Language Reference: INDEX Statement

INDEX Statement

INDEX variables $\text{[math]}$ NONE ;

The INDEX statement creates an index for the named variables in the current input SAS data set. An index is created for each variable listed, provided that the variable does not already have an index. Current retrieval is set to the last variable indexed. Subsequent I/O operations such as LIST, READ, FIND, and DELETE can use this index to retrieve observations from the data. The indices are automatically updated when a data set is edited with the APPEND, DELETE, or REPLACE statements. Only one index is in effect at any given time. The SHOW CONTENTS command indicates which index is in use.

For example, the following statements copy the Sasuser.Class data set and create indexes for the Name and Sex variables. Current retrieval is set to use the Sex variable, as shown in Figure 23.138.

 data class;
    set sashelp.class;
 run;

proc iml;
use class;
index name sex;
list all;
close class;

Figure 23.138 Result of Listing Observations of an Indexed Data Set

   OBS Name     Sex       Age    Height    Weight                               
------ -------- --- --------- --------- ---------                               
     2 Alice    F     13.0000   56.5000   84.0000                               
     3 Barbara  F     13.0000   65.3000   98.0000                               
     4 Carol    F     14.0000   62.8000  102.5000                               
     7 Jane     F     12.0000   59.8000   84.5000                               
     8 Janet    F     15.0000   62.5000  112.5000                               
    11 Joyce    F     11.0000   51.3000   50.5000                               
    12 Judy     F     14.0000   64.3000   90.0000                               
    13 Louise   F     12.0000   56.3000   77.0000                               
    14 Mary     F     15.0000   66.5000  112.0000                               
     1 Alfred   M     14.0000   69.0000  112.5000                               
     5 Henry    M     14.0000   63.5000  102.5000                               
     6 James    M     12.0000   57.3000   83.0000                               
     9 Jeffrey  M     13.0000   62.5000   84.0000                               
    10 John     M     12.0000   59.0000   99.5000                               
    15 Philip   M     16.0000   72.0000  150.0000                               
    16 Robert   M     12.0000   64.8000  128.0000                               
    17 Ronald   M     15.0000   67.0000  133.0000                               
    18 Thomas   M     11.0000   57.5000   85.0000                               
    19 William  M     15.0000   66.5000  112.0000

The INDEX NONE statement can be used to set retrieval back to physical order.

When a WHERE clause is being processed, the SAS/IML language automatically determines which index to use, if any. The decision is based on the variables and operators involved in the WHERE clause, and the decision criterion is based on the efficiency of retrieval.