R: Conversion between Lower Triangular Matrix and Vector

This post shows how to convert a lower triangular matrix into a vector or vice versa using R code.

R code

The following R code efficiently converts between a lower triangular matrix and a vector, supporting both row-wise and column-wise operations for diverse purposes.

#----------------------------------------------- # 1) Extract the lower triangular part (LTP) #    of matrix as a vector #----------------------------------------------- mat <- matrix(1:9, nrow = 3)   # column-wise v_cw <- mat[lower.tri(mat, diag = TRUE)]  # row-wise v_rw <- c(t(mat))[c(t(lower.tri(mat, diag = TRUE)))]    cat("Input matrix:\n"); print(mat)   # vector containing the LTP print(v_cw) print(v_rw)   #----------------------------------------------- # 2) Insert a vector into  #    the lower triangular part of matrix #----------------------------------------------- A <- B <- matrix(0,3,3)   # column-wise A[lower.tri(A, diag = TRUE)] <- v_cw # row-wise B[t(lower.tri(B, diag = TRUE))] <- v_rw; B <- t(B)   # reconstructed lower triangular matrix  print(A)  print(B)    Colored by Color Scripter

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Output for the above code is as follows.

Input matrix:      [,1] [,2] [,3] [1,]    1    4    7 [2,]    2    5    8 [3,]    3    6    9 >  > # vector containing the LTP > print(v_cw) [1] 1 2 3 5 6 9 > print(v_rw) [1] 1 2 5 3 6 9 >  >  > # reconstructed lower triangular matrix  > print(A)       [,1] [,2] [,3] [1,]    1    0    0 [2,]    2    5    0 [3,]    3    6    9 > print(B)       [,1] [,2] [,3] [1,]    1    0    0 [2,]    2    5    0 [3,]    3    6    9   Colored by Color Scripter

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