11. Reading SAS datasets. GIORGIO RUSSOLILLO - Cours de prépara)on à la cer)fica)on SAS «Base Programming» 242

Transcription

1 11. Reading SAS datasets 242

2 Reading a single SAS dataset DATA SAS-dataset; SET SAS-dataset; <more SAS statements>; - SAS-dataset in the DATA statement is the name (libref.filename) of the dataset to be created - SAS-dataset in the SET statement is the name (libref.filename) of the dataset to be read DATA work.subset1; SET Lib9_3.sales; IF Country="AU" AND Salary>50000; PROC PRINT DATA=subset1; 243

3 Where to specify DROP= and KEEP= op)ons You can specify DROP and KEEP in either the DATA statement or in the SET statement, depending on whether you want to drop (keep) variables of the output or input file: - If you never reference certain variables and you don t want them to appear in the new dataset, you can use a DROP= op)on in the SET statement - If you need to reference a variable in the original dataset, you can specify the variable in the DROP= or KEEP= op)on in the DATA statement DATA work.subset1 (DROP=Employee_ID Salary); SET Lib9_3.sales ; IF Country='AU'; PROC PRINT DATA=subset1; DATA work.subset1; SET Lib9_3.sales (KEEP=Employee_ID Salary); IF Country='AU'; PROC PRINT DATA=subset1; (This script will not do its job because SAS will not read Country in Lib9_3.Sales) DATA work.subset1; SET Lib9_3.sales (KEEP=Employee_ID Salary); IF Salary>50000; PROC PRINT DATA=subset1; 244

4 How SAS processes the Data step when reading from a SAS dataset 245

5 Compila)on Phase During the compilabon phase: - Syntax error are checked in each statement of the DATA step - PDV is created: - a slot is added for each variable in the input SAS dataset when the SET statement is compiled; - any variables created in the DATA step are also added to the PDV. - The descrip)on por)on of the new SAS dataset is created 246

6 Execu)on Phase: First Itera)on 247

7 Second itera)on (1) N.B. : SAS retains the variables that: - were read from the input SAS dataset in the PDV - were created by a SUM statement All the other variables (e.g. variables created in the data steps using an assignment statement) are set to missing 248

8 Second itera)on (2) 249

9 Using the BY statement with the SET statement in a DATA step DATA SAS-dataset; SET SAS-dataset; BY <DESCENDING> variable(s); - N.B.: The datasets that are listed in the set statement must be sorted by the values of the BY variable(s) PROC PRINT DATA=Lib9_3.specialsals; PROC SORT DATA=Lib9_3.specialsals OUT=salsort; BY Dept; PROC PRINT DATA=salsort; 250

10 Accumula)ng totals for groups of data How much is the total salary for each department? 251

11 How SAS processes By groups DATA deptsals (KEEP=Dept DeptSal); SET Salsort; BY Dept; - The DATA step creates two temporary variables for each by variable: - FIRST.variable : it equals 1 for the first observa)on in a by group, otherwise is 0 - LAST.variable : it equals 1 for the last observa)on in a by group, otherwise is 0 252

12 Accumula)ng totals for groups of data: First step Sezng the value of the accumula)ng variable to zero DATA deptsals (KEEP=Dept DeptSal); SET Salsort; BY Dept; IF First.Dept THEN DeptSal=0; PROC PRINT DATA=deptsals; Each )me a new group begins (First.Dept=1), DeptSal is set to zero 253

13 Accumula)ng totals for groups of data: Second Step Incremen)ng the accumula)ng variable with the SUM statement DATA deptsals (KEEP=Dept DeptSal); SET Salsort; BY Dept; IF First.Dept THEN DeptSal=0; DeptSal+Salary; PROC PRINT DATA=deptsals; 254

14 Accumula)ng totals for groups of data: Third Step Outpuzng only the last observa)on of each BY group DATA deptsals (KEEP=Dept DeptSal); SET Salsort; BY Dept; IF First.Dept THEN DeptSal=0; DeptSal+Salary; IF Last.Dept; PROC PRINT DATA=deptsals; Only when Last.Dept = 1 SAS con)nues processing ans writes the output! (implicit OUTPUT statement) 255

15 Accumula)ng totals: a more complicated example We want to create two accumula)ng variables, NumEmps and DeptSal, in which we put the number of Employees and the total salary of each department for each project PROC PRINT DATA=Lib9_3.projsals; 256

