## Hi everyone,

I think this is very simple question but I failed to fix it. I am defining my ODE in stan where I get an error when I assign the first element of pop_init (which is a vector) to the variable L which is a real. But I get the incompatibility error. Any help is appreciated. Thanks

```
5: vector[] theta, // parameters
6: array[] real x_r, array[] int x_i) { // unused
7: real L = pop_init[1];
^
8: real H = pop_init[2];
9: real bh = theta[1];
```

Ill-typed arguments supplied to assignment operator =: lhs has type real and rhs has type vector

This is the entire stan code:

```
functions {
vector[] MyODE( real t, // time
vector[] pop_init, // initial states
vector[] theta,
array[] real x_r, array[] int x_i) { // unused // parameters
real L = pop_init[1];
real H = pop_init[2];
real bh = theta[1];
real mh = theta[2];
real ml = theta[3];
real bl = theta[4];
// differential equations
real dH_dt = (bh - mh * L) * H;
real dL_dt = (bl * H - ml) * L;
return { dL_dt , dH_dt };
}
}
data{
int N;
vector[N] L;
vector[N] H;
}
parameters{
vector<lower=0>[4] theta; //{bH, mH, bL, mL}
vector<lower=0>[2] sigma;
}
model{
matrix[N, 2] pop;
pop[1,1] = H[1];
pop[1,2] = L[1];
vector[20] SEQ;
for(i in 1:20) SEQ[i] = i;
pop[2:N,1:2] = ode_rk45(
MyODE, pop[1], 0, SEQ, theta,
rep_array(0.0, 0), rep_array(0, 0),
1e-5, 1e-3, 5e2);
for ( t in 2:N ){
target += lognormal_lpdf(H[t] | log(pop[t, 1]), sigma[1]);
target += lognormal_lpdf(L[t] | log(pop[t, 2]), sigma[2]);
}
sigma ~ gamma(2, 30);
theta[1] ~ gamma(4, 2);
theta[2] ~ gamma(2, 4);
theta[3] ~ gamma(3, 2);
theta[4] ~ gamma(1, 10);
}
'
```