Date:         Tue, 7 Oct 1997 17:20:52 -0500
Reply-To:     "Matheson, David" <davidm@SPSS.COM>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@UGA.CC.UGA.EDU>
From:         "Matheson, David" <davidm@SPSS.COM>
Subject:      Re: sequence of integers
Comments: To: Melvin Klassen <KLASSEN@UVVM.UVIC.CA>

Actually, if N is a sequence of 1, 2, 3, 4, 5, 6
  1 + MOD(N,3)  returns
2, 3, 1, 2, 3, 1   and
 4 - MOD(N,3) returns
3, 2, 4, 3, 2, 4
 
 
I'd suggest
 
>compute A = 1 + mod(N-1,3).
>compute B = 3 - mod(N-1,3).
>execute.
 
Whenever N is a multiple of 3, MOD(N,3) will return a 0.
For the A series, the other numbers (that aren't mult of 3)
are exactly what you want at that point. Adding 1 to this result
gives you a repeating 1,2,3 series, but not starting at 1. For the B
series, you get a 4, rather than a 1, in positions 3, 6, ...when
subtracting
from 4. With N defined as above
N-1 = 0,1,2,3,4,5    and
MOD(N-1,3) = 0,1,2,0,1,2
Adding 1 then gives you the desired A series and subtracting from 3
gives you the desired B series with the suggested starting points.
 
If N is not defined, but you have an active data set, you can use
 
>compute A = 1 + mod($casenum-1,3).
>compute B = 3 - mod($casenum-1,3).
>execute.
 
David Matheson
SPSS Technical Support
 
>----------
>From:  Melvin Klassen[SMTP:KLASSEN@UVVM.UVIC.CA]
>Sent:  Tuesday, October 07, 1997 3:00 PM
>To:    SPSSX-L@UGA.CC.UGA.EDU
>Subject:       Re: sequence of integers
>
>Anders Alexandersson & Maria Krol <mkrol+@PITT.EDU> writes:
>>On large SPSS datasets, how do you compute sequences of integers that
>>restart? For instance, how do you compute variables A and B, where
>>A=(1,2,3,1,2,3,1,2,3,...) and B=(3,2,1,3,2,1,3,2,1,...)?
>
>Given values of 1 or 2 or 3 or 4 or 5 or 6 or ... in the variable 'N', try:
>
> COMPUTE A = 1 + MOD(N,3)
> COMPUTE B = 4 - MOD(N,3)
>
>That's all!
>