Question

I have two symmetrical matrices of the same lengths (one contains correlation coefficients and the other contains p-values).

I’m trying to make a matrix such that the upper.tri contains correlation coefficients and the lower.tri contains the relevant p-values.

Answer 1

Assuming your matrices are correl and pval

# create a new matrix that is the pvalues
new <- pval
# not sure what you want the diagonal to be, lets make it NA
diag(new) <- NA
# replace the upper triangle of this new matrix with the 
# upper triangle of the correlation matrix
new[upper.tri(new)] <- correl[upper.tri(correl)]