If you want to remain in the IML environment and mimic DATA step processing, you need to learn the basic differences between IML and the SAS DATA step:
With SAS/IML software, you start with a CREATE statement instead of a DATA statement. You must explicitly set up all your variables with the correct attributes before you create a data set. This means that you must define character variables to have the desired string length beforehand. Numeric variables are the default, so any variable not defined as character is assumed to be numeric. In the DATA step, the variable attributes are determined from context across the whole step.
With SAS/IML software, you must use an APPEND statement to output an observation; in the DATA step, you either use an OUTPUT statement or let the DATA step output it automatically.
With SAS/IML software, you iterate with a DO DATA loop. In the DATA step, the iterations are implied.
With SAS/IML software, you have to close the data set with a CLOSE statement unless you plan to exit the IML environment with a QUIT statement. The DATA step closes the data set automatically at the end of the step.
The DATA step usually executes faster than IML.
In short, the DATA step treats the problem with greater simplicity, allowing shorter programs. However, IML has more flexibility because it is interactive and has a powerful matrix-handling capability.