Vectorize double loops in R

Question

I am new to R and am trying to vectorize my codes below.

What is a better way to do this? Thanks so much!

*

l_mat <- data.frame(matrix(ncol = 4, nrow = 4))
datax <- data.frame("var1"= c(1,1,1,1), "Var2" = c(2,2,2,2), "Var3"=c(3,3,3,3), "Var4"=c(4,4,4,4))
for (i in 1:4) {
      for (j in 1:4) {
             if (datax[i, 2] == datax[j, 2]) {
                   l_mat[i, j] <- 100
               } else {
                     l_mat[i, j] <- 1
                 }
         }
   }

*

Answer 1

It can be better done with outer. As we are checking if all the values in the second column against itself, create the logical expression with outer, convert it to a numeric index and then replace the values with 1 or 100

out <- 1 + (outer(datax[,2], datax[,2], `==`))
out[] <- c(1, 100)[out]

---

Or in a single line

ifelse(outer(datax[,2], datax[,2], `==`), 100, 1)

Or use a variation with pmax and outer

do.call(pmax, list(outer(datax[,2], datax[,2], `==`) * 100, 1))