How to subset values in matrix based on conditions in R

Question

I have a matrix:

A = matrix(1:20,nrow = 5,ncol = 4,byrow = TRUE)

and a vector:

B = c(1,4,11,14,15,20)

I want to get one value from each row under these conditions: if a row of A has multiple matches with B, only take the first match from the row; if a row of A has only one match, just take this matched value; if a row of A has no match, take the column 1 of this row. So the expected result is appended in the last column of A (as data frame).

      V1 V2 V3 V4 result
    1  1  2  3  4      1
    2  5  6  7  8      5
    3  9 10 11 12     11
    4 13 14 15 16     14
    5 17 18 19 20     20

Thank you for any helps!

Answer 1

i1 <- max.col(array(A %in% B, dim(A)), 'first')
data.frame(A, result = A[cbind(seq_len(nrow(A)), i1)])

  X1 X2 X3 X4 result
1  1  2  3  4      1
2  5  6  7  8      5
3  9 10 11 12     11
4 13 14 15 16     14
5 17 18 19 20     20

---

i2 <- stack(data.frame(t(A)))
cbind(A, aggregate(values~ind, i2, \(x) c(x[x%in%B],x[1])[1])[2])

   1  2  3  4 values
1  1  2  3  4      1
2  5  6  7  8      5
3  9 10 11 12     11
4 13 14 15 16     14
5 17 18 19 20     20