R由行号子集

Question

I have a large matrix “A” (14 columns but 250 000 rows). It has no id column.

I have another matrix “B” (14x1) with 14 numbers.

1   1   3   7   15  31  63  127 255 511 1023    2047    4095    8191

I would like to subset “A” taking only the rows that have a number in B. Eg. Row 1, 3, 7, 15, 31, etc.

I have tried the following:

newA <- A[nrow(A) %in% c(B),]

I didn't work. I have also tried to add an id column to the large matrix A (eg A$id <- 1:nrow(A) but it didn't work either.

Answer 1

Try converting B to a numeric vector and use that to index A - this works with matrices regardless of how many rows and columns they have.

B<-as.numeric(as.character(B))

newA<-A[B,]

Edit - refined for clarity and information on applicability to matrices of all dimensions.