Modifying matrix values based on another matrix without using a loop or apply function

Question

I have two matrices S and A and I would like to modify values of A based on values of S. Matrix S contains indices of matrix A which I would like to modify. For instance

S <- matrix(c(2,3,3,1,1,2),nrow=3, byrow=TRUE)

   [,1] [,2]
[1,]    2    3
[2,]    3    1
[3,]    1    2

The resulting matrix A would be

     [,1] [,2] [,3]
[1,]    0    1    1
[2,]    1    0    1
[3,]    1    1    0

In other words, In matrix S every row represents a graph vertex. Values in every row (say ith) are indices of vertices which are connected to ith vertex. Matrix A is an adjacency matrix of the graph.

Question: How to solve the problem, i.e. obtain matrix A, without using a for loop or apply/sapply/mapply etc. functions? What is the fastest way to do this?

EDIT: An example

S <- matrix(c(2,3,3,1,1,2,1,2),nrow=4, byrow=TRUE)

    [,1] [,2]
[1,]    2    3
[2,]    3    1
[3,]    1    2
[4,]    1    2

The result should be:

     [,1] [,2] [,3] [,4]
[1,]    0    1    1    1
[2,]    1    0    1    1
[3,]    1    1    0    0
[4,]    1    1    0    0

In your new example, where is the index for the 4th column? THe numbers are only till 3 — akrun
For the first row, I assume, 2, 3 implies the 'A' would change 0 to 1, but why it is filling for 4th column when there is no index — akrun
@akrun There is a filling because in the 4th row there is 1 in the first column, which means 4th and 1th vertices are connected — treskov

akrun akrun · Accepted Answer · 2020-11-10T22:49:11

We can use the 'S' as row/column index to change the values in A

A <- matrix(0, 4, 4)
A[cbind(c(row(S)), c(S))] <- 1
+(A|t(A))
#     [,1] [,2] [,3] [,4]
#[1,]    0    1    1    1
#[2,]    1    0    1    1
#[3,]    1    1    0    0
#[4,]    1    1    0    0

Modifying matrix values based on another matrix without using a loop or apply function

1 Answers