Date:         Sat, 3 May 1997 11:19:36 EDT
Reply-To:     whitloi1@WESTATPO.WESTAT.COM
Sender:       "SAS(r) Discussion" <SAS-L@UGA.CC.UGA.EDU>
From:         Ian Whitlock <whitloi1@WESTATPO.WESTAT.COM>
Subject:      Re: ARRAY STATEMENT
Comments: To: taharris@IX.NETCOM.COM

    Subject: ARRAY STATEMENT
    Summary: Arrays allow data values to specify variables.
    Respondent: Ian Whitlock <whitloi1@westat.com>
 
    Terri Harris <taharris@IX.NETCOM.COM> wrote:
 
    >I am an 'experienced' SAS user who must confess to having never used
    >an ARRAY statement.  Could someone tell me how they use the statement,
    >what particular value or advantage as a data step shortcut it offers,
    >and so on?  Much appreciated, Terri Harris
 
    From an abstract point of view, SAS arrays allow data values to specify
    variables.  This means that tedious code can often be exchanged by the
    introduction of data values to drive the code.
 
    The principle can be important whenever
 
        1) A few lines of code are needed for each variable and there are a
           lot of variables.
 
           I have 36 problem flag FLAG1 - FLAG36.  Set the flags to 0.  I
           can write 36 statements
 
                flag1 = 0 ;
                flag2 = 0 ;
                ....
 
           or use
 
                array flags (36) flag1 - flag36 ;
                do i = 1 to dim ( flags ) ;
                   flag ( i ) = 0 ;
                end ;
 
        2) The number of variables to change is specified in the data.
 
           N is a number on the input data set.  Change the first N flag
           variables to 1.
 
           Now I can write 36 statements
 
                if 1 <= n then flag1 = 1 ;
                if 2 <= n then flag2 = 1 ;
                ...
 
           or use
 
                do i = 1 to n ;
                   flag ( i ) = 1 ;
                end ;
 
        3) Which variable is specified in the data.
 
           N2 is a number on the input data set.  Change the flag variable
           corresponding to this number to 99.
 
           I can write 36 statements
 
                 if n2 = 1 then flag1 = 99 ;
                 if n2 = 2 then flag2 = 99 ;
                 ....
 
           or I write
 
                 flag ( n2 ) = 99 ;
 
           This example if very revealing because it shows that an array
           can serve as a master if switch when some condition names the
           variable to be changed.  I often find simple code cluttered up
           with many IF statements can be shortened using arrays because
           the IF statements are passing judgement on "is this the variable
           I want"  instead of going directly to the variable.
 
 
    In studying the above examples it should be intuitively clear that one
    can always eliminate the array at the expense of writing a lot more
    "make work" type code when the list of variables is known.  Of course
    at some point it becomes simpler to learn about arrays.
 
    Two of the three examples above require iterative DO-loops.  I would
    like to see other examples of array usage that don't involve iterative
    DO-loops and conversely examples of iterative DO-loops that don't
    involve arrays.
 
    Ian Whitlock <whitloi1@westat.com>