[R] how to code it??

[Gabor Grothendieck]

On Wed, Jul 28, 2010 at 2:10 PM, Raghu <r.raghuraman at gmail.com> wrote:
> Hi
>
> I have say a large vector of 3500 digits. Initially the digits are 0s and
> 1s. I need to check for a rule to change some of the 0s to -1s in this
> vector. But once I change a 0 to -1 then I need to start applying the rule
> to change the next 0 only after I see the next 1 in the vector.
>
> Say for example x = (0,0,0,0,0,0,0,1,0,0,1,1,0,0,1,0,0,0,1)
> I need to traverse from the 9th element to the last ( because the first
> occurrence of 1 is at 8) . Let us assume that according to our rule we
> change the 13th element (only 0s can be changed) to -1. Now we need to go to
> the next occurrence of 1 (which is 15) and begin the rule application from
> the 16th till the end of the vector and once replaced a 0 to a -1 then start
> again from the next 1. How do we code this? I 'feel' recursion is the best
> possible solution but I am not a programmer and will await experts' views.
> If this is not a typical R-forum question then my advance apologies.
>

If you are asking to replace the first 0 after any string of 1s with a
-1 then try this:


> replace(x, c(FALSE, diff(x) < 0), -1)
 [1]  0  0  0  0  0  0  0  1 -1  0  1  1 -1  0  1 -1  0  0  1