An index is a separate
file from a data set that contains information about observations
within the data set. Specifically, an index contains value/identifier
pairs that indicate the location of observations within the data set
and the value of one or more key variables in that observation.
Correct answer: c
To create an index at
the same time that you create a data set, you use the INDEX= option
in the DATA statement. You must assign a unique name to a composite
index, while a simple index is automatically assigned the name of
the key variable as its name. You can set the value of the MSGLEVEL=
system option to I in order to see messages about indexes in the SAS
log.
Correct answer: a
For many maintenance
tasks that you perform on a data set, SAS automatically performs corresponding
tasks to the index file. For example, if you delete a data set, the
index file is deleted as well. If you rename a data set with the CHANGE
statement in the DATASETS procedure, SAS automatically renames the
index file. If you copy a data set to a new location with the COPY
statement in the DATASETS procedure, SAS automatically reconstructs
the index file in the new location.
Correct answer: d
You can use the DATASETS
procedure or the SQL procedure to create or delete an index from an
existing data set. You can also rebuild the index with a DATA step
and use the INDEX= option to create an index on the rebuilt data set.
However, rebuilding a data set uses more system resources than adding
an index to an existing data set with either the DATASETS or the SQL
procedure.
Correct answer: a
You use the CREATE INDEX
statement of the SQL procedure to create an index on an existing data
set. In the SQL procedure, you must name the index in the CREATE INDEX
statement; for a simple index, the index name must match the name
of the key variable.
Correct answer: b
You can use either the
CONTENTS procedure or the CONTENTS statement in the DATASETS procedure
to generate a list of information about a data set, including a list
of existing indexes. All indexes for a data set are stored in a single
file that is separate from but has the same name as the data set.
Correct answer: b
An index can improve
the efficiency with which SAS is able to access certain observations
in a data set. However, an index is not always useful. SAS will not
use an index to process subsetting IF statements, or other statements
that SAS determines might be more efficiently processed without an
index.