SAS 9.4 Macro Language, 5th Edition

Figure 2.3 The Word Scanner Obtains Tokens

The compiler continues to pull tokens until it recognizes the end of the DATA step (in

this case, the RUN statement), which is called a DATA step boundary, as shown in the

following figure. When the DATA step compiler recognizes the end of a step, the step is

executed, and the DATA step is complete.

Figure 2.4 The Word Scanner Sends Tokens to the Compiler

In most SAS programs with no macro processor activity, all information that the

compiler receives comes from the submitted program.

How SAS Processes Statements with Macro

Activity

In a program with macro activity, the macro processor can generate text that is placed on

the input stack to be tokenized by the word scanner. The example in this section shows

you how the macro processor creates and resolves a macro variable. To illustrate how the

compiler and the macro processor work together, the following figure contains the macro

processor and the macro variable symbol table. SAS creates the symbol table at the

beginning of a SAS session to hold the values of automatic and global macro variables.

SAS creates automatic macro variables at the beginning of a SAS session. For the sake

of illustration, the symbol table is shown with only one automatic macro variable,

SYSDAY.

18 Chapter 2 • SAS Programs and Macro Processing

Figure 2.5 The Macro Processor and Symbol Table

Whenever the word scanner encounters a macro trigger, it sends information to the

macro processor. A macro trigger is either an ampersand (&) or percent sign (%)

followed by a nonblank character. As it did in the previous example, the word scanner

begins to process this program by examining the first characters in the input stack. In

this case, the word scanner finds a percent sign (%) followed by a nonblank character.

The word scanner recognizes this combination of characters as a potential macro

language element, and triggers the macro processor to examine % and LET, as shown in

the following figure.

Figure 2.6 The Macro Processor Examines LET

When the macro processor recognizes a macro language element, it begins to work with

the word scanner. In this case, the macro processor removes the %LET statement, and

writes an entry in the symbol table, as shown in the following figure.

How SAS Processes Statements with Macro Activity 19

Figure 2.7 The Macro Processor Writes to the Symbol Table

From the time the word scanner triggers the macro processor until that macro processor

action is complete, the macro processor controls all activity. When the macro processor

is active, no activity occurs in the word scanner or the DATA step compiler.

When the macro processor is finished, the word scanner reads the next token (the DATA

keyword in this example) and sends it to the compiler. The word scanner triggers the

compiler, which begins to pull tokens from the top of the queue, as shown in the

following figure.

Figure 2.8 The Word Scanner Resumes Tokenization

As it processes each token, SAS removes the protection that the macro quoting functions

provide to mask special characters and mnemonic operators. For more information, see

Chapter 7, “Macro Quoting,” on page 81.

If the word scanner finds an ampersand followed by a nonblank character in a token, it

triggers the macro processor to examine the next token, as shown in the following figure.

20 Chapter 2 • SAS Programs and Macro Processing

Figure 2.9 The Macro Processor Examines &FILE

The macro processor examines the token and recognizes a macro variable that exists in

the symbol table. The macro processor removes the macro variable name from the input

stack and replaces it with the text from the symbol table, as shown in the following

figure.

Figure 2.10 The Macro Processor Generates Text to the Input Stack

The compiler continues to request tokens, and the word scanner continues to supply

them, until the entire input stack has been read as shown in the following figure.

How SAS Processes Statements with Macro Activity 21

Figure 2.11 The Word Scanner Completes Processing

If the end of the input stack is a DATA step boundary, as it is in this example, the

compiler compiles and executes the step. SAS then frees the DATA step task. Any macro

variables that were created during the program remain in the symbol table. If the end of

the input stack is not a step boundary, the processed statements remain in the compiler.

Processing resumes when more statements are submitted to the input stack.

22 Chapter 2 • SAS Programs and Macro Processing