1. Introduction to SAS Essentials Mastering SAS for Data Analytics
Alan Elliott and Wayne Woodward

2. Intro to SAS Chapter 3 Part 2

3. 3.9 GOING DEEPER: UNDERSTANDING HOW THE DATA STEP READS AND STORES DATA
To understand how SAS works, it can be helpful to "look under the hood" and see what is happening in all those bits and bytes as SAS reads and processes data. Knowing how SAS handles data is a little bit like knowing how the motor in your car works. You can usually drive around okay without knowing anything about pistons, but sometimes, it is good to know what that knocking sound means.

4. How SAS Thinks UNDERSTANDING HOWTHE DATA STEP READS AND STORES DATA
Data Step Processing
The DATA Step vs The PROC Step
More about reading data files
Review of how to read data into SAS

5. Consider the following program
Using the following SAS program:
DATA NEW;
INPUT ID $ AGE TEMPC;
TEMPF=TEMPC*(9/5)+32;
DATALINES; ;
RUN;
PROC PRINT;RUN;
How does SAS read in this data and create a SAS data set?
This code calculates a value and creates a variable named TEMPF. We’ll learn more about calculations later…
Alan Elliott, stattutorials.com

6. Overview of SAS Data Step
Compile Phase
(Look at Syntax)
Execution Phase
(Read data, Calculate)
Output Phase
(Create Data Set)

7. Concepts…
COMPILE - SAS reads the syntax of the SAS program to see if there are any errors in the code. If there are no errors found, SAS “compiles” this code – that is, it transforms the SAS code into a code used internally by SAS. (You don’t need to know this internal code.) EXECUTION - If the code syntax checks out, SAS begins performing the tasks specified by the code. For example, the first line of code is DATA NEW, so during the execution phase, SAS creates a “blank” dataset (no data in it) named NEW that it will use to put the data into as it is read. OUTPUT - SAS reads in each data line. It interprets this line of data into the values for each variable and stores them into the data set one line at a time until all data have been output into the specified data set.

8. Compile Phase SAS Checks the syntax of the program.
Identifies type and length of each variable
Does any variable need conversion?
If everything is okay, proceed to the next step.
If errors are discovered, SAS attempts to interpret what you mean. If SAS can’t correct the error, it prints an error message to the log.
DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*(9/5)+32; DATALINES; ; run; proc print;run;

9. Create Input Buffer SAS creates an input buffer
INPUT BUFFER contains data as it is read in
DATALINES; ;
INPUT BUFFER 1 2 3 4 5 6 7 8 9 10 11 12 .

10. Execution Phase
PROGRAM DATA VECTOR (PDV) is created and contains information about the variables
Two automatic variables _N_ and _ERROR_ and a position for each of the four variables in the DATA step.
Sets  _N_ = 1 _ERROR_ = 0 (no initial error) and remaining variables to missing.
_N_
_ERROR_ ID
AGE
TEMPC
TEMPF 1 .

11. Buffer to PDV Buffer 1 2 3 4 5 6 7 8 9 10 11 12 . PDV Reads 1st record.
PDV
Reads 1st record
_N_
_ERROR_ ID
AGE
TEMPC
TEMPF 1
0001 24
37.3 .
Initially missing
Processes the code TEMPF=TEMPC*(9/5)+32;
If there is an executable statement…
_N_
_ERROR_ ID
AGE
TEMPC
TEMPF 1
0001 24
37.3
99.14
Calculated value

12. Output Phase
The values in the PDV are written to the output data set (NEW) as the first observation:
From PDV
_N_
_ERROR_ ID
AGE
TEMPC
TEMPF 1
0001 24
37.3
99.14
Write data to data set.
This is the first record in the output data set named “NEW.”
Note that _N_ and _ERROR_ are dropped. ID
AGE
TEMPC
TEMPF
0001 24
37.3
99.14

13. Exceptions to Missing in PDV
Initial values usually set to missing in PDV
_N_
_ERROR_ ID
AGE
TEMPC
TEMPF 1 .
Some data values are not initially set to missing in the PDV
variables in a RETAIN statement
variables created in a SUM statement
data elements in a _TEMPORARY_ array
variables created with options in the FILE or INFILE statements
These exceptions are covered later.

14. Know the Difference INPUT BUFFER PROGRAM DATA VECTOR 1 2 3 4 5 6 7 8 910 11 12 .
_N_
_ERROR_ ID
AGE
TEMPC
TEMPF 1 .

15. Next data record read
Once SAS finished reading the first data record, it continues the same process, and reads the second record…sending results to output data set (named NEW in this case.)
…and so on for all records. ID
AGE
TEMPC
TEMPF
0001 24
37.3
99.14
0002 35
38.2
100.76

16. Descriptor Information
For the data set, SAS creates and maintains a description about each SAS data set:
data set attributes
variable attributes
the name of the data set
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.

