FINAL SAS PAPER

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SAS Essentials
How SAS Thinks
Neil.Howard_at_amgen.com
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The DATA step is your most powerful programming
tool.So understand and use it well.
Socrates
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Objectives

• understand DATA step
• processes
• internals
• defaults

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• compilation of DATA step source code
• execution of resultant machine code

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• compile and execute phases of
• INPUT (non SAS data)
• SET

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Compile Time Activities

• syntax scan
• source code translation to machine language
• definition of input and output files

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Compile TimeActivities

• input buffer
• LPDV (logical program data vector)
• data set descriptor information

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Creation of LPDV

• Variables added in the order seen by the compiler
• during parsing and interpretation of source
  statements

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Compile Time Statements

• location critical
• BY
• WHERE
• ARRAY
• ATTRIB
• FORMAT
• INFORMAT
• LENGTH

• location irrelevant
• DROP
• KEEP
• LABEL
• RENAME
• RETAIN

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Retained Variables

• all SAS special variables
• _N_
• _ERROR_
• all vars in RETAIN statement
• all vars from SET, MERGE, or UPDATE
• accumulator vars in SUM statement(s)

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Variables Not Retained

• Variables from input statement
• user defined variables (other
  than SUM statement)

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Type and Length of Variables

• determined at compile time
• by first reference to the compiler
  (in the DATA step)
• Numerics
• length is 8 during DATA step processing
• length is an output property

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INPUT statement

• reading non-SAS data

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Compile Loop and LPDV

• data a
• put _all_ write LPDV to LOG
• input idnum
• diagdate mmddyy8.
• sex
• rx_grp 10.
• time intck (year, diagdate, today() )
• put _all_ write LPDV to LOG
• cards
• 1 09-09-52 F placebo
• 2 11-15-64 M 300 mg.
• 3 04-07-48 F 600 mg.
• run

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input buffer
logical program data vector
idnum diagdate sex rx_grp
time
numeric numeric char
char numeric
8 8
8 10 8
Building descriptor portion of SAS data set
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logical program data vector
DKR keep keep
keep keep
keep drop drop
Drop/keep/rename
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Execution of a DATA Step
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Execution of a DATA Step
_N_ 1
Initialization of LPDV
read input file
Y
next step
end of file?
N
process statements in step
termination
implied output
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DATA Step Execution

• Implied read/write loop, stopped by
• no more data to read
• explicit STOP
• no input data
• some execution time errors

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Execution Time Activities

• execute initialize-to-missing (ITM)
• read from input source
• modify data using user-controlled statements
• supply values of variables to LPDV
• output observation to SAS data set

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Initialization

• _N_ set to loop count
• _ERROR_ set to 0
• user variables set to missing

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Execution Loop - raw data

• data a
• put _all_ write LPDV to LOG
• input idnum
• diagdate mmddyy8.
• sex
• rx_grp 10.
• time intck (year, diagdate, today() )
• put _all_ write LPDV to LOG
• cards
• 1 09-09-52 F placebo
• 2 11-15-64 M 300 mg.
• 3 04-07-48 F 600 mg.
• run
• proc contents run
• proc print run

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LPDV
IDNUM DIAGDATE SEX RX_GRP
TIME _N_

• . . . 1
• 1 -2670 F placebo 48 1
• . . . 2
• 2 1780 M 300 mg. 36 2
• . . . 3
• 3 -4286 F 600 mg. 52 3
• . . . 4
• (over all executions of DATA step..)

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2 data a 3 put _all_ write LPDV
to LOG 4 input idnum 5 diagdate
mmddyy8. 6 sex 7 rx_grp
10. 8 time intck ('year', diagdate,
today() ) 9 put _all_ write LPDV to
LOG 10 cards IDNUM. DIAGDATE. SEX
RX_GRP TIME. _ERROR_0 _N_1 IDNUM1
DIAGDATE-2670 SEXF RX_GRPplacebo TIME49
_ERROR_0 _N_1 IDNUM. DIAGDATE. SEX RX_GRP
TIME. _ERROR_0 _N_2 IDNUM2 DIAGDATE1780
SEXM RX_GRP300 mg. TIME37 _ERROR_0
_N_2 IDNUM. DIAGDATE. SEX RX_GRP TIME.
_ERROR_0 _N_3 IDNUM3 DIAGDATE-4286 SEXF
RX_GRP600 mg. TIME53 _ERROR_0 _N_3 IDNUM.
DIAGDATE. SEX RX_GRP TIME. _ERROR_0
_N_4 NOTE The data set WORK.A has 3
observations and 5 variables. NOTE The DATA
statement used 0.59 seconds. 14 run 15 16
proc contents run NOTE The PROCEDURE CONTENTS
used 0.39 seconds.
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Data Set Name WORK.A
Observations 3 Member Type DATA
Variables 5 Engine
V612
Indexes 0 Created 1118
Saturday, January 20, 2001 Observation
Length 42 Last Modified 1118 Saturday,
January 20, 2001 Deleted Observations
0 Protection
Compressed NO Data Set Type
Sorted
NO Label -----Engine/Host Dependent
Information----- Data Set Page Size
8192 Number of Data Set Pages 1 File Format
607 First Data Page 1 Max Obs
per Page 194 Obs in First Data Page
3 -----Alphabetic List of Variables and
Attributes----- Variable Type Len
Pos 5 TIME
Num 8 34 2 DIAGDATE Num
8 8 1 IDNUM Num 8 0 4
RX_GRP Char 10 24 3 SEX
Char 8 16
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PROC PRINT
IDNUM DIAGDATE SEX RX_GRP TIME 1
-2670 F placebo 48 2
1780 M 300 mg. 36 3 -4286
F 600 mg. 52
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SET statement

