The following bit of SAS code is supposed to read from a dataset which contains a numeric variable called 'Radvalue'. Radvalue is the temperature of a radiator, and if a radiator is switched off but then its temperature increases by 2 or more it's a sign that it has come on, and if it is on but its temperature decreases by 2 or more it's a sign that it's gone off. Radstate is a new variable in the dataset which indicates for every observation whether the radiator is on or off, and it's this I'm trying to fill in automatically for the whole dataset. So I'm trying to use the LAG function, trying to initialise the first row, which doesn't have a dif_radvalue, and then trying to apply the algorithm I just described to row 2 onwards. Any idea why the columns Radstate and l_radstate come out completely blank?

Thanks everso much!! Let me know if I haven't explained the problem clearly.

Data work.heating_algorithm_b;
 Input ID Radvalue; 
 Datalines; 
  1 15.38 
  2 15.38 
  3 20.79 
  4 33.47 
  5 37.03 
 ; 

DATA temp.heating_algorithm_c;
 SET temp.heating_algorithm_b;

 DIF_Radvalue = Radvalue - lag(Radvalue);

 l_Radstate = lag(Radstate);

 if missing(dif_radvalue) then  
  do;
   dif_radvalue = 0;
   radstate = "off"; 
  end;                            
 else if l_Radstate = "off"  &  DIF_Radvalue > 2    then Radstate = "on";
 else if l_Radstate = "on" &  DIF_Radvalue < -2  then  Radstate = "off";
 else  Radstate = l_Radstate;
run;