替换给定 R 中另一个向量的负数位置的列？

Question

A<-matrix(c(1,2,1,1,1,2,1,1,1),nrow=3,ncol=3)
     [,1] [,2] [,3]
[1,]    1    1    1
[2,]    2    1    1
[3,]    1    2    1

B<-c(1,2,-1)

For example I want to replace all of the third column with all NA's because the third element in B is negative. I was thinking something like this

NegNum<-length(B[B<0])
A[,tail(NegNum)]<-NA

The negative numbers are always in the last n positions of B so i tried to use tail but im not sure how it works.

Answer 1

I would suggest indexing based on B :

#Matrix 1
A<-matrix(c(1,2,1,1,1,2,1,1,1),nrow=3,ncol=3)
#Vector
B<-c(1,2,-1)
#Solution
A[,which(B<0)]<-NA

Output:

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

Answer 2

Here is another base R option

A %*% diag(replace(B, B < 0, NA))

or

replace(A,cbind(seq(nrow(A)),rep(which(B<0),nrow(A))),NA)

which gives

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