Try this and see if it does what you want:
data = rand(1336, 89700); % Create sample data.
sums = zeros(1336, 299); % Preallocate.
for col = 1 : size(data, 2) / 299
% Extract every 300th column, starting with column "col", into a new matrix.
extractedMatrix = data(:, col : 300 : end); % A new matrix 1/300th as wide as the original
% Sum all the values in that matrix, and put that sum into means.
% Get the sums for just this one set of columns.
theseSums = sum(extractedMatrix, 2);
% Store these sums in the column "col" of our master array.
sums(:, col) = theseSums; % Get a single sum from the entire 2-D matrix.
end
whos sums
