Finding average of every nth row?
75 views (last 30 days)
Show older comments
I have a column vector that is 9985x1. I want to create a new vector that lists the average of every 6 rows in the column. How can I do this? Is there a way to do it with a loop?
Thank you!
2 Comments
Accepted Answer
Jan
on 13 Nov 2017
Edited: Jan
on 4 Apr 2019
This is the mean over 6 subsequent rows:
x = rand(9985, 1);
S = numel(x);
xx = reshape(x(1:S - mod(S, 6)), 6, []);
y = sum(xx, 1).' / 6;
The trailing rows are ignored.
[EDITED] General method for matrices:
x = rand(9985, 14);
p = 6;
n = size(x, 1); % Length of first dimension
nc = n - mod(n, p); % Multiple of p
np = nc / p; % Length of result
xx = reshape(x(1:nC, :), p, np, []); % [p x np x size(x,2)]
y = sum(xx, 1) / p; % Mean over 1st dim
y = reshape(y, np, []); % Remove leading dim of length 1
See also FEX: BlockMean
6 Comments
More Answers (3)
Andrei Bobrov
on 13 Nov 2017
Edited: Andrei Bobrov
on 13 Nov 2017
Let A - your array (9985 x 1).
out = splitapply(@mean,A,ceil((1:numel(A))'/6));
or
out = accumarray(ceil((1:numel(A))'/6),A(:),[],@mean);
Joe S
on 26 Dec 2018
These answers work when dim=1 but appears to fail for ndims =2 (multiple columns). To average nRows together (variable "gps") for ndims=2, see below. You have the option of including the average of the "modulus rows" with: includeMods=1.
%create simple matrix, easier to check vs. random numbers
nRows = 11;
nCols = 3;
includeMods=1;
x = reshape(1:(nRows*nCols),[nRows,nCols]);
%provide row number to average over
grps = 3;
%determine remainder rows from Modulo operation
exRows = mod(nRows,grps);
% work with the top portion that group "cleanly"
topM = x(1:end-exRows,:);
%gather dims for reshaping
nRowsTop = size(topM,1);
numTop = numel(topM);
%Reshape so nrows = grps (row number to average over)
flatTop = reshape(topM,[grps numTop./grps]);
%take averages
meanTop = mean(flatTop,1);
% reshape back to original number of columns
final = reshape(meanTop,[nRowsTop./grps nCols]);
%optional, average the modulus rows, add back into final answer
if includeMods
if ~(exRows==0)
exMat= x(end-exRows+1:end,:);
meanEx = mean(exMat,1);
final = vertcat(final,meanEx);
end
end
0 Comments
See Also
Categories
Find more on Resizing and Reshaping Matrices in Help Center and File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!