Homework class help

OK, so I have learned so far about getchar/putchar, arrays, varialbes, and control flows. and my homework is writing a program that gets an input from the user and read it char by char. The string has three parts:
it has two non-negative numbers and an operator (*,-,/,+).
I need to read this string and output the result of the given input.

For example, a legal input 3.5+12 will get the output 15.5.

I have to do this without using arrays and scanf function.
Also, I am allowed to assume that the input is legal and that there
is at least one space (' ') between the numbers and the operator.

Okay, now you may ask-- so what is the problem?

Well, everything went fine but for example when I enter: 10.345+10
I get this: 10.345000+10.000000=20.345001.
Another example is: 10.34 + 123.34, gives the output 10.340000+123.340004=133.680008.

What Every Computer Scientist Should Know About Floating-Point Arithmetic

I haven't read all of this, but from a glance, I think you mean that it is a relative error, and thus I don't have much to do, right?

Interesting article, though.
Thanks.