)-numrows+1;
lowermean = mean(A(1:numrows,:),2);
uppermean = mean(A(lastrowstart:end,:),2);
uppermean-lowermean
Or did you mean take the mean over all elements in the bottom 20% of rows and upper 20% of rows, so you end up with a single number?
If that is the case:
numrows = round(0.2*size(A,1));
lastrowstart = size(A,1)-numrows+1;
lowermean = mean(mean(A(1:numrows,:),2));
uppermean = mean(mean(A(lastrowstart:end,:),2));
uppermean-lowermean
5 Comments
Azzi Abdelmalek
on 30 Jul 2012
Edited: Azzi Abdelmalek
on 30 Jul 2012
%replacing nan by zero
x(find(isnan(x)))=0 % add this to replace nan by zero
% the previous code
n=size(x,1);n1=round(n/5);row_mean=mean(x(n-n1+1:end,:))-mean(x(1:n1,:))
result_mean=mean(row_mean)
%in case you want calculate the mean, ignoring nan , that means: if i have [1 3 nan 4]; the mean will not (1+3+0+4)/4, but (1+3+4)/3. in this case add this code
ind=arrayfun(@(y) ~isnan(y),x)
x(find(isnan(x)))=0;
n=size(x,1);n1=round(n/5);
row_mean1=sum(x(n-n1+1:end,:))./sum(ind(n-n1+1:end,:))-sum(x(1:n1,:))./sum(ind(1:n1,:))
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