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Understanding SAS Data Step Processing

Understanding SAS Data Step Processing. Alan C. Elliott stattutorials.com. Reading Raw Data. Using the following SAS program: DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*( 9 / 5 )+ 32 ; DATALINES; 0001 24 37.3 0002 35 38.2 ; run ; proc print ; run ;. Overview of SAS Data Step.

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Understanding SAS Data Step Processing

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  1. Understanding SAS Data Step Processing Alan C. Elliott stattutorials.com

  2. Reading Raw Data • Using the following SAS program: DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*(9/5)+32; DATALINES; 0001 24 37.3 0002 35 38.2 ; run; procprint;run; Alan C. Elliott, stattutorials.com

  3. Overview of SAS Data Step Compile Phase (Look at Syntax) Execution Phase (Read data, Calculate) Output Phase (Create Data Set) Alan C. Elliott, stattutorials.com

  4. 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; 0001 24 37.3 0002 35 38.2 ; run; procprint;run; Alan C. Elliott, stattutorials.com

  5. Create Input Buffer • SAS creates an input buffer • INPUT BUFFER contains data as it is read in DATALINES; 0001 24 37.3 0002 35 38.2 ; INPUT BUFFER Alan C. Elliott, stattutorials.com

  6. 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. Alan C. Elliott, stattutorials.com

  7. Buffer to PDV Buffer PDV Reads 1st record Initially missing • Processes the code TEMPF=TEMPC*(9/5)+32; If there is an executable statement… Calculated value Alan C. Elliott, stattutorials.com

  8. Output Phase • The values in the PDV are written to the output data set (NEW) as the first observation: From PDV This is the first record in the output data set named “NEW.” Note that _N_ and _ERROR_ are dropped. Write data to data set. Alan C. Elliott, stattutorials.com

  9. Exceptions to Missing in PDV Initial values usually set to missing in PDV • 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. Alan C. Elliott, stattutorials.com

  10. 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. Alan C. Elliott, stattutorials.com

  11. 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. Alan C. Elliott, stattutorials.com

  12. Data Set Description procdatasets ; contents data=new; run; Contents output… (abbreviated) Alternate program proccontentsdata= new; run; Alan C. Elliott, stattutorials.com

  13. Description output continued… Alan C. Elliott, stattutorials.com

  14. Description output continued… Alan C. Elliott, stattutorials.com

  15. Original Program DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*(9/5)+32; DATALINES; 0001 24 37.3 0002 35 38.2 ; run; procprint;run; Alan C. Elliott, stattutorials.com

  16. Original Program DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*(9/5)+32; DATALINES; 0001 24 37.3 0002 35 38.2 ; run; procprint;run; Program output Alan C. Elliott, stattutorials.com

  17. Example of Error DATA NEW; INPUT ID $ AGE TEMPC; TEMPF=TEMPC*(9/5)+32 DATALINES; 00012437.3 00023538.2 ; run; procprint;run; procdatasets ; contents data=new; run; Missing Semi-colon Alan C. Elliott, stattutorials.com

  18. Error found during compilation 76 DATA NEW; 77 INPUT ID $ AGE TEMPC; 78 TEMPF=TEMPC*(9/5)+32 79 DATALINES; --------- 22 80 0001 24 37.3 ---- 180 ERROR 22-322: Syntax error, expecting one of the following: !, !!, &, *, **, +, -, /, <, <=, <>, =, >, ><, >=, AND, EQ, GE, GT, IN, LE, LT, MAX, MIN, NE, NG, NL, NOTIN, OR, ^=, |, ||, ~=. ERROR 180-322: Statement is not valid or it is used out of proper order. 81 0002 35 38.2 82 ; 83 run; ERROR: No DATALINES or INFILE statement. Alan C. Elliott, stattutorials.com

  19. Summary - Compilation Phase • During Compilation • Check 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= Alan C. Elliott, stattutorials.com

  20. 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. Alan C. Elliott, stattutorials.com

  21. End Alan C. Elliott, stattutorials.com

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