rand_char = @(n) char(ceil(94*rand(1,n))+31);

for ii = 1:3    
    N = ceil(40*rand(1))+9;
    a1 = ceil(150*rand(1,N)) + 50;
    a2 = ceil(200*rand(1,N));
    str1 = arrayfun(@(x) repmat(rand_char(1),1,a1(x)), 1:numel(a1), 'UniformOutput',0);
    str2 = arrayfun(@(x) rand_char(a2(x)), 1:numel(a2),'UniformOutput',0);
    str_cell = reshape([str1; str2],1,[]);
    rand_string{ii} = [str_cell{:}]; 
end