Find last row in a sequence of consecutive values

Question

I am working with a series of TRUE and FALSE sequences that are organized by group and stretching over some span of time (e.g. days). Below, I have taken the time to recreate an example and convert the data.frame into a data.table using the data.table package.

> dput(df)
structure(list(day = c(1L, 2L, 3L, 4L, 5L, 6L, 7L, 1L, 2L, 3L, 
4L, 5L, 6L, 7L), group = structure(c(1L, 1L, 1L, 1L, 1L, 1L, 
1L, 2L, 2L, 2L, 2L, 2L, 2L, 2L), .Label = c("a", "b"), class = "factor"), 
    condition = structure(c(1L, 1L, 2L, 2L, 2L, 1L, 1L, 1L, 2L, 
    2L, 1L, 1L, 2L, 2L), .Label = c("FALSE", "TRUE"), class = "factor")), .Names = c("day", 
"group", "condition"), row.names = c(NA, -14L), class = "data.frame")
> library(data.table)
> df <- as.data.table(df)
> setkey(df, group)
> df
    day group condition
 1:   1     a     FALSE
 2:   2     a     FALSE
 3:   3     a      TRUE
 4:   4     a      TRUE
 5:   5     a      TRUE
 6:   6     a     FALSE
 7:   7     a     FALSE
 8:   1     b     FALSE
 9:   2     b      TRUE
10:   3     b      TRUE
11:   4     b     FALSE
12:   5     b     FALSE
13:   6     b      TRUE
14:   7     b      TRUE

Using data.table, I know how to select the first day for each group where condition is equal to "TRUE":

> df[condition == "TRUE", day[1], by = group]
   group V1
1:     a  3
2:     b  2
> df
    day group condition
 1:   1     a     FALSE
 2:   2     a     FALSE
 3:   3     a      TRUE     <~~~~~~~~
 4:   4     a      TRUE
 5:   5     a      TRUE
 6:   6     a     FALSE
 7:   7     a     FALSE
 8:   1     b     FALSE
 9:   2     b      TRUE     <~~~~~~~~
10:   3     b      TRUE
11:   4     b     FALSE
12:   5     b     FALSE
13:   6     b      TRUE
14:   7     b      TRUE

What I would like to accomplish is a final output that identifies the last row for the first sequence of each group where the condition is equal to "TRUE". Please see my output below:

       group V1
    1:     a  5
    2:     b  3
> df
    day group condition
 1:   1     a     FALSE
 2:   2     a     FALSE
 3:   3     a      TRUE
 4:   4     a      TRUE
 5:   5     a      TRUE     <~~~~~
 6:   6     a     FALSE
 7:   7     a     FALSE
 8:   1     b     FALSE
 9:   2     b      TRUE
10:   3     b      TRUE     <~~~~~
11:   4     b     FALSE
12:   5     b     FALSE
13:   6     b      TRUE
14:   7     b      TRUE

Any advice, suggestions, or other material would be much appreciated. Thank you for your time and feel free to ask any clarifying questions!

Answer 1

Here we can look for changes in the condition, specifically a transition from TRUE to FALSE.

a <- df[diff(c(condition,1)) == -1, day[1], by=group]

#    group V1
# 1:     a  5
# 2:     b  3

Thus does assume that the condition column is a factor with FALSE as the first level.