append_struct_array(struct, field, elem, idx_elem)

function new_struct = append_struct_array(struct, field, elem, idx_elem)
% APPEND_STRUCT_ARRAY Append an element to a structure array in a (parent) structure.
% NEW_STRUCT = APPEND_STRUCT_ARRAY(STRUCT, FIELD, ELEM, IDX_ELEM) Append
% element ELEM to the structure array named FIELD in parent structure
% STRUCT. Returns a new parent structure NEW_STRUCT. If this field does
% not exist, create new one. 
%
% Optional IDX_ELEM parameter causes the specified element number to be replaced.
%
% Misha Koshelev
% July 29th, 2009
% Montague Laboratory

    if ~isfield(struct, field) || ...
            isempty(struct.(field))
        struct.(field) = elem;

    else
        if ~exist('idx_elem', 'var')

            % New element
            idx_elem = length(struct.(field))+1;
        end
        
        % Field names in new element
        elem_field = fieldnames(elem);
        for idx_elem_field=1:length(elem_field)
            
            elem_field_this = elem_field{idx_elem_field};
            struct.(field)(idx_elem).(elem_field_this) = ...
                elem.(elem_field_this);
        end
    end
    
    new_struct = struct;