how to subset columns of data frame in r

Question

I have lots of data frame like the one below. I need to find the Total in last sample just before weight value is zero. here for example in the sample of 7 I need to find the weight that is 6.

Sample Weight Total
1       2     0
2       3     0 
3       7     0
4       5     4
5       4     3
6       3     8
7       6     12
8       0     11
9       0     8
10      0     9

Here is what i have tried:

New.DF <- DF$Total [ Weight > 0, Sample=max]

Can u show the expected otuput

akrun
– akrun

2017-06-01 09:50:27 +00:00
Commented Jun 1, 2017 at 9:50 — akrun
– akrun, Commented Jun 1, 2017 at 9:50

akrun · Accepted Answer · 2017-06-01 09:54:13Z

1

We need to extract the column

 with(DF, max(Sample[Weight!=0]))
 #[1] 7

Or may be

library(data.table)
setDT(DF)[Weight!=0, .SD[.N] ,.(grp = rleid(Weight==0))]
#      grp Sample Total
#1:     1      7    12

edited Jun 1, 2017 at 9:54

answered Jun 1, 2017 at 9:47

akrun

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2 Comments

Mori Over a year ago

Thank you for answer, i tried first but why these two give me different answer?b <-with(df, max(Sample[EZ!=0])) c<- df$Sample[ with(df, max(Sample[EZ!=0]))]

akrun Over a year ago

@Mori It is because you are subsetting the Sample and then use it as an index in the 2nd case, which is not right. INstead you may need i1 <- with(df, Sample == max(Sample[EZ !=0])) and use that to subset

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