function S=normalizeRows(S,q)
% function S=normalizeRows(S,q)
%
% Normalizes the rows of the (possibly sparse) matrix S.
% The normalization is done with respect to L_q norm.
% If a row is all-zero, it is left unchanged.
%
% -- Guy Lebanon, 2004

if nargin==1, q=1; end
I=find(sum(S,2)==0);
if issparse(S),
    if q~=1,
        sums=(sum(abs(S).^q,2)).^(1/q);
    else sums = sum(abs(S),2);
    end
    S = spdiags(1./sums,0,size(S,1),size(S,1)) * S; 
else
    if q~=1,
        sums=(sum(abs(S).^q,2)).^(1/q);
    else sums=sum(abs(S),2);
    end
    S=S./repmat(sums,[1 size(S,2)]);
end
S(I,:)=0;