SAS 官网上的一段关于Set的point选项的解释：

http://support.sas.com/documentation/cdl/en/lrdict/64316/HTML/default/viewer.htm#a000173782.htm

POINT=variable

    specifies a temporary variable whose numeric value determines which observation is read. POINT= causes the SET statement to use random (direct) access to read a SAS data set.
    Requirement:         a STOP statement
    Restriction:         You cannot use POINT= with a BY statement, a WHERE statement, or a WHERE= data set option. In addition, you cannot use it with transport format data sets, data sets in sequential format on tape or disk, and SAS/ACCESS views or the SQL procedure views that read data from external files.
    Restriction:         You cannot use POINT= with KEY=.
    Tip:         You must supply the values of the POINT= variable. For example, you can use the POINT= variable as the index variable in some form of the DO statement.
    Tip:         The POINT= variable is available anywhere in the DATA step, but it is not added to any new SAS data set.
    Featured in:         Combining One Observation with Many and Reading a Subset by Using Direct Access

    CAUTION:
        Continuous loops can occur when you use the POINT= option.

        When you use the POINT= option, you must include a STOP statement to stop DATA step processing, programming logic that checks for an invalid value of the POINT= variable, or both. Because POINT= reads only those observations that are specified in the DO statement, SAS cannot read an end-of-file indicator as it would if the file were being read sequentially. Because reading an end-of-file indicator ends a DATA step automatically, failure to substitute another means of ending the DATA step when you use POINT= can cause the DATA step to go into a continuous loop. If SAS reads an invalid value of the POINT= variable, it sets the automatic variable _ERROR_ to 1. Use this information to check for conditions that cause continuous DO-loop processing, or include a STOP statement at the end of the DATA step, or both.

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