In the IRanges vignette, it says:

"The * operator symmetrically magnifies a Ranges object by a factor, where positive contracts (zooms in) and negative expands (zooms out)." 

Then there is the code example with a confusing comment (IMO).

> ir * -2 # half the width

because this is doubling the width of the original ir.

Hi Michael,

Thanks for spotting this. Will correct. Other than  that, I've never been a big fan of these arithmetic operations on ranges. They don't meet any of the basic expectations one would have about arithmetic operations in general:

(a) When multiplying by some value, the interpretation of the sign of the value is counter intuitive:

  ir <- IRanges(120, 131)

  ir * 4
  # IRanges of length 1
  #     start end width
  # [1]   124 126     3

  ir * -4
  # IRanges of length 1
  #     start end width
  # [1]   102 149    48

So using a positive value a divides the "half width" of the original object by a and using a negative value actually multiplies it by abs(a).

(b) It turns out that multiplying by 0.25 works and produces the same result as multiplying by -4:

  ir * 0.25
  # IRanges of length 1
  #     start end width
  # [1]   102 149    48

and multiplying by -0.25 also works and produces the same result as multiplying by 4.

Ranges should really expand when multiplied by a value > 1 and shrink when the value is < 1. Multiplying by a negative value doesn't need to be supported. Division should probably be supported as a convenient way to shrink ranges e.g. ir / 4 (same as what ir * 4 currently does).

(c) Addition is inconsistent with multiplication: adding a positive value expands, a negative one shrinks:

  > ir + 2
  IRanges of length 1
      start end width
  [1]   118 133    16

  > ir - 2
  IRanges of length 1
      start end width
  [1]   122 129     8

The result of this is that interaction between addition and multiplication is not what I would expect. I would expect this

  (ir + 3) * -2
  # IRanges of length 1
  #     start end width
  # [1]   108 143    36

to be equivalent to this:

  (ir * -2) - 6
  # IRanges of length 1
  #     start end width
  # [1]   120 131    12

but it's not. It's in fact equivalent to this:

  (ir * -2) + 6
  # IRanges of length 1
  #     start end width
  # [1]   108 143    36

I think these were introduced a long time ago to provide some convenience in the context of interactive visualization but I wished they had adopted a more conventional semantic. I would discourage people from using them programmatically since we might want to revisit their semantic at some point. I'll add a note about this in the man page.

H.