Hi guys,

I have to admit that I am new to Stan. I am planning to do linear interpolation in Stan, but I have problems in indexing a matrix in Stan. For example, I can easily locate the maximum and minimum value that is smaller/larger than the new x value and their corresponding y values, see the sample code below.

```
##x and its corresponding y values
xy<-matrix(rep(seq(1,10,1),2),ncol=2)
##A new data point whose y value needs to be estimated
new_x<-8.23
##Lower bound
x1<-max(xy[xy[,1]<new_x,1])
y1<-xy[xy[,1]==x1,2]
##Upper bound
x2<-min(xy[xy[,1]>new_x,1])
y2<-xy[xy[,1]==x2,2]
##new_y corresponds to new_x
new_y<-(y2-y1)/(x2-x1)*(new_x-x1)+y1
```

However, how can I do it with `function`

and `generated quantities`

block in Stan? Many thanks!

Best,

Hongchao