creating time to improvement of +1 variable in r? - r

Question

I want to create a "time to improvement of +1" variable I have a longitudinal in a long format at baseline, 3, 6 and 9 months. How do I go about it in r? The improvement from the baseline.
The data is like this:
sno time WHZ
1 0 -0.5
1 3 1.4
1 6 -0.7
1 9 2.2
2 0 -0.63
2 3 0.7
2 6 -2.64
2 9 2.1
expected output
sno time WHZ impr First time to imp
1 0 -0.5 0 3
1 3 1.4 1.9 3
1 6 -0.7 -0.2 3
1 9 2.2 2.7 3
2 0 -0.63 0 3
2 3 0.7 1.33 3
2 6 -2.64 -2.01 3
2 9 2.1 2.73 3
Codes I was trying to use to first create the improvement variable:
library(dplyr)
data %>%
group_by(sno)%>%
mutate(ImprvWHZ = data$WHZ - lag(data$WHZ, default = data$WHZ[1]))

Answer 1

If I understand the question correctly, here is a dplyr solution.
library(dplyr)
dat %>%
group_by(sno) %>%
mutate(Improv = WHZ - WHZ[1],
TimeToImprov = ifelse(Improv > 1, time - time[1], NA))
## A tibble: 8 x 5
## Groups: sno [2]
# sno time WHZ Improv TimeToImprov
# <int> <int> <dbl> <dbl> <int>
#1 1 0 -0.5 0 NA
#2 1 3 1.4 1.9 3
#3 1 6 -0.7 -0.200 NA
#4 1 9 2.2 2.7 9
#5 2 0 -0.63 0 NA
#6 2 3 0.7 1.33 3
#7 2 6 -2.64 -2.01 NA
#8 2 9 2.1 2.73 9
And here is a base R solution.
res <- lapply(split(dat, dat$sno), function(DF){
DF$Improv <- DF$WHZ - DF$WHZ[1]
DF$TimeToImprov <- ifelse(DF$Improv > 1, DF$time - DF$time[1], NA)
DF
})
res <- do.call(rbind, res)
row.names(res) <- NULL
res
# sno time WHZ Improv TimeToImprov
#1 1 0 -0.50 0.00 NA
#2 1 3 1.40 1.90 3
#3 1 6 -0.70 -0.20 NA
#4 1 9 2.20 2.70 9
#5 2 0 -0.63 0.00 NA
#6 2 3 0.70 1.33 3
#7 2 6 -2.64 -2.01 NA
#8 2 9 2.10 2.73 9
DATA.
dat <- read.table(text = "
sno time WHZ
1 0 -0.5
1 3 1.4
1 6 -0.7
1 9 2.2
2 0 -0.63
2 3 0.7
2 6 -2.64
2 9 2.1
", header = TRUE)