CODEWITHSUNDEEP
rdata.table
Burgiled
Burgiled

Reputation: 33

R - Find indices of first non zero elements in data.table

I've got pannel data in the following form:

DT = data.table(Id=1:5, Time1=c(0,0,1,2,0), Time2=c(1,0,0,0,0), Time3=c(0,1,2,0,0), Time4=c(0,1,2,5,4))

Id  Time1 Time2 Time3 Time4
1   0     1     0     0
2   0     0     1     1
3   1     0     2     2
4   2     0     0     5
5   0     0     0     4

I want to extrac the first time for every ID which is not 0. The dataset is large, therefore I was looking for a solution with data.table.

I tried something using which.min(which!=0), but could work it out.

The output should look as follows:

Id  Time1 Time2 Time3 Time4 Output
1   0     1     0     0     2
2   0     0     1     1     3
3   1     0     2     2     1
4   2     0     0     5     1
5   0     0     0     4     4

Thanks for your help.

Upvotes: 3

Views: 1305

Answers (1)

Rich Scriven
Rich Scriven

Reputation: 99351

You could do

DT[, Output := which.max(.SD != 0), by = Id][]
#    Id Time1 Time2 Time3 Time4 Output
# 1:  1     0     1     0     0      2
# 2:  2     0     0     1     1      3
# 3:  3     1     0     2     2      1
# 4:  4     2     0     0     5      1
# 5:  5     0     0     0     4      4

which.max finds the index of the first occurrence of the maximum value. So when we do .SD != 0 it finds the first occurrence of TRUE.

Equivalently, we could also use

DT[, Output := match(TRUE, .SD != 0), by = Id]
# or 
DT[, Output := which(.SD != 0)[1], by = Id]

Upvotes: 3

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