How can I perform a piece of macro for each observation in sas data step?

Question

Suppose I allow user to write his own variable calculation macro using a common user interface:

%macro calculate(var_name, var_value);
%* Some user-defined calculation;
%mend calculate;

Then in a data step, I can calculate a new variable using the user-defined macro:

data dataset;
    set dataset;
    new_var = %calculate('variable1', variable1); * This doesn't work. It just shows my indication.
run;

Where variable1 is a variable in dataset. Here, I want to pass in the variable name and the actual value of the variable. After the calculation, put the value in new_var.

How can I achieve this?

Answer 1

Is it required that you achieve this with macros? This sounds like a situation where PROC FCMP would be most useful as it allows you to define your own functions or subroutines (fcmp="function compiler") that can used in a data step just like a built-in function.

Here is a simple example:

proc fcmp outlib=sasuser.funcs.math;
  function calc(var);
     newvar=log(var); /*user defined stuff here - can be simple or complex*/
     return(newvar);
  endsub;
run;

option cmplib=sasuser.funcs; /*tell SAS where to look for functions*/
data _null_;
  set sashelp.class;
  newvar=calc(height); /*call your new function */
  put newvar=;
run;

Answer 2

You can make this work, but you are probably writing the macro incorrectly. You have to remember that SAS macros are essentially text preprocessors: they input the code you have written and output code to feed to SAS [1].

So here is a simple "addition" macro:

%macro calculate (var_name, var_value);
  &var_name + &var_value;
%mend;

data one;
  input a@@;
  b = %calculate(a, 3);
  c = %calculate(a, a);
cards;
1 3 -2 4
;
run;

proc print data=one;
run;

The macro facility will replace the %calculate bits with the code generated by the macro, and SAS will actually see the following:

%macro calculate (var_name, var_value);
  &var_name + &var_value;
%mend;

data one;
  input a@@;
  b = a + 3;
  c = a + a;
cards;
1 3 -2 4
;
run;

proc print data=one;
run;

You can see it for yourself on the log file with the options mprint statement

Unfortunately, this will not cover all potential calculations. You might want to look up PROC FCMP for a way to generate custom functions/subroutines usable in the DATA step.

[1] I know it is more complicated than that, but you can get far thinking like this.