[R] selection based on dates

Rui Barradas ruipbarradas at sapo.pt
Tue Jul 30 15:08:38 CEST 2013


Try the following.

idx <- which(c(TRUE, diff(as.integer(x[,2])) != 0))

Also, note that in constructs such as
a <-c("1/1/13",15,20)
both 15 and 20 are coerced to character. So your matrix is a matrix of 
chars. For different types of data, use data.frames

Hope this helps,

Rui Barradas

Em 30-07-2013 13:13, Andras Farkas escreveu:
> Dear All
> please provide your insigths on the following:
> I have:
> a <-c("1/1/13",15,20)
> b <-c("1/5/13",15,25)
> c <-c("1/9/13",15,28)
> d <-c("2/1/13",18,30)
> e <-c("2/5/13",18,35)
> f <-c("2/9/13",18,38)
> x <-matrix(c(a,b,c,d,e,f),ncol=3,byrow=TRUE)
> What I would like to do is to eliminate certain rows of this matrix based on the date column values. As you can see, in the second column my values (15 and 18) repeat 3 times each, so this column serves as an ID number if you will. Thus each ID numbers show up with 3 different date values in the first column. Now I would like to eliminate the rows with the earliest date per ID number. My result should look like this:
> z <-x[-c(1,4),]
> as allways, your help is greatly appreciated,
> thanks,
> Andras
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