R help - May 2005 - setting elements of matrix

This is, no doubt, an easy problem, but I need help. I want to set
values in a matrix based on entries in a vector. Here is an example:
 
D is an r x m matrix initilized to all zeroes. r=6, m=5 in the
example.
 
Ro <- c(1, 3, 4, 4, 6)
Co <- c(1, 2, 3, 4, 5)
 
I want to set D[Ro[j], Co[j] ] to 1. So, D becomes
 
1 0 0 0 0
0 0 0 0 0
0 1 0 0 0
0 0 1 1 0
0 0 0 0 0
0 0 0 0 1
 
A loop works but is slow, which matters since this is being done many
times in a simulation.
 
Thanks,
rv

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Richard Valliant wrote:
> This is, no doubt, an easy problem, but I need help. I want to set
> values in a matrix based on entries in a vector. Here is an example:
>  
> D is an r x m matrix initilized to all zeroes. r=6, m=5 in the
> example.
>  
> Ro <- c(1, 3, 4, 4, 6)
> Co <- c(1, 2, 3, 4, 5)
>  
> I want to set D[Ro[j], Co[j] ] to 1. So, D becomes
>  
> 1 0 0 0 0
> 0 0 0 0 0
> 0 1 0 0 0
> 0 0 1 1 0
> 0 0 0 0 0
> 0 0 0 0 1
>  
> A loop works but is slow, which matters since this is being done many
> times in a simulation.
>  

How about:

D <- matrix(0, 6, 5)
D[cbind(Ro, Co)] <- 1

--sundar