Overview of DATA Step Processing :: SAS(R) 9.3 Language Reference: Concepts, Second Edition

Flow of Action

When you submit a DATA step for execution, it is first compiled and then executed. The following figure shows the flow of action for a typical SAS DATA step.

Flow of Action in the DATA Step

The Compilation Phase

When you submit a DATA step for execution, SAS checks the syntax of the SAS statements and compiles them, that is, automatically translates the statements into machine code. In this phase, SAS identifies the type and length of each new variable, and determines whether a variable type conversion is necessary for each subsequent reference to a variable. During the compilation phase, SAS creates the following three items:

input buffer

is a logical area in memory into which SAS reads each record of raw data when SAS executes an INPUT statement. Note that this buffer is created only when the DATA step reads raw data. (When the DATA step reads a SAS data set, SAS reads the data directly into the program data vector.)

program data vector (PDV)

is a logical area in memory where SAS builds a data set, one observation at a time. When a program executes, SAS reads data values from the input buffer or creates them by executing SAS language statements. The data values are assigned to the appropriate variables in the program data vector. From here, SAS writes the values to a SAS data set as a single observation.

Along with data set variables and computed variables, the PDV contains two automatic variables, _N_ and _ERROR_. The _N_ variable counts the number of times the DATA step begins to iterate. The _ERROR_ variable signals the occurrence of an error caused by the data during execution. The value of _ERROR_ is either 0 (indicating no errors exist), or 1 (indicating that one or more errors have occurred). SAS does not write these variables to the output data set.

descriptor information

is information that SAS creates and maintains about each SAS data set, including data set attributes and variable attributes. For example, it contains the name of the data set and its member type, the date and time that the data set was created, and the number, names, and data types (character or numeric) of the variables.

The Execution Phase

By default, a simple DATA step iterates once for each observation that is being created. The flow of action in the Execution Phase of a simple DATA step is described as follows:

The DATA step begins with a DATA statement. Each time the DATA statement executes, a new iteration of the DATA step begins, and the _N_ automatic variable is incremented by 1.
SAS sets the newly created program variables to missing in the program data vector (PDV).
SAS reads a data record from a raw data file into the input buffer, or it reads an observation from a SAS data set directly into the program data vector. You can use an INPUT, MERGE, SET, MODIFY, or UPDATE statement to read a record.
SAS executes any subsequent programming statements for the current record.
At the end of the statements, an output, return, and reset occur automatically. SAS writes an observation to the SAS data set, the system automatically returns to the top of the DATA step, and the values of variables created by INPUT and assignment statements are reset to missing in the program data vector. Note that variables that you read with a SET, MERGE, MODIFY, or UPDATE statement are not reset to missing here.
SAS counts another iteration, reads the next record or observation, and executes the subsequent programming statements for the current observation.
The DATA step terminates when SAS encounters the end-of-file in a SAS data set or a raw data file.

Note: The figure shows the default processing of the DATA step. You can place data-reading statements (such as INPUT or SET), or data-writing statements (such as OUTPUT), in any order in your program.