[R] Ceiling to the nearest ten?

[Barry Rowlingson]

Rob Robinson wrote:
> Perhaps not the most elegant but 10*ceiling(x/10) usually works for me...

  I thought maybe adding five and rounding to the nearest -1 decimal 
places might be quicker:

  > round(a+5,-1)
   [1] 10 10 20 20 20 30 30 40 40 40

  gives same as:

  > ceiling(a/10)*10
   [1] 10 10 20 20 20 30 30 40 40 40

but it's not quicker:

  > system.time(for(i in 1:1000000){round(a+5,-1)})
  [1] 17.505  0.116 19.538  0.000  0.000
  > system.time(for(i in 1:1000000){ceiling(a/10)*10})
  [1] 5.212 0.048 5.726 0.000 0.000

which surprised me, given that the ceiling example has a multiply and a 
divide to do before working out the ceiling, whereas the round just has 
an add to do before working out the rounding, and I can't immediately 
see why ceilinging should be faster than roundinging:

  > system.time(for(i in 1:1000000){ceiling(a)})
  [1] 2.724 0.044 3.053 0.000 0.000
  > system.time(for(i in 1:1000000){round(a)})
  [1] 12.397  0.092 14.339  0.000  0.000

  but maybe if I think about it a bit more I'll see...

Barry