statistics: some bugs

[sjamaan]

The scrutinizer found a few bugs in statistics. The first one in wilcoxon-signed-rank-test initially looked like a simple mistake of swapping the arguments to list-ref, but even after fixing that I was unable to get it to do anything but error out.

#1;> (use statistics srfi-1)
#2;> (wilcoxon-signed-rank-test (iota 17))
;; ERROR

After adding some debugging code, it looks like the indexes are wrong; last is determined counting from the end of the list, but it *looks* like the code is treating it as if it's counted from the front. After changing that accordingly I still kept getting errors, so I'll leave that to you. I decided to let my patch simply fix these warnings and nothing more.

The second message about wilcoxon-signed-rank-test refers to the fact that var-r1 is being applied as if it were a procedure. However, it's just a number, so this would fail.

The warning about mcnemars-test is due to the fact that the return value of set! is undefined in Scheme (this is different from Common Lisp, where setf returns the last value set, I think). I don't really understand the purpose of the named let, so I didn't touch it (instead just doing a simple transformation that's always safe), but it really looks like it doesn't do anything at all.

Attached is a patch that gets rid of these warnings.