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SAS 9.4 Macro Language, 5th Edition by

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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

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