Sum of lag functions - r

Question

Within one person's data from a behavioral task, I am trying to sum the clock time at which a target appears (data$onset) and the reaction time of their response (data$Latency) to find the clock time at which they entered their response at. For future data processing reasons, these calculated values will have to be placed in the data$onset column two values down from when the target appeared on the screen. In the example below:
Item
onset
Latency
Prime
9.97
0
Target
10.70
0.45
Mask
11.02
0
Response
NA
0
Onset is how many seconds into the task the stimuli appeared, and latency is reaction time to the target. latency for non-targets will always be 0, as subjects don't respond to them. in the "NA" under onset, I need that value to be the sum of the onset of the target+reaction time to the target (10.70+0.45). Here is the code I have tried:
data$onset=if_else(is.na(data$onset), sum(lag(data$onset, n = 2)+lag(data$Latency, n = 2)), data$onset)
If any clarification is needed please let me know.

Answer 1

since you used if_else I'm adding a dplyr solution;
library(dplyr)
data %>%
mutate(onset=ifelse(is.na(onset),lag(onset,n =2)+lag(Latency,n = 2),onset))
output;
Item onset Latency
<fct> <dbl> <dbl>
1 Prime 9.97 0
2 Target 10.7 0.45
3 Mask 11.0 0
4 Response 11.1 0
Also note that, if you want to stick to your own syntax;
data$onset=if_else(is.na(data$onset), lag(data$onset, n = 2)+lag(data$Latency, n = 2), data$onset)