R: make symmetric matrix from lower diagonal

Question

I have a lower triangular of a matrix that I'm trying to convert to a dissim matrix and thus it needs to be symmetric.

print(rdf)

         X0        X1        X2        X3 X4
0 0.0000000        NA        NA        NA NA
1 0.5340909 0.0000000        NA        NA NA
2 0.5340909 0.0000000 0.0000000        NA NA
3 0.3200000 0.5227273 0.5227273 0.0000000 NA
4 0.6263736 0.4945055 0.4945055 0.5384615  0

library(gdata)
upperTriangle(rdf) <- lowerTriangle(rdf)
isSymmetric(rdf)
       X0        X1        X2        X3        X4
0 0.0000000 0.5340909 0.5340909 0.6263736 0.4945055
1 0.5340909 0.0000000 0.3200000 0.0000000 0.5227273
2 0.5340909 0.0000000 0.0000000 0.5227273 0.4945055
3 0.3200000 0.5227273 0.5227273 0.0000000 0.5384615
4 0.6263736 0.4945055 0.4945055 0.5384615 0.0000000
[1] FALSE

What is going on? Note, I don't need to use gdata

Seems like [this](http://stackoverflow.com/questions/18165320/creating-a-symmetric-matrix-in-r) or [this](http://stackoverflow.com/questions/29857378/make-a-matrix-symmetric?lq=1) may be duplicates. — aosmith, Oct 08 '15 at 21:36

score 26 · Accepted Answer · answered Oct 08 '15 at 21:29

You need to make sure that the elements you're copying are ordered correctly:

m <- matrix(NA,4,4)
m[lower.tri(m,diag=TRUE)] <- 1:10
     [,1] [,2] [,3] [,4]
[1,]    1   NA   NA   NA
[2,]    2    5   NA   NA
[3,]    3    6    8   NA
[4,]    4    7    9   10

makeSymm <- function(m) {
   m[upper.tri(m)] <- t(m)[upper.tri(m)]
   return(m)
}
makeSymm(m)

Or you can use the built-in

Matrix::forceSymmetric(m,uplo="L")

R: make symmetric matrix from lower diagonal

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