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
generated quantities block in Stan? Many thanks!