function defarg( arg, val, nullok )

% DEFARG  Assign a default value to an argument.
%
% defarg( arg, val )
%     - <arg> is the string name of the argument
%     - <val> is the value to be assigned to <arg> if it doesn't exist
%
% e.g., defarg( 'arg1', -1 )
%       defarg( 'arg2', 'none' )

% 18-Feb-98 -- created (RFM)
% 20-Feb-98 -- added ability to handle fields of structures (RFM)
% 10-Dec-98 -- replaced evalin with assignin, as suggested by CPT (RFM)

% This could be cleaned up a lot.

if nargin<3,
	nullok=0;
end

% are we assigning a field of a structure array?
p=strchr(arg,'.');
if p==0,
	if (evalin('caller',sprintf('exist(''%s'',''var'')',arg))==0) | ...
		( (evalin('caller',sprintf('isempty(%s)',arg))==1) & (nullok==0) ),
		assignin('caller',arg,val);
	end

% yes, we are
else
	f1=arg(1:p-1);
	f2=arg(p+1:end);
	baseexists=evalin('caller',sprintf('exist(''%s'',''var'')',f1));
	if baseexists,
		fieldexists=evalin('caller',sprintf('isfield(%s,''%s'')',f1,f2));
	else
		fieldexists=0;
	end
	if ~( baseexists & fieldexists ),
		if ischar(val),
			evalin('caller',sprintf('%s=''%s'';',arg,val));
		else
			evalin('caller',sprintf('%s=%f;',arg,val));
		end
	end
end

return


% STRCHR  Find first occurrence of a character in a string
function pos = strchr( str, chr )
p=find(str==chr(1));
if isempty(p),
	pos=0;
else
	pos=p(1);
end
return