[R] How hist() decides breaks?

[(Ted Harding)]

Hi Folks,
I'd like to know how hist() decides how many cells to use
when it ignores my "suggestion" to use say 'hist(...,breaks=50)'.

More specifically, I have the results of 10000 simulations,
each returning an 8-vector, therefore 8 variables each with
10000 values. Some of these 8 have somewhat skew distributions.
Say one of these 8 variables is X.

I ask for H <- hist(X,breaks=50), and get a histogram which
usually has a different number of cells than what I intended.

For instance, for one of these simulations, the 8 different
values of length(H$breaks) are:

  70, 44, 38, 68, 50, 40, 46, 45

?hist tells me

A)
  breaks: one of:
    *  a vector giving the breakpoints between histogram
       cells,
    *  a single number giving the number of cells for the
       histogram,
    *  a character string naming an algorithm to compute the
       number of cells (see Details),
    *  a function to compute the number of cells.

    In the last three cases the number is a suggestion only. 

B)
  The default for 'breaks' is '"Sturges"': see 'nclass.Sturges'.

If I look at the code for nclass.Sturges() I see

  function (x) ceiling(log2(length(x)) + 1)

and, for length(X) = 10000, this gives 15. This is not related
to any of the numbers of breaks I actually got, in any way obvious
to me.

So:
Question 1: hist() has apparently ignored my "suggestion" of
  "break=50". Why? What is the criterion for ignoring?

Question 2: Presumably, if it ignores the "suggestion", it
  does something else, of its choice. I would then, perhaps,
  expect it to fall back to its default, which is (allegedly)
  Sturges. But the result from nclass.Sturges looks different
  from what it actually did. So what did it actually do, and
  how did it decide on this?

With thanks,
Ted.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
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Date: 19-May-08                                       Time: 10:31:20
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