16 Remember to work on sorted data.. PROC SORT DATA=Lib9_3.projsals OUT=projsort; BY Proj Dept; PROC PRINT DATA=projsort; 257

17 Mul)ple BY-variables processing (1) DATA pdsals; SET projsort; BY Proj Dept; 258

18 (2) When you specify mul)ple BY-variables, SAS respects two rules: 1) FIRST.variable for each BY variable is set to 1 at the first occurrence of a new value for the primary BY variable 2) A value of 1 for the primary BY variable forces LAST.variable to a value of 1 for the secondary BY variable Rule 1 Rule 2 259

19 First Step DATA pdsals (DROP=Employee_ID Salary); SET projsort; BY Proj Dept; IF First.Dept THEN DO; DeptSal=0; NumEmps=0; END; Secng the value of the accumula)ng variables to zero In order to make a unit the two statement in which SAS sets the two variables to zero, we use a DO group PROC PRINT DATA=pdsals; 260

20 Second Step Incremen)ng the accumula)ng variables by means of the sum statement DATA pdsals (DROP=Employee_ID Salary); SET projsort; BY Proj Dept; IF First.Dept THEN DO; DeptSal=0; NumEmps=0; END; DeptSal+Salary; NumEmps+1; PROC PRINT DATA=pdsals; 261

21 DATA pdsals (DROP=Employee_ID Salary); SET projsort; BY Proj Dept; IF First.Dept THEN DO; DeptSal=0; NumEmps=0; END; DeptSal+Salary; NumEmps+1; IF Last.Dept; PROC PRINT DATA=pdsals; Third Step Outpuzng only the totals (that is the last observa)on for each group) 262

22 The POINT= op)on You can access an observa)ons directly, without having to process each observa)on that preceeds it: POINT=variable - Specifies a temporary numeric variable that contains the observa)on number of the observa)on to read. - You must give a value to this variable before execu)ng the SET statement The DATA step con)nues to read observa)ons un)l it reaches an end-of-file marker. If we force SAS to read only the specified observa)on(s), SAS cannot read an end-offile marker and the program causes con)nuous looping DATA subset1; obsnum=4; SET Lib9_3.Employee_addresses POINT=obsnum; This program would produce a con)nuous loop 263

23 The STOP statement STOP ; It causes SAS to stop processing the current DATA step. PROC PRINT DATA=Lib9_3.Employee_addresses; DATA subset1; obsnum=4; SET Lib9_3.Employee_addresses POINT=obsnum; STOP; This program doesn t output anything, due to the fact that the implicit OUTPUT statement has not been executed 264

24 The OUTPUT statement The DATA step writes observa)on to output by default. You can make this ac)on explicit using the OUTPUT statement: OUTPUT <SAS-dataset(s)> ; - SAS-dataset(s) names the dataset(s) to which the observa)on is wri{en. - All dataset names that are specified in the OUTPUT statement must also appear in the DATA statement. If no dataset is specified, the current observa)on is wri{en to the dataset(s) specified in the DATA stastement DATA subset1; obsnum=4; SET Lib9_3.Employee_addresses POINT=obsnum; OUTPUT; STOP; PROC PRINT DATA=subset1; 265

25 Outpuzng mul)ple datasets If you create several datasets in the DATA statement, you can choose which dataset produce I create three datasets, but I fill only the third one : DATA Usa Australia Other; SET Lib9_3.Employee_addresses; OUTPUT Other; DATA Usa Australia Other; SET Lib9_3.Employee_addresses; IF Country='AU' THEN OUTPUT Australia; ELSE IF Country = 'US' THEN OUTPUT Usa; ELSE OUTPUT Other; DATA Usa Australia Other; SET Lib9_3.Employee_addresses; SELECT (Country); WHEN ('AU') OUTPUT Australia; WHEN ('US') OUTPUT Usa; OTHERWISE OUTPUT Other; END; 266

26 END= op)on It creates a temporary variable that contains an end-of-file marker END=variable - variable : is a temporary variable which is ini)alized to 0 and it is set to 1 when the SET statement reads the last observa)on of the dataset - This variable is not added to the dataset DATA totsals (KEEP=NumEmp DeptSal); SET Lib9_3.projsals END=last; NumEmp+1; DeptSal+Salary; IF last; PROC PRINT DATA=totsals; N.B.: END= and POINT= op)ons are incompa)ble in the same SET statement 267