I would use a while loop. Something like this maybe: (If your function produces a 'new' value every time you call it.)
value1 = myfunctionvalue()
error = 1;
while error ~= 0
value2 = myfunctionvalue();
error = value2 - value1
value1 = value2
end
Although, really you should probably have some tolerance on the while loop (error<tolerance). If the subsequent calls don't match exactly, then the above approach could get stuck.
