I am going through the Lotka-Volterra example, trying to reproduce it.

```
library(rstan)
rstan_options(auto_write = TRUE)
options(mc.cores = parallel::detectCores(logical = FALSE))
lynx_hare_df <-
read.csv("hudson-bay-lynx-hare.csv", comment.char="#")
head(lynx_hare_df, n = 3)
lynx_hare_melted_df <- melt(as.matrix(lynx_hare_df[, 2:3]))
colnames(lynx_hare_melted_df) <- c("year", "species", "pelts")
lynx_hare_melted_df$year <-
lynx_hare_melted_df$year +
rep(1899, length(lynx_hare_melted_df$year))
head(lynx_hare_melted_df, n=3)
tail(lynx_hare_melted_df, n=3)
N <- length(lynx_hare_df$Year) - 1 # num observations
ts <- 1:N # observation time
y0 <- c(lynx_hare_df$Hare[1], lynx_hare_df$Lynx[1]) # first observation
y<- as.matrix(lynx_hare_df[2:(N + 1), 2:3]) # remaining observ
y <- cbind(y[ , 2], y[ , 1]); # reverse order
lynx_hare_data <- list(N, ts, y0, y)
model <- stan_model("lotka-volterra.stan")
fit <- sampling(model, data = lynx_hare_data)
```

I get the following error:

```
> fit <- sampling(model, data = lynx_hare_data)
Error in new_CppObject_xp(fields$.module, fields$.pointer, ...) :
Exception: variable does not exist; processing stage=data initialization; variable name=N; base type=int (in 'model6d34530f74d5_lotka_volterra' at line 21)
failed to create the sampler; sampling not done
```

But the variable N does exist, the data for the stan model is a class list with N in the first position as a num.

lynx_hare_data <- list(N, ts, y0, y)

Any help would be much appreciated, thank you.