R style subset/replace in Rcpp - c++

Question

In R I have a loop which runs through a data frame of people coming and leaving a site and assigns them to a parking lot until they leave when it releases the spot.
activity = structure(list(ID = c(1, 2, 3, 3, 1, 2, 3, 2, 3, 1, 2, 1),
Lot = structure(c(1L, 3L, 6L, 6L, 1L, 3L, 2L, 5L, 2L, 4L, 5L, 4L),
.Label = c("a", "A", "C", "d", "f", "z"), class = "factor"),
time = c(1, 3, 6, 7, 8, 9, 10, 12, 13, 14, 15, 21),
dir = c(1, 1, 1, -1, -1, -1, 1, 1, -1, 1, -1, -1),
Here = c("no","no","no","no","no","no","no","no","no","no","no","no")),
class = "data.frame",
row.names = c(NA, -12L), .Names = c("ID", "Lot", "time", "dir","Here"))
lots = structure(c(30, 175, 170, 160, 300, 300, 35, 160, 85, 400, 200,
110, 60, 130, 100, 80, 1000, 320, 330, 350, 320, 250, 250, 370,
20, 40, 0, 140, 185, 200, 185, 120, 55, 105, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0), .Dim = c(34L, 2L),
.Dimnames = list(c("a", "b", "c", "d", "e", "f", "g", "h", "i", "j",
"k", "l", "m", "n", "o", "p", "q", "r", "s", "t",
"u", "v", "x", "y", "z", "A", "B", "C", "D", "E",
"F", "G", "H", "I"), c("Capacity", "Utilization")))
for(i in 1:nrow(activity)){
id = activity$ID[i]
l = activity$Lot[i]
d = activity$dir[i]
t = activity$time[i]
lots[l,"Utilization"] = lots[l,"Utilization"] + d
if(d == -1) activity$Here[activity$ID == id & activity$time <= t] = "no"
}
What I want to determine is, in Rcpp is there a way to do the R style subset and replace? e.g., lots[l,"Utilization"] = ... and activity$Here[activity$ID == id & activity$time <= t] = .... I'm specifically wanting to be able to load in 3 matrices, the master record table, the matrix which manages lots and the matrix which manages current locations (in this example I know the lot already, in practice I need to determine where they are likely to park). I have functioning code but it takes ~160 seconds to run and I'm trying to learn how to leverage Rcpp to make this much faster.