Execution Phase
Initializing Variables
At the
beginning of the execution phase, the value of _N_ is 1. Because there
are no data errors, the value of _ERROR_ is 0.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Figure 7.6 Program Data Vector: Initializing Variables
The remaining variables
are initialized to missing. Missing numeric values are represented
by periods, and missing character values are represented by blanks.
SET Statement
The SET statement identifies the location of the input
data set. Columns are added to the PDV in the order in which they
appear in the input table. Attributes are inherited from the input
table.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Sequentially Process Statements
After the SET statement,
SAS executes the remaining statements sequentially and updates the
values in the PDV.
data work.update; set cert.invent; Total=instock+backord; /*#1*/ SalePrice=(CostPerUnit*0.65)+CostPerUnit; /*#2*/ format CostPerUnit SalePrice dollar6.2; run;
| 1 | SAS processes the first assignment statement to create the new variable, Total. The values of InStock and BackOrd are added together to create a value for Total. See PDV: Create a New Variable, Total below for a visual representation of how the PDV processes the first assignment statement. |
| 2 | SAS processes the second assignment statement to create the new variable, SalePrice. The value of CostPerUnit is multiplied by 0.65, and the resulting value is added to the value of CostPerUnit to create a value for SalePrice. See PDV: Create a New Variable, SalePrice below for a visual representation of how the PDV processes the second assignment statement. |
Figure 7.7 PDV: Create a New Variable, Total
Figure 7.8 PDV: Create a New Variable, SalePrice
The formats for each
variable are applied before SAS adds the values to the PDV.
End of the DATA Step
At the end of the DATA
step, several actions occur. First, the values in the PDV are written
to the output data set as the first observation.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Figure 7.9 Program Data Vector and Output Data Set
Next, control returns
to the top of the DATA step, and the value of _N_ increments from
1 to 2. Finally, the variable values in the PDV are reset to missing.
Notice that the automatic variable _ERROR_ is reset to 0 if necessary.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Figure 7.10 Program Data Vector and Output Data Set
Iterations of the DATA Step
You can see that the DATA step works
like a loop, repetitively executing statements to read data values
and create observations one by one. At the beginning of the second
iteration, the value of _N_ is 2, and _ERROR_ is still 0. Each loop
(or cycle of execution) is called an iteration.
Figure 7.11 Iterations of the DATA Step
As the SET statement executes
for the second time, the values from the second record are read from
the input table into the PDV.
Figure 7.12 Program Data Vector and Output Data Set
Next, the value for
Total is calculated based on the current values for InStock and BackOrd.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Figure 7.13 Program Data Vector and Output Data Set
Next, the value for
SalePrice is calculated based on the values for CostPerUnit, multiplied
by 0.65, and added to the value of CostPerUnit.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Figure 7.14 Program Data Vector and Output Data Set
The RUN statement indicates
the end of the DATA step loop. At the bottom of the DATA step, the
values in the PDV are written to the data set as the second observation.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Next, the value of _N_
increments from 2 to 3, control returns to the top of the DATA step,
and the values for Item, IDnum, InStock, BackOrd, CostPerUnit, Total,
and SalePrice are reset to missing.
data work.update;
set cert.invent;
Total=instock+backord;
SalePrice=(CostPerUnit*0.65)+CostPerUnit;
format CostPerUnit SalePrice dollar6.2;
run;
Figure 7.15 Program Data Vector and Output Data
When PROC IMPORT reads
raw data, SAS sets the value of each variable in the DATA step to
missing at the beginning of each cycle of execution, with these exceptions:
-
variables that are named in a RETAIN statement
-
variables that are created in a sum statement
-
automatic variables
In contrast, when reading
variables from a SAS data set, SAS sets the values to missing only
before the first cycle of execution of the DATA step. Therefore, the
variables retain their values until new values become available (for
example, through an assignment statement or through the next execution
of a SET or MERGE statement). Variables that are created with options
in a SET or MERGE statement also retain their values from one cycle
of execution to the next.
End-of-File Marker
The execution phase continues in this
manner until the end-of-file marker is reached in the input data file.
When there are no more records in the input data file to be read,
the data portion of the new data set is complete and the DATA step
stops.
This is the output data
set that SAS creates:
Figure 7.16 SAS Data Set Work.Update
End of the Execution Phase
At the end of the execution
phase, the SAS log confirms that the input data file was read, and
it displays the number of observations and variables in the data set.
Log 7.1 SAS Log
NOTE: There were 9 observations read from the data set
CERT.INVENT.
NOTE: The data set WORK.UPDATE has 9 observations and 7
variables.
Recall that you can
display the data set with the PRINT procedure.
proc print data=work.update; run;
Output 7.1 Output from the PRINT Procedure
Last updated: August 23, 2018
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