[R] Sums based on values of other matrix

[arun]

Hi,
Try:
A<- structure(c(1, 2, 1, 2, 2, 1, 2, 1), .Dim = c(2L, 4L))
B<- structure(c(3, 1, 4, 1, 2, 2, 1, 2), .Dim = c(2L, 4L))
 B1<- matrix(0,nrow(B),ncol(B))
B1[A==1]<-B[A==1]
 rowSums(B1)
#[1] 7 4
A.K.




----- Original Message -----
From: tobias schlager <tobebryant at me.com>
To: "r-help at r-project.org" <r-help at r-project.org>
Cc: 
Sent: Thursday, September 26, 2013 5:51 PM
Subject: [R] Sums based on values of other matrix

Dear all, 

I have a big problem: 
- I got two matrices, A and B 
- A shows identifies the value of B, however the values of B must be summed 
- For instance, 
1 1 2 2 
2 2 1 1 
gives matrix a 
3 4 2 1 
1 1 2 2 
gives matrix b 

Now the result for the value 1 would be 
7 
4 
which are the rowsums of the values of matrix B given that matrix A has the value 1. 


How can I do this automatically? I am really puzzled here. Thanks for your help guys, 
Tobi
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