Convert matrix into symmetrical matrix in R

Question

I currently have a matrix output from a program that looks like the following where the bottom left has all 1s:

  B C D E
A 0 1 2 3
B 1 1 3 3
C 1 1 1 3
D 1 1 1 0

Is there a way to convert it into a symmetrical matrix instead of having all the 1s?

Answer 1

I do not think that the solution of @RonakShah is correct.

M = matrix(1:16, nrow=4)
M
     [,1] [,2] [,3] [,4]
[1,]    1    5    9   13
[2,]    2    6   10   14
[3,]    3    7   11   15
[4,]    4    8   12   16

M[lower.tri(M)] <- M[upper.tri(M)]
M
     [,1] [,2] [,3] [,4]
[1,]    1    5    9   13
[2,]    5    6   10   14
[3,]    9   13   11   15
[4,]   10   14   15   16

This is not symmetric. Instead, use

M = matrix(1:16, nrow=4)
M[lower.tri(M)] <- t(M)[lower.tri(M)]
M
     [,1] [,2] [,3] [,4]
[1,]    1    5    9   13
[2,]    5    6   10   14
[3,]    9   10   11   15
[4,]   13   14   15   16

Answer 2

You can copy the upper triangular values to lower triangle.

mat[lower.tri(mat)] <- mat[upper.tri(mat)]
mat

#  B C D E
#A 0 1 2 3
#B 1 1 3 3
#C 2 3 1 3
#D 3 3 3 0