One possible way:
S.f1 = zeros(1,10)
S.f2 = ones(1,10)
X.f1 = 55;
X.f2 = 56;
repindex = 2
for field = fieldnames(X)'
%field is a 1x1 cell array that iterates over the fields of X
S.(field{1})(repindex) = X.(field{1});
end
Edit: Actually, it may just be simpler to convert your structure into a matrix, fill it up, and convert it back to a structure. This assumes that all fields are row vectors of the same size, though:
%convert S to matrix:
Smat = cell2mat(struct2cell(S));
%load the data element-by-element:
Smat(:, repindex) = cell2mat(struct2cell(X));
%when all done convert back to struct:
S = cell2struct(num2cell(Smat, 2), fieldnames(S))
I'll leave both solution up for you to choose.
