Processing Statements Iteratively
Overview
Many macro applications require
iterative processing. With the iterative %DO statement that you can
repeatedly
-
execute macro programming code
-
generate SAS code.
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General form, iterative
%DO statement with %END statement:
%DO index-variable=start %TO stop <%BY increment>;
text
%END;
index-variable
is either the name
of a macro variable or a text expression that generates a macro variable
name.
start and stop
specify either integers
or macro expressions that generate integers to control how many times
the portion of the macro between the iterative %DO and %END statements
is processed.
increment
specifies either an
integer (other than 0) or a macro expression that generates an integer
to be added to the value of the index variable in each iteration of
the loop. By default, increment is
1.
text
can be
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%DO and %END statements
are valid only inside a macro definition. The index-variable is
created in the local symbol table if it does not appear in any existing
symbol table.
The iterative %DO statement
evaluates the value of the index variable at the beginning of
each loop iteration. The loop stops processing when the index variable
has a value that is outside the range of the start and stop values.
Example
You can use a macro
loop to create and display a series of macro variables.
This example creates
a series of macro variables named
teach1-teachn,
one for each observation in the Sasuser.Schedule data set, and assigns
teacher names to them as values. Then the Putloop macro
uses a %DO statement and a %END statement to create a loop that writes
these macro variables and their values to the SAS log, as follows:
data _null_;
set sasuser.schedule end=no_more;
call symput('teach'||left(_n_),(trim(teacher)));
if no_more then call symput('count',_n_);
run;
%macro putloop;
%local i;
%do i=1 %to &count;
%put TEACH&i is &&teach&i;
%end;
%mend putloop;
%putloopTip
It is a good idea to specifically
declare the index variable of a macro loop as a local variable to
avoid the possibility of accidentally changing the value of a macro
variable that has the same name in other symbol tables.
When the
Putloop macro
is executed, no code is sent to the compiler, because the %PUT statements
are executed by the macro processor. The following messages are written
to the SAS log.
TEACH1 is Hallis, Dr. George TEACH2 is Wickam, Dr. Alice TEACH3 is Forest, Mr. Peter TEACH4 is Tally, Ms. Julia TEACH5 is Hallis, Dr. George TEACH6 is Berthan, Ms. Judy TEACH7 is Hallis, Dr. George TEACH8 is Wickam, Dr. Alice TEACH9 is Forest, Mr. Peter TEACH10 is Tally, Ms. Julia TEACH11 is Tally, Ms. Julia TEACH12 is Berthan, Ms. Judy TEACH13 is Hallis, Dr. George TEACH14 is Wickam, Dr. Alice TEACH15 is Forest, Mr. Peter TEACH16 is Tally, Ms. Julia TEACH17 is Hallis, Dr. George TEACH18 is Berthan, Ms. Judy |
You can also use a macro
loop to generate statements that can be placed inside a SAS program
step.
Example
The following macro
generates a series of statements within a DATA step. On each iteration,
the macro writes a message to the SAS log that puts the current value
of the index variable into HEX6. format.
%macro hex(start=1,stop=10,incr=1);
%local i;
data _null_;
%do i=&start %to &stop %by &incr;
value=&i;
put "Hexadecimal form of &i is " value hex6.;
%end;
run;
%mend hex; Note: The HEX6. format converts
a number to hexadecimal format.
Suppose you submit the
following call:
options mprint mlogic; %hex(start=20,stop=30,incr=2)
Some of the messages
that are written to the SAS log when Hex executes are shown below.
Notice that according to the MLOGIC messages, the loop stops processing
when the value of the index variable is 32 (which is beyond the value
that is specified for
Stop).
MLOGIC(HEX): %DO loop index variable I is now 30; loop will iterate again. MPRINT(HEX): value=30; MPRINT(HEX): put "Hexadecimal form of 30 is " value hex6.; MLOGIC(HEX): %DO loop index variable I is now 32; loop will not iterate again. MPRINT(HEX): run; Hexadecimal form of 20 is 000014 Hexadecimal form of 22 is 000016 Hexadecimal form of 24 is 000018 Hexadecimal form of 26 is 00001A Hexadecimal form of 28 is 00001C Hexadecimal form of 30 is 00001E NOTE: DATA statement used: real time 0.06 seconds cpu time 0.06 seconds MLOGIC(HEX): Ending execution. |
Example
Suppose course offerings
for several years are stored in a series of external files that are
named by year, such as Raw1999.dat and Raw2000.dat. All the files
have the same record layout. Suppose you want to read each file into
a separate SAS data set.
The following macro
uses a %DO statement to create a loop that creates a data set from
each of the specified external files:
%macro readraw(first=1999,last=2005); %local year; %do year=&first %to &last; data year&year; infile "raw&year..dat"; input course_code $4. location $15. begin_date date9. teacher $25.; run; proc print data=year&year; title "Scheduled classes for &year"; format begin_date date9.; run; %end; %mend readraw;
Suppose you submit the
following call to the
Readraw macro:
%readraw(first=2000,last=2002)
The macro creates three
data sets named Year2000, Year2001, and Year2002. Remember that in
order for this program to run properly, the raw data files must be
named appropriately, and they must be stored in the location that
the program specifies. The generated DATA step is shown in the log.
336 %readraw(first=2000,last=2002)
MLOGIC(READRAW): Beginning execution.
MLOGIC(READRAW): Parameter FIRST has value 2000
MLOGIC(READRAW): Parameter LAST has value 2002
MLOGIC(READRAW): %LOCAL YEAR
MLOGIC(READRAW): %DO loop beginning; index variable YEAR; start value is 2000;
stop value is 2002; by value is 1.
MPRINT(READRAW): data year2000;
MPRINT(READRAW): infile "raw2000.dat";
MPRINT(READRAW): input course_code $4. location $15. begin_date date9.
teacher $25.;
MPRINT(READRAW): run;
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