How could I read let\'s say 10 floats and store them in an array without wasting any memory?
You'll have to be more specific about the problem.
Unless you have something else that you need to do with these numbers, then the easiest way to save memory is not to store them in an array to begin with.
It sounds like what you want is something like.
sum = 0;
do
read current
sum += current
while (you haven't yet read 10 numbers);