Partial Answer: "Double Trouble"

  • Article

I am so sorry for not posting the answer to "Double Trouble" sonner. Vacation and busy work schedule has prevented me from doing so, but that's no excuse!

Before I dive it why this won't work, I have to admit, you can't really tell without compiling the program. Again, not the best written quiz,:( .... maybe I should just stop making them... but it's a fun way to discuss interesting things in the .NET Framework. What do you think?

One of the main concepts I want to highlight in that post is what Tommy (who was the first person) who posted back to my comments said:

Tuesday, May 09, 2006 2:23 AM by Tommy Carlier

No, I can't. But I have a feeling that it will say 'false' for all 4 the cases. I know that doubles and == don't like each other very much.

 

Double by definition is inexact, and doing equality comparison on it is bad news. Yes, most of the time it'll work, but as I have highlighted here. There are some random numbers where it just won't work!

 

To understand why this is happening try running dumping each of the string to a byte array like this:

 

byte[] dIncorrect = BitConverter.GetBytes(d);

byte[] dCorrect = BitConverter.GetBytes(4.170404);

You will see that the least significant bit is incorrect:

 

dIncorrect {Dimensions:[0x00000008]}
[0x00000000] 0x2a byte
[0x00000001] 0x6e byte
[0x00000002] 0xdc byte
[0x00000003] 0x62 byte
[0x00000004] 0x7e byte
[0x00000005] 0xae byte
[0x00000006] 0x10 byte
[0x00000007] 0x40 byte
dCorrect {Dimensions:[0x00000008]}
[0x00000000] 0x2b byte
[0x00000001] 0x6e byte
[0x00000002] 0xdc byte
[0x00000003] 0x62 byte
[0x00000004] 0x7e byte
[0x00000005] 0xae byte
[0x00000006] 0x10 byte
[0x00000007] 0x40 byte

The next step to the investigation is to see why the bit was parsed incorrectly, which also leads me to my real reason why I haven't posted this earlier. I want to debug down on our code to see what are the "gotcha" numbers that will cause this to be inexact. I still haven't found the time to do it yet... so I will have to post a follow up to the answer.