reduce the length by summing the bins

dear all,

Let say I have matrix of Histograms, where each column is one histogram vector. What I want is to sum two bins each other, i.e., first bin + second bin, Third bin + forth bin.

Let say

A =
 [ 1   2  3
   4   5  6
   7   8  9
  10  11 12]

output should be

B =
 [ 5    7  9
   17   19  21 ]

Is it possible without using loops.

Answer 1

[Walter Roberson]

   B = A(1:2:end, :) + A(2:2:end,:);

[Junaid]

Thank you.

Answer 2

[Jan]

Faster for large arrays, because it does not create temporary arrays, e.g. 3 times faster for [1e5 x 3] in Matlab 2009a/64/Win:

B = reshape(sum(reshape(A, 2, 2, 3)), 2, 3);

[EDITED] General approach:

newLen = size(A, 1) / 2;
B = reshape(sum(reshape(A, 2, newLen, 3)), newLen, 3);

See also: FEX: BlockMean

[Junaid]

Thank you. It worked fine with given example.

Could you please modify the code so that I can scale it to large arrays. I tried but there was logical error.

[Jan]

Then please post what you have tried and a copy of the error message.

[Andrei Bobrov]

B = squeeze(sum(reshape(A,2,[],size(A,2))));

[Tamara del Águila]

... and how could I possibly downsample an array binning the data? With resample is not possible because it calculate averages, so I was wondering if it would be possible to do it with something like this answer, so that the matrix can be resize (let's say, to the half or less) without loosing the counts.

[Tamara del Águila]

About what I further commented on the topic, In any case anyone is interested, has the same problem, or would like to share its knowledge about the specific issue, I have posted a formal question, because I am stuck with it...

https://nl.mathworks.com/matlabcentral/answers/837388-how-to-bin-data-from-a-matrix-2d-array?s_tid=srchtitle

thanks!