17. Data Set Description
Alternate programs
proc contents data= new;
To see all data sets use
proc contents data=_ALL_;
proc datasets ; contents data=new; run; Contents output… (abbreviated) #
Name
Member Type
File Size
Last Modified 1
NEW
DATA
5120
20Nov13:08:59:32

18. Description output continued…
Data Set Name
WORK.NEW
Observations 2
Member Type
DATA
Variables 4
Engine V9
Indexes
Created
Wed, Nov 20, :59:32 AM
Observation Length 32
Last Modified
Deleted Observations
Protection
Compressed NO
Data Set Type
Sorted
Label
Data Representation
WINDOWS_64
Encoding
wlatin1 Western (Windows)

19. Description output continued…
Alphabetic List of Variables and Attributes #
Variable
Type
Len 2
AGE
Num 8 1 ID
Char 3
TEMPC 4
TEMPF

20. Example -- How Errors are Found During Compilation
Review this program:
DATA NEW;
INPUT ID $ AGE TEMPC;
TEMPF=TEMPC*(9/5)+32;
DATALINES; ;
run;
proc print;run;

21. Original Program DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*(9/5)+32;
DATALINES; ;
run;
proc print;run;
Program output
Obs ID
AGE
TEMPC
TEMPF 1
0001 24
37.3
99.14 2
0002 35
38.2
100.76

22. Example of Error DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*(9/5)+32
Review this program – what’s the error?:
DATA NEW;
INPUT ID $ AGE TEMPC;
TEMPF=TEMPC*(9/5)+32
DATALINES; ;
run;
proc print;run;
Missing Semi-colon

23. Error found during compilation
76 DATA NEW; 77 INPUT ID $ AGE TEMPC; 78 TEMPF=TEMPC*(9/5) DATALINES; ERROR : Syntax error, expecting one of the following: !, !!, &, *, **, +, - , /, <, <=, <>, =, >, ><, >=, AND, EQ, GE, GT, IN, LE, LT, MAX, MIN, NE, NG, NL, NOTIN, OR, ^=, |, ||, ~=. ERROR : Statement is not valid or it is used out of proper order ; 83 run; ERROR: No DATALINES or INFILE statement.
Error found during compilation

24. Summary - Compilation Phase
During Compilation
Check code syntax
Identify type and length of each new variable (is a data type conversion needed?)
Creates INPUT BUFFER if there is an INPUT statement for an external file
Creates the Program Data Vector (PDV)
Creates descriptor information for data sets and variable attributes
Other options not discussed here: DROP; KEEP; RENAME; RETAIN; WHERE; LABEL; LENGTH; FORMAT; ARRAY; BY; ATTRIB; END=, IN=, FIRST, LAST, POINT=
Does not read any data….

25. Summary – Execution Phase
The DATA step iterates once for each observation being created.
Each time the DATA statement executes, _N_ is incremented by 1.
Newly created variables set to missing in the PDV.
SAS reads a data record from a raw data file into the input buffer (there are other possibilities not discussed here).
SAS executes any other programming statements for the current record.
At the end of the data statements (RUN;) SAS writes an observation to the SAS data set (OUTPUT PHASE)
SAS returns to the top of the DATA step (Step 3 above)
The DATA step terminates when there is no more data.

26. Quiz - Find Syntax Errors
DATA MYDATA;
INPUT ID $ SBP DBP GENDER $ AGE WT;
DATALINES; M F M F
F ;
PROC PRINT;
RUN;
Where is the syntax error?

27. Quiz - Find Syntax Errors
DATA MYDATA;
INPUT ID $ SBP DBP GENDER $ AGE WT;
DATALINES; M F M F
F ;
PROC PRINT;
RUN;
Where is the syntax error?

28. Find Syntax Errors DATA MYDATA; INFILE 'C:\SASDATA\EXAMPLE.DAT';
INPUT ID $ 1-3 GP $ 5 AGE 6-9
TIME TIME TIME ;
DATALINES;
PROC MEANS;
RUN;
Where is the syntax error?

29. Find Syntax Errors DATA MYDATA; INFILE 'C:\SASDATA\EXAMPLE.DAT';
INPUT ID $ 1-3 GP $ 5 AGE 6-9
TIME TIME TIME ;
DATALINES;
PROC MEANS;
RUN;
Where is the syntax error?

