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List:       sas-l
Subject:    Re: IML: comparing two matrices by cell values
From:       Dale McLerran <stringplayer_2 () YAHOO ! COM>
Date:       2005-10-31 21:06:16
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--- "von-hippel.1@osu.edu" <von-hippel.1@OSU.EDU> wrote:

> I am calculating a matrix C from the values in matrices A and B.
>
> In each matrix, I use the first two columns as a kind of index. When
> I
> am looking at a row of C, is there a convenient way to find the rows
> of
> A and B that have the same index?
>
> Here's an example.... Suppose I have two 3-column matrices A and B,
> where B has more rows than A. E.g.:
>
> A =
>  {1 1 1,
>   2 2 2};
>
> B =
>  {1 1 0,
>   2 1 0,
>   2 2 0};
>
> I want to create a third matrix C with the same dimension as B, and
> cells filled according to the following rule.
>
>  C[i,j,1] = B[i,j,1]
>  C[i,j,2] = B[i,j,2]
>  C[i,j,3] = A[i,j,3] if A[i,j,1]=C[i,j,1] and A[i,j,2]=C[i,j,2]
>           = 0 otherwise.
>
> So
>
> C =
>  {1 1 1,
>   2 1 0,
>   2 2 2};
>
> How would you go about doing this?
>
> Thanks!
> Paul
>

Paul,

I don't know if there is a more efficient way to accomplish what
you need, but the following code does work.  We have to condition
on the i-th row of A and then find all rows of B where the first
two columns match A[i,1] and A[i,2].  The LOC function is employed
to find the rows where this condition is true.


  /* First initialize C to B, then initialize column 3 of C to zero */
  C = B;
  C[,3]=0;

  /* Loop over rows of A and find rows of B that match A[i,1]  */
  /* and A[i,2].  For rows of B which match, set B[j,3]=A[i,3] */
  nrow_A = nrow(A);
  do i=1 to nrow_A;
    j = loc(B[,1]=A[i,1] & B[,2]=A[i,2]);
    if nrow(j)>0 then do;
      C[j,3] = A[i,3];
    end;
  end;

  print C;


Note that if there are rows in A which do not have matching first
two columns in B, then the index j cannot be constructed and the
number of rows for the symbol j will be zero.  We only set the
value of C[j,3]=A[i,3] when we do find (at least) one row of
B which has the same values for columns 1 and 2 as are observed
for the current row of A.

HTH,

Dale

---------------------------------------
Dale McLerran
Fred Hutchinson Cancer Research Center
mailto: dmclerra@NO_SPAMfhcrc.org
Ph:  (206) 667-2926
Fax: (206) 667-5977
---------------------------------------




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