I would recommend you use fullfile instead of strcat to build paths. This way you don't have to worry about path separators.
Assuming that all files have the number of rows, this is how I'd do it:
source_folder = 'C:\Users\Desktop\Analyze'; %much more meaningful name than D
numfiles = 250;
data = []; %will be preallocated upon reading the first file, once we know the size of a column
for fileidx = 1:numfiles
filedata = readmatrix(fullfile(source_folder, sprintf('Data_%d.xlsx', fileidx)));
if fileidx == 1
data = zeros(size(filedata, 1), numfiles); %preallocate to the height of columns x num files
end
data(:, fileidx) = filedata(:, 7); %copy column 7 into output matrix
end
end
