Row-wise correlations in R

Question

I have two matrices of the same size. I would like to calculate the correlation coefficient between each pair of rows in these matrices; row 1 from A with row 1 B, row 2 from A

Answer 1

This should be fast:

cA <- A - rowMeans(A)
cB <- B - rowMeans(B)
sA <- sqrt(rowMeans(cA^2))
sB <- sqrt(rowMeans(cB^2))

rowMeans(cA * cB) / (sA * sB)