I'm trying to calculate how much water weight was lost by 13 frogs and toads (5 frogs, 8 toads) placed in front of a fan for a total of 45 minutes (don't worry! all toads and frogs are ok!).
I weighed each individual(g) after 9 minutes, so I have a total of 5 measurements. I want to know the rate of water lost of each at 9 minutes interval.
When I run a for loop, I only get a rate for the first measurement.
How to get a rate for each 9 minute interval for all 13 animals?
This is what my code looks like:
for(i in 1: length(EWL)) {
for(j in 1:13) {
ft <- EWL[which(EWL$Animal == as.character(j)), ]
wr <- (ft[6, "Weight"] - ft[1, "Weight"]) / 45
EWL$WR[EWL$Animal == as.character(j)] <- wr
}
}
For toad 1, this code gave me -.06, which is repeated for all 5 measurements. Same thing for toad 2, I get -.08 for each interval. The math is correct for the first interval calculation, it's just repeated for all intervals.
My data looks like this:
> EWL
Time Weight Posture Animal Size Species
1 0 204.4 3 1 L toad
2 9 199.6 3 1 L toad
3 18 197.6 4 1 L toad
4 27 196.3 3 1 L toad
5 36 194.5 4 1 L toad
6 45 192.8 4 1 L toad
7 0 30.7 2 2 S toad
8 9 30.1 2 2 S toad
9 18 29.4 3 2 S toad
10 27 29.1 4 2 S toad
11 36 28.8 5 2 S toad
12 45 28.3 5 2 S toad
dput:
structure(list(Time = c(0, 9, 18, 27, 36, 45, 0, 9, 18, 27, 36,
45), Weight = c(204.4, 199.6, 197.6, 196.3, 194.5, 192.8, 30.7,
30.1, 29.4, 29.1, 28.8, 28.3), Posture = c(3, 3, 4, 3, 4, 4,
2, 2, 3, 4, 5, 5), Animal = c(1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2,
2), Size = c("L", "L", "L", "L", "L", "L", "S", "S", "S", "S",
"S", "S"), Species = c("toad", "toad", "toad", "toad", "toad",
"toad", "toad", "toad", "toad", "toad", "toad", "toad")), class = "data.frame", row.names = c(NA,
-12L))
Your code calculates weight loss rate wr per minute from a difference between the first measurement at time 0 and last measurement at time 45. This is a single number per animal. The last line then inputs that one number to all lines for the animal.
To calculate wr per minute for all intervals in a for cycle, use:
for(i in 1:13) {
ft <- EWL[which(EWL$Animal == as.character(i)), ]
# difference between subsequent weight divided by interval length
wr <- diff(ft[, "Weight"], lag = 1) / 9
# first value is NA, because toads did not lose weight at time 0
EWL$WR[EWL$Animal == as.character(i)] <- c(NA, wr)
}
print(EWL)
# Time Weight Posture Animal Size Species WR
# 1 0 204.4 3 1 L toad NA
# 2 9 199.6 3 1 L toad -0.53333333
# 3 18 197.6 4 1 L toad -0.22222222
# 4 27 196.3 3 1 L toad -0.14444444
# 5 36 194.5 4 1 L toad -0.20000000
# 6 45 192.8 4 1 L toad -0.18888889
# 7 0 30.7 2 2 S toad NA
# 8 9 30.1 2 2 S toad -0.06666667
# 9 18 29.4 3 2 S toad -0.07777778
# 10 27 29.1 4 2 S toad -0.03333333
# 11 36 28.8 5 2 S toad -0.03333333
# 12 45 28.3 5 2 S toad -0.05555556
However, note that a linear mixed model might be more suitable for the analysis of this data.
# assumes different intercept for each animal
fit <- lme4::lmer(Weight ~ Time + (1 | Animal), data = EWL)
# assumes different slope for each animal
fit <- lme4::lmer(Weight ~ Time + (Time | Animal), data = EWL)