How can I compare a value in a column to the previous one using R?

Question

I have a large dataframe in R composed of 4 columns and thousands of lines. Here are the first lines as an example:

     ID     V1      V2     Stimulus
[1,]          


        

Answer 1

Firstly, create the new colum (dat is the name of your dataset):

Status <- ave(dat[ , "Stimulus"], c(0, cumsum(abs(diff(dat[ , "Stimulus"])))),
              FUN = function(x)
                      if(!x[1]) "PRE" else c(rep("OK", min(2, length(x))),
                                             rep("POST", length(x) - 2)))

Now, combine both objects:

cbind(dat, Status)

The result:

      ID    V1     V2 Stimulus Status
[1,]   1 74.80  803.0        0    PRE
[2,]   1 75.98  790.9        0    PRE
[3,]   1 75.95  791.1        0    PRE
[4,]   1 65.70  918.7        0    PRE
[5,]   1 59.63 1005.6       13     OK
[6,]   1 59.44 1012.0       13     OK
[7,]   1 59.62 1010.0       13   POST
[8,]   1 63.85  942.4       13   POST
[9,]   1 60.75  992.9        0    PRE
[10,]  1 59.62 1010.0        0    PRE
[11,]  1 61.68  974.0        0    PRE
[12,]  1 65.21  921.4       15     OK
[13,]  1 59.23 1012.0       15     OK
[14,]  1 61.23  979.5       15   POST
[15,]  1 70.80  849.2        0    PRE