Changes in Handling of Real Zeros

Question

I recently upgraded from Mathematica 5 to Mathematica 8 and I have a number of notebooks that no longer work correctly. The problems relate to the use of units. I don't need complicated unit conversions so I typically just tag values with symbols for the units. Under Mathematica 5, I could write the following (I understand that this example isn't physically meaningful):

3. in + 4. in - 7. in + 5. lb

And Mathematica would return

5. lb

Now, under Mathematica 8, I get

0. + 5. lb

This breaks a large number of notebooks. Can someone explain what is happening, and what if anything I can do to get the old behavior back?

Thanks!

Answer 1

Only precise zero coefficients are eliminated. Use Chop:

3. in + 4. in - 7. in + 5. lb // Chop

5. lb

---

Leonid admonished me for posting a method overloading Times. I didn't honestly expect anyone to use that and I think I made that pretty clear in the original post.

Nevertheless, here is a safer method that only affects uses of specified units:

units = {in, lb};

(# /: 0. # := 0 #) & /@ units;

3. in + 4. in - 7. in + 5. lb

5. lb