30. Find Syntax Errors DATA MYDATA; INFILE 'C:\SASDATA\EXAMPLE.CSV';
DLM=',' FIRSTOBS=2 OBS=26;
INPUT GROUP $ AGE TIME2 TIME3 Time4 SOCIO;
PROC MEANS;
RUN;
Where is the syntax error?

31. Find Syntax Errors DATA MYDATA; INFILE 'C:\SASDATA\EXAMPLE.CSV';
DLM=',' FIRSTOBS=2 OBS=26;
INPUT GROUP $ AGE TIME2 TIME3 Time4 SOCIO;
PROC MEANS;
RUN;
Where is the syntax error?

32. Character Variable LENGTH in SAS
By default, character variables have a length of 8.
DATA NAMES;
INPUT FIRST $ LAST $ AGE;
DATALINES;
GEORGE WASHINGTON 30
JAMES ADAMS 34
BERNIE RUMPELSTILTSKIN 55 ;
proc print;
run;
What’s the problem with this code?

33. Results Obs FIRST LAST AGE 1 GEORGE WASHINGT 30 2 JAMES ADAMS 34 3
BERNIE
RUMPELST 55
NOTE THE PROBLEM

34. Solution: Use LENGTH Statement
data names; LENGTH LAST $15.; input FIRST $ LAST $ AGE; Etc…
You could also use a FORMAT statement here.
Obs
LAST
FIRST
AGE 1
WASHINGTON
GEORGE 30 2
ADAMS
JAMES 34 3
RUMPELSTILTSKIN
BERNIE 55
Problem corrected…

35. Problem: Missing Data in Freeform
data names; LENGTH LAST $15. input FIRST $ LAST $ AGE; DATALINES; GEORGE WASHINGTON 30 JAMES ADAMS BERNIE RUMPELSTILTSKIN 55 ; proc print; run;
Note: No AGE for JAMES Adams

36. Did not read all of the data!
Results are
Obs
LAST
FIRST
AGE 1
WASHINGTON
GEORGE 30 2
ADAMS
JAMES
Did not read all of the data!

37. Solution: Indicate missing data
data names;
LENGTH LAST $15.
input FIRST $ LAST $ AGE;
DATALINES;
GEORGE WASHINGTON 30
JAMES ADAMS .
BERNIE RUMPELSTILTSKIN 55 ;
proc print;
run;
Note: Note missing value denoted as dot (.)

38. Results Obs LAST FIRST AGE 1 WASHINGTON GEORGE 30 2 ADAMS JAMES . 3
RUMPELSTILTSKIN
BERNIE 55

39. Missing Data in Column Read in DCOLUMN.SAS
DATA MYDATA; INPUT ID $ 1 SBP 2-4 DBP 5-7 GENDER $ 8 AGE 9-10 WT 11-13; DATALINES; 1120 M F F F20110 ; RUN; PROC PRINT;
Read in DCOLUMN.SAS
Delete the 80 in the first record , the 150 in the 4th record, and M in record 3 (preserve columns)
Change PROC MEANS to PROC PRINT
Run the program and observe output

40. Resulting Output – it’s okay!
Obs ID
SBP
DBP
GENDER
AGE WT 1
120 . M 15
115 2
130 70 F 25
180 3
140
100 89
170 4 80 30 5
125 20
110
Note the blanks in the data set are read as missing values – the numeric missing values are indicated by dot (.) and text missing values are indicated with a blank.
This works if data are read using column or formatted input.

41. Location of created Data sets
In the left window in SAS, click on the Explore tab. Notice the Contents of “SAS Environment.”
Click on the Libraries icon.
You will see several “Libraries” including Work.
Click on Work.

42. Work Library
The WORK library contains all of the SAS data sets we’ve created so far. (You may have a little different list.)
Double Click on the one named Employees.

43. SAS Viewtable SAS Viewtable displays the contents of a data set.
Click on “x” to close.
Note that the name is work.employees.
Close viewer.

44. 3.10 SUMMARY
This chapter defined the difference between temporary and permanent data sets and illustrated several methods for importing data sets into SAS using either the SAS Wizard or PROC IMPORT. Finally, the way SAS "thinks" as it is inputting data is explained.

45. These slides are based on the book:
Introduction to SAS Essentials Mastering SAS for Data Analytics, 2nd Edition
By Alan C, Elliott and Wayne A. Woodward
Paperback: 512 pages
Publisher: Wiley; 2 edition (August 3, 2015)
Language: English
ISBN-10:  X
ISBN-13: 
These slides are provided for you to use to teach SAS using this book. Feel free to modify them for your own needs. Please send comments about errors in the slides (or suggestions for improvements) to Thanks.