[R] Collapsing a vector/data-frame based on the previous values

[jim holtman]

Here is another way of doing it:

> states <- structure(list(Date = c("24/07/2012", "25/07/2012", "26/07/2012",
+ "27/07/2012", "28/07/2012", "24/07/2012", "25/07/2012", "26/07/2012",
+ "27/07/2012", "28/07/2012"), State = c(1L, 1L, 1L, 1L, 1L, -1L,
+ -1L, -1L, 1L, -1L)), .Names = c("Date", "State"), class = "data.frame",
+ row.names = c(NA,
+ -10L))
> # mark the state changes
> states$diff <- c(TRUE, diff(states$State) != 0)
> states[states$diff,]
         Date State diff
1  24/07/2012     1 TRUE
6  24/07/2012    -1 TRUE
9  27/07/2012     1 TRUE
10 28/07/2012    -1 TRUE


On Tue, Jul 24, 2012 at 2:40 AM, Raghuraman Ramachandran
<optionsraghu at gmail.com> wrote:
> Hello
>
> I have a data frame like this:
> dput(states)
> structure(list(Date = c("24/07/2012", "25/07/2012", "26/07/2012",
> "27/07/2012", "28/07/2012", "24/07/2012", "25/07/2012", "26/07/2012",
> "27/07/2012", "28/07/2012"), State = c(1L, 1L, 1L, 1L, 1L, -1L,
> -1L, -1L, 1L, -1L)), .Names = c("Date", "State"), class = "data.frame",
> row.names = c(NA,
> -10L))
>
>> State
>
>       Date State
> 1  24/07/2012     1
> 2  25/07/2012     1
> 3  26/07/2012     1
> 4  27/07/2012     1
> 5  28/07/2012     1
> 6  24/07/2012    -1
> 7  25/07/2012    -1
> 8  26/07/2012    -1
> 9  27/07/2012     1
> 10 28/07/2012    -1
>
> I wish to collapse it into a smaller one based on the state value. If state
> is 1 already then the second state (and the row) is to be removed till a
> new state is found and so on.. States can be 1 or -1 only. So in the
> previous example the new data frame should be:
>
>   Date State  24/07/2012 1  24/07/2012 -1  27/07/2012 1  28/07/2012 -1  Can
> someone help?
>
> Thx
> Raghu
>
>         [[alternative HTML version deleted]]
>
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-- 
Jim Holtman
Data Munger Guru

What is the problem that you are trying to solve?
Tell me what you want to do, not how you want to do it.