# Create a new array by summing the columns of old array

3 views (last 30 days)
Shanice Kelly on 5 Jan 2021
Answered: Michael on 5 Jan 2021
I have an array that is 512x64, I would like to sum each consecutive columns until I have a new array that is 512x32,
so old_array column 1 + old_array column 2 = new_ array column 1
old_array column 3 + old_array column 4 = new_ array column 2
old_array column 5 + old_array column 6 = new_ array column 3
etc.
I have tried using a nested for loop but I am unsure what to exceute inside. Thank you for any help!

Stephen23 on 5 Jan 2021
Edited: Stephen23 on 5 Jan 2021
The MATLAB approach, where M is your matrix:
new = M(:,1:2:end) + M(:,2:2:end);
Shanice Kelly on 5 Jan 2021
Thank you!

Walter Roberson on 5 Jan 2021
reshape( sum( reshape(YourArray, 512, 2, 32), 2), 512, 32)
Shanice Kelly on 5 Jan 2021
Thank you!

Daniel Catton on 5 Jan 2021
a = YourArray;
b = [];
for i = 1:2:32
b = [b a(:,i)+a(:,i+1)];
end
Shanice Kelly on 5 Jan 2021
Thank you!

Michael on 5 Jan 2021
By calling up the matrix elements using their linear indices and skipping by the number of rows in your matrix you can do what you are trying. I have a simple example below using a 2x10 matrix that produces a 2x5 result. Note that this isn't the most efficient method, as it does some summations that aren't necessary and just throws them away in the last line of code. Unless you are worried about speed or memory, this is probably just fine though. I hope this helps.
%Build the matrix
blah = [1 2 3 4 5 6 7 8;...
9 10 11 12 13 14 15 16];
%Get sizes
[rows,cols] = size(blah);
els = numel(blah);
%Create an list of the linear indices of blah that excludes the last column
allbutlastcol = 1:(els-rows);
%Do the summation, but get the result as a vector
%(Here is where we skip with the +rows)
blahsum_vector = blah(allbutlastcol)+blah((allbutlastcol)+rows);
%Reshape the result to get the matrix result
blahsum_matrix = reshape(blahsum_vector,rows,cols-1);
%Remove the columns we don't need
result = blahsum_matrix(:,1:2:end)
result =
3 7 11 15
19 23 27 31

R2020b

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