• reading existing SAS data

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DATA Step Compile

• no input buffer
• compiler reads descriptor portion of input SAS
  data set to build the LPDV
• returns same variables/attributes, including new
  variables

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SET

• determine which SAS data set to be read
• identify next observation to be read
• copy variable values to LPDV

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Execution Loop - SAS data

• data sas_a
• put _all_
• set a
• tot_rec 1
• put _all_
• run

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Building LPDV from descriptor portion of old SAS
data set
logical program data vector
idnum diagdate sex rx_grp
time tot_rec
numeric numeric char
char numeric numeric
8 8
8 10 8
8
Building descriptor portion of new SAS data set
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LPDV
IDNUM DIAGDATE SEX RX_GRP TIME TOT_REC
_N_

• . .
  . 0 1
• 1 -2670 F placebo 48 1
  1
• 1 -2670 F placebo 48 1
  2
• 2 1780 M 300 mg. 36 2
  2
• 2 1780 M 300 mg. 36 2
  3
• 3 -4286 F 600 mg. 52 3
  3
• 3 -4286 F 600 mg. 52 3
  4
• (over all executions of DATA step..)

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LOG

• idnum. diagdate. sex rx_grp time.
  tot_rec0 _ERROR_0 _N_1
• idnum1 diagdate-2670 sexF rx_grpplacebo
  time48 tot_rec1 _ERROR_0 _N_1
• idnum1 diagdate-2670 sexF rx_grpplacebo
  time48 tot_rec1 _ERROR_0 _N_2
• idnum2 diagdate1780 sexM rx_grp300 mg.
  time36 tot_rec2 _ERROR_0 _N_2
• idnum2 diagdate1780 sexM rx_grp300 mg.
  time36 tot_rec2 _ERROR_0 _N_3
• idnum3 diagdate-4286 sexF rx_grp600 mg.
  time52 tot_rec3 _ERROR_0 _N_3
• idnum3 diagdate-4286 sexF rx_grp600 mg.
  time52 tot_rec3 _ERROR_0 _N_4

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PROC PRINT

• IDNUM DIAGDATE SEX RX_GRP TIME
  TOT_REC
• 1 -2670 F placebo
  48 1
• 2 1780 M 300 mg. 36
  2
• 3 -4286 F 600 mg. 52
  3

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Logic of a MERGE

• compile
• execute

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• data left
• input ID X Y
• cards
• 1 88 99
• 2 66 77
• 44 55

data right input ID A B cards 1
A14 B32 3 A53 B11
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proc sort dataleft by ID run proc sort
dataright by ID run data both merge left
(ininleft) right (ininright)
by ID run
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logical program data vector first iteration MATCH
ID X Y A B
INLEFT INRIGHT _N_ _ERROR_
1 88 99 A14 B32 1 1
1 0
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logical program data vector second iteration NO
MATCH
ID X Y A B
INLEFT INRIGHT _N_ _ERROR_
2 66 77 1
0 2 0
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logical program data vector third iteration MATCH
ID X Y A B
INLEFT INRIGHT _N_ _ERROR_
3 44 55 A53 B11 1 1
3 0
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Lets try this again

• data left
• input ID X Y
• cards
• 1 88 99
• 2 66 77
• 44 55

data right input ID A B cards 1
A14 B32 3 A53 B11
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proc sort dataleft by ID run proc sort
dataright by ID run data both merge left
(ininleft) right (ininright)
by ID (one-on-one merge) run
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logical program data vector first iteration 11
MATCH
ID X Y A B
_N_ _ERROR_
1 88 99 A14 B32 1 0
1
OVERWRITTEN value came from data set right
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logical program data vector second iteration 11
MATCH
ID X Y A B
_N_ _ERROR_
2 66 77 A53 B11 2 0
3
OVERWRITTEN value came from data set right
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logical program data vector third iteration 11
NO MATCH
ID X Y A B
_N_ _ERROR_
3 44 55 3
0
MISSING no values from right
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Output SAS data set
ID X Y A B 1 88 99
A14 B32 3 66 77 A53 B11 3
44 55
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DATA Step Conclusions

• Understanding internals and default activities
  allows you to
• make informed coding decisions
• write flexible and efficient code
• debug and test effectively
• interpret results readily

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Remember

• We have discussed DEFAULTS
• As soon as you add options, statements, features,
  etc., the default actions change TEST them!
• You can use these same tools to track whats
  happening.