Question

I am running a for loop with two matrices. One matrix(A) has ~100 strings (such as, name1, name2, …, name100) and only has one column. The other matrix(B) is bigger than A with rows and columns of both values and strings. In some places in B matrix, each name of A matrix is matched. I would like to extract and stack matched entire rows with a particular string of matrix A on output matrix.

So, I am running as below,

output <- NULL
for(K in 1:nrow(A)){
  print(K)
  for(cc in 1:nrow(B)){
    for(dd in 1:ncol(B)){
      if(toupper(A[K])==toupper(B[cc,dd])){
        output <- rbind(output,B[cc,])
      }
    }
  }
}

But it is too slow. How do you make this for loop more efficient in terms of running time?

Answer 1

Here some idea:

A <- matrix(c('a','b','c','d'), ncol=1)
B <- data.frame(z1=c('a','g','f','c'), z2=rnorm(4), z3=c('a','b','f','f'))

id <- apply(B, 2, function(x) A %in% x)
newB <- B[apply(id,1,sum)>0, ]