How to apply function in each row in data.table

Question

Supposed I have the following

dt <- data.table(a=c(T,T,F,F), b= c(T,F,T,F))

return,

       a     b
1:  TRUE  TRUE
2:  TRUE FALSE
3: FALSE  TRUE
4: FALSE FALSE

I have tried to use function(x) min(which(x)) to determine the first TRUE of each row in dt, but it did not work. The result that I expect will be

       a     b index
1:  TRUE  TRUE     1
2:  TRUE FALSE     1
3: FALSE  TRUE     2
4: FALSE FALSE  9999

,where the index column represents the position of first TRUE and 9999 is used when that row contains only FALSE

FYI: In the real data, I have around 50 columns that contains TRUE and FALSE

Could you please give me suggestions?

Answer 1

For 50 columns, it is better to use max.col

dt$index <- max.col(dt, 'first') *(!!rowSums(dt))

Or as @David Arenburg mentioned, more idiomatic code would be

dt[, indx := max.col(.SD,ties.method="first")*(!!rowSums(.SD))]

If we need 9999

 (max.col(dt)*(!!rowSums(dt))) + (!rowSums(dt))*9999