Seems like it should work, but it doesn't. However this works:
%% Goal is cell array 'cellArray' containing 'High' for certain elements, and 'Low' for other elements
% Create the index
a = rand(100, 1);
cellArray = cell(100, 1); % Initialize cell array
iHigh = a > 0.5; % Index to many values
iLow = a <= 0.5; % Index to many other values
% Assign the cell values based on the index.
% Put a cell into an element of an array that is a cell.
cellArray(iHigh) = {'High'};
cellArray(iLow) = {'Low'};
