C float and double comparisons

Question

I'm comparing simple floats and doubles in C, specifically the value 8.7 for both of them. Now I assign 8.7 to each variable, when I print I get a result of 8.7000 for both values. Why has the compiler added these zeros. And the main question I wanted to ask was is there any further numbers that I'm not seeing, as in hidden after the trailing zeros. I read that I shouldn't do comparisons like this with float because of a lack of precision, but I thought with such a small value surely it can store 8.7 with a degree of accuracy needed to compare itself with another 8.7 value?

My only worry is that its actually being represented somewhere in memory as eg 8.70000003758 or something, which is throwing my comparisons off? I tried to printf with %.20f to see any further numbers that might be hiding but I think that just created numbers that were otherwise not there as the whole accuracy of the number changed to 8.6918734634834929 or something similar.

Answer 1

I'm comparing simple floats and doubles in C, specifically the value 8.7 for both of them.

Bad choice, since 8.7 has no exact binary representation.

Now I assign 8.7 to each variable, when I print I get a result of 8.7000 for both values. Why has the compiler added these zeros.

It hasn't, your print routine has.

And the main question I wanted to ask was is there any further numbers that I'm not seeing, as in hidden after the trailing zeros.

Definitely, since 8.7 has no exact binary representation. (Try to write it out as the sum of integer powers of 2, you can't do it.)

I read that I shouldn't do comparisons like this with float because of a lack of precision, but I thought with such a small value surely it can store 8.7 with a degree of accuracy needed to compare itself with another 8.7 value?

You thought wrong. 1/3 is small but has no exact decimal representation with a finite number of digits. Whether a value is big or small has nothing to do with whether it can be represented exactly with a finite number of digits in a particular base.

My only worry is that its actually being represented somewhere in memory as eg 8.70000003758 or something, which is throwing my comparisons off?

Exactly, just as representing 1/3 as 0.333333333 would do.

I tried to printf with %.20f to see any further numbers that might be hiding but I think that just created numbers that were otherwise not there as the whole accuracy of the number changed to 8.6918734634834929 or something similar.

That's probably just a bug. Show us that code. Perhaps you tried to output a double and left out the l.