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.