Previously, I shared with the world a function to create a pairwise matrix of the number of transitions and transversions between two DNA sequences. Klaus Schliep kindly pointed out the possibility of a bug in the function and offered a faster, more accurate version. Thanks Klaus!

titv<-function(dat){
mat<-as.matrix(dat)
res<-matrix(NA, ncol=dim(mat)[1], nrow=dim(mat)[1], dimnames=list(x=names(dat), y=names(dat)))
for(i in 1:(dim(mat)[1] – 1)){
for(j in (i+1):dim(mat)[1]){
vec<-as.numeric(mat[i,])+as.numeric(mat[j,])-8
res[j,i]<-sum(!is.na(match(vec,c(200,56))))#Transitions
res[i,j]<-sum(!is.na(match(vec,c(152,168,88,104))))#Transversions
}
}
res
}

The previous version of the function considered the difference between an unknown base (coded as N) and a T as a transition. The new version does not detect this difference.