You can sort and then keep just the 512 with the highest values:
data = rand(1, 2048);
subplot(2, 1, 1);
plot(data);
% Sort the data
[sortedData, sortIndices] = sort(data, 'descend');
% Find the first 512 - they will be the greatest value.
elementsToKeep = sortIndices(1:512);
% Get them back in their original order:
elementsToKeep = sort(elementsToKeep);
% Extract those elements
extractedData = data(elementsToKeep);
subplot(2, 1, 2);
plot(extractedData);
% Enlarge figure to full screen.
set(gcf, 'units','normalized','outerposition',[0 0 1 1]);
That's one way to do it.
