How to find first non-zero element and last non-zero element and TRIM vector

Question

I have a data frame:

d<-data.frame(time = factor(c("00:00","00:15","00:30","00:45", "01:00","01:15","01:30","01:45","02:00","02:40" )), q=c(0,0,100,0,0,100,0,0,0,0),p=c(.25,.25,.25,.25,.25,.25,.25,.25,.25,.25))
d


    time   q    p
1  00:00   0 0.25
2  00:15   0 0.25
3  00:30 100 0.25
4  00:45   0 0.25
5  01:00   0 0.25
6  01:15 100 0.25
7  01:30   0 0.25
8  01:45   0 0.25
9  02:00   0 0.25
10 02:40   0 0.25

I would like to eliminate rows of the data frame that are BEFORE the first non-zero index of column "q" AND AFTER the last non-zero index of column "q". In the case above the results should look like this:

  00:30 100 0.25
  00:45   0 0.25
  01:00   0 0.25
  01:15 100 0.25

What's the best way to do this?

Answer 1

You can use which

 indx <- which(d$q!=0)
 d[indx[1L]:indx[length(indx)],]
 #   time   q    p
 #3 00:30 100 0.25
 #4 00:45   0 0.25
 #5 01:00   0 0.25
 #6 01:15 100 0.25

As @Frank mentioned in the comments, if all the values are '0', then we may need a condition. The below function will return the whole dataset in that case.

 f1 <- function(dat, col){
   if(sum(dat[,col])!=0){
     indx <- which(dat[,col]!=0)
     dat[indx[1L]:indx[length(indx)],]
     }
   else{
    dat
    }
  }

 f1(d, 'q')
 #    time   q    p
 #3 00:30 100 0.25
 #4 00:45   0 0.25
 #5 01:00   0 0.25
 #6 01:15 100 0.25

Answer 2

Here is my suggestion.

d[d$q!=0,]