Reading Repeating Blocks of Data
The Basics of Reading Repeating Blocks of Data
Consider this example data. Each record in the file
Tempdata contains three blocks of data. Each block contains a date
followed by the day's high temperature in a small city located in
the southern United States.
Figure 21.1 Raw Data File Tempdata
You could write a DATA
step that reads each record and creates three different Date and Temp
variables.
Figure 21.2 Three Date and Temp Variables
Alternatively, you could
create a separate observation for each block of data in a record.
This data set is better structured for analysis and reporting with
SAS procedures.
Figure 21.3 Separate Observations for Each Block of Data in a Record
Holding the Current Record with a Line-Hold Specifier
You need to hold the current record so that the INPUT
statement can read, and SAS can generate output from, repeated blocks
of data in the same record. This is easily accomplished by using a
line-hold specifier in the INPUT statement.
SAS provides two line-hold
specifiers.
-
The trailing at-sign (@) holds the input record for the execution of the next INPUT statement.
-
The double trailing at-sign (@@) holds the input record for the execution of the next INPUT statement, even across iterations of the DATA step.
The term trailing indicates
that the @ or @@ must be the last item that is specified in the INPUT
statement. Here is an example.
input Name $20. @; or input Name $20. @@;
Using the Double Trailing At-Sign (@@) to Hold the Current Record
Normally, each time a DATA step executes, the INPUT
statement reads the next record. But when you use the trailing @@,
the INPUT statement continues reading from the same record.
Here are several facts
about the double trailing at-sign (@@):
-
It works like the trailing @ except it holds the data line in the input buffer across multiple executions of the DATA step.
-
It typically is used to read multiple SAS observations from a single data line.
-
It should not be used with the @ pointer control, with column input, or with the MISSOVER option.
A record that is held
by the double trailing at-sign (@@) is not released until either of
the following events occurs:
-
The input pointer moves past the end of the record. Then the input pointer moves down to the next record.
-
An INPUT statement that has no trailing @ executes.
input ID $ @@; . . input Department 5.;
The following example
requires only one INPUT statement to read the values for Date and
HighTemp, but the INPUT statement must execute three times for each
record.
The INPUT statement
reads a block of values for Date and HighTemp, and holds the current
record by using the trailing @@. The values in the program data vector
are written to the data set as an observation, and control returns
to the top of the DATA step.
data perm.april10;
infile tempdata;
input Date : date. HighTemp @@;Figure 21.4 Control Returned to the Top of the DATA Step
In the next iteration,
the INPUT statement reads the next block of values for Date and HighTemp
from the same record.
Figure 21.5 Date and High Temp
Completing the DATA Step
You can add a FORMAT statement
to the DATA step to display the date or time values with a specified
format. The FORMAT statement below uses the DATEw.
format to display the values for Date in the form ddmmmyyyy.
data perm.april10;
infile tempdata;
input Date : date. HighTemp @@;
format date date9.;
run;Figure 21.6 Displaying Dates in a Specified Format
DATA Step Processing of Repeating Blocks of Data
Complete DATA Step
data perm.april10; infile tempdata; input Date : date. HighTemp @@; format date date9.; run;
Example: How the DATA Step Processes Repeated Blocks of Data
As the execution phase
begins, the input pointer rests on column 1 of record 1.
Figure 21.7 Input Pointer on Column 1 of Record 1
During the first iteration
of the DATA step, the first block of values for Date and HighTemp
are read into the program data vector.
Figure 21.8 Reading the First Block of Values
The first observation
is written to the data set.
Figure 21.9 Control Returns to the Top of the DATA Step
Control returns to the
top of the DATA step, and the values are reset to missing.
Figure 21.10 Reset Values
During the second iteration,
the @@ prevents the input pointer from moving down to the next record.
Instead, the INPUT statement reads the second block of values for
Date and HighTemp from the first record.
Figure 21.11 Reading the Second Block of Values from the First Record
The second observation
is written to the data set, and control returns to the top of the
DATA step.
Figure 21.12 Writing the Second Observation to the Data Set
During the third iteration,
the last block of values is read and written to the data set as the
third observation.
Figure 21.13 Writing the Third Observation to the Data Set
During the fourth iteration,
the first block of values in the second record is read and written
as the fourth observation.
Figure 21.14 Writing the Fourth Observation to the Data Set
The execution phase
continues until the last block of data is read.
Figure 21.15 Writing the Last Observation to the Data Set
You can display the
data set with the PRINT procedure.
proc print data=perm.april10; run;
Figure 21.16 PROC PRINT Output of the Data Set.
Last updated: January 10, 2018
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