Understand errors when using trimmed mean inside plyr functions called from knitr

[jennybc]

In homework 4, I get intermittent problems around the trimmed mean. Here's an example:

The user-defined function myTrimMean() is not being found here. But this code works fine in the Console and when I compile via RStudio's Knit HTML button. It also worked fine in this very compilation, run by fileherd, in the previous R chunks. Why does this happen?

[jennybc]

If I define the function myTrimMean() in my active R session, then compile via fileherd(), I get a new error:

Now the problem is not being able to see jTrim.

[jennybc]

And then, after compiling successfully with RStudio and knit2html() ... it compiles successfully with fileherd(). I am mystified. Need to strip an example down to just taking a trimmed mean in these various ways and get to the bottom of this.

[jennybc]

Is there something funky about scoping and ddply() that's only exposed when using knitr?

BTW happens with hw03 as well, which has some trimmed mean stuff.

Is this thread on the knitr Google Group relevant?
https://groups.google.com/forum/#!searchin/knitr/plyr/knitr/qZKxMrhPzEY/WC0JFrXepwwJ