Tree list to reverse-lower triangular matrix in R

Question

How would I go about converting

m = list(1,2:3,4:6,7:10)

to

     [,1] [,2] [,3] [,4]
[1,]    0    0    0   10
[2,]    0    0    6    9
[3,]    0    3    5    8
[4,]    1    2    4    7

An idea or some guidance are appreciated! Thank you for your patience, in case the question is too naive or requires additional information (I'll gladly provide).

Answer 1

Ill chuck a base R method forward

# Create matrix with dimensions defined by the length of your list 
mat <- matrix(0, length(m), length(m)) 
# Fill in desired order
mat[upper.tri(mat, TRUE)] <- unlist(m)
# Order rows 
mat[length(m):1, ]

Answer 2

1) Below the lapply appends n zeros to each component of m and the sapply takes the first n elements of each component of m reshaping the result into a matrix. Finally we reverse the order of the rows of the resulting matrix. This works even if m does not define a triangular matrix:

n <- length(m)
sapply(lapply(m, c, numeric(n)), head, n)[n:1, ]

giving:

     [,1] [,2] [,3] [,4]
[1,]    0    0    0   10
[2,]    0    0    6    9
[3,]    0    3    5    8
[4,]    1    2    4    7

If n can be zero then use rev(seq_len(n)) in place of n:1 .

2) A straight forward sapply also works. It prepends each reversed component of m with the appropriate number of zeros and reshapes into a matrix:

sapply(m, function(v) c(numeric(n - length(v)), rev(v)))