Addition errors in IEEE754 floating point representation

Question

So in class, we were talking about the idea of floating point precision in IEEE754 format, and how, when some numbers are added, precision is lost. My professor then gave the following example of a large absolute error between what we think the answer might be, and what the answer actually is when our program (written in Java) actually compiles it:

$0.4 \times10^{30} + 1.1 \times10^{30}$

Compiling this in Java, i get $1.5000001 \times 10^{30}$

My question is, why does this happen? Printing out the IEEE754 format of the two numbers, I get:

$\space\space\space0[11100010]10111100010010010101011=+1.10111100010010010101011\times2^{99}$ $+0[11100001]01000011000111100001000=+1.01000011000111100001000\times2^{98}$

So, I suppose when you add these, you end up loosing precision, but exactly how? Moreover, how can I even tell, before adding, that the addition of two numbers will make them loose precision? Or is the only way to just plus into a program and see what happens? None of this was really covered in class unfortunately.

Answer 1

Printing out the IEEE754 format of the two numbers, I get:

$\space\space\space0[11100010]10111100010010010101011=+1.10111100010010010101011\times2^{99}$ $+0[11100001]01000011000111100001000=+1.01000011000111100001000\times2^{98}$

Compare the exact values:

$$ 1.1011110001001001010101011000111111010110011111001110000010110110100111 \times 2^{99} \\ 1.0100001100011110000011111010111001101101011100100001011111001010101 \times 2^{98} $$

Observe that both of the numbers are already rounded to represent them in IEEE 754, and it happens that both of them have rounded up.

Also note that since the values have different exponents, they've been rounded at different places.

Now add the rounded values:

  1.10111100010010010101011 ×299
+ 0.101000011000111100001000×299
 -------------------------------
 10.01011101110110000101111 x299
 11  1         1111

There's overflow, so the answer will have exponent $2^{100}$ and we have to round. Again, we're rounding up, to $1.00101110111011000011000\times2^{100}$.

So, I suppose when you add these, you end up loosing precision, but exactly how?

You will nearly always lose precision when adding two floating point numbers. If they're both positive and have the same exponent, the output will have a larger exponent and you lose some bits from the end. If they have different exponents, the smaller one will lose bits from the end. And if they have opposite signs and the same exponent, the output has a smaller exponent and you lose precision because you need bits beyond the end which were lost earlier. Essentially the only cases where you don't lose precision are when you add a number to itself, or when the input numbers and the result can all be represented exactly in the width of the type.