If variables already exist in the input dataset, how can you reference them?

Referencing existing variables in an input dataset can be effectively accomplished using either a WHERE or IF statement. Both of these statements allow you to filter or conditionally operate on the variables contained within the dataset without the need for renaming or modifying the structure of the dataset itself.

When you use a WHERE statement in particular, it enables the application of conditions for data selection directly within the DATA step or PROC step. This allows you to specify criteria that must be met for a record to be processed, ultimately referencing and manipulating the existing variables as needed.

Similarly, an IF statement can be used within a DATA step to create new variables or conditionally execute statements based on the values of the existing variables. This method gives you the flexibility to work with your dataset dynamically.

The other statements mentioned, while useful in their own contexts, serve different purposes. The RENAME statement is intended for changing the names of variables, the SET statement is used for reading in data from existing datasets, and the KEEP statement controls which variables to retain in the output dataset. None of these options specifically address the direct referencing of existing variables for operations or conditions as effectively as the WHERE or IF statements do.