Reconstruct symmetric matrix from values in long-form

An igraph solution where you read in the dataframe, with the value assumed as edge weights. You can then convert this to an adjacency matrix

dat <- read.table(header=T, text=" one   two   value
  a     b     30
  a     c     40
  a     d     20
  b     c     10
  b     d     05
  c     d     30")

library(igraph)

# Make undirected so that graph matrix will be symmetric
g <- graph.data.frame(dat, directed=FALSE)

# add value as a weight attribute
get.adjacency(g, attr="value", sparse=FALSE)
#   a  b  c  d
#a  0 30 40 20
#b 30  0 10  5
#c 40 10  0 30
#d 20  5 30  0

Yet another approach is reshape::cast

df.long = data.frame(one=c('a','a','a','b','b','c'),
                     two=c('b','c','d','c','d','d'),
                     value=c(30,40,20,10,05,30) )

# cast will recover the upper/lower-triangles...
df <- as.matrix( cast(df.long, one ~ two, fill=0) )
#    b  c  d
# a 30 40 20
# b  0 10  5
# c  0  0 30

So we construct matrix with full indices, and insert:

df <- matrix(nrow=length(indices), ncol=length(indices),dimnames = list(indices,indices))    
diag(df) <- 0
# once we assure that the full upper-triangle is present and in sorted order (as Robert's answer does), then we
df[upper.tri(df)] <- as.matrix( cast(df.long, one ~ two, fill=0) )
df[lower.tri(df)] <- df[upper.tri(df)]

---

UPDATE: the original sketch included these manual kludges

Then the same approaches to add the missing row 'd' and column 'a', and fill the lower triangle by adding the transpose t(df) :

df <- cbind(a=rep(0,4), rbind(df, d=rep(0,3)))
#   a  b  c  d
# a 0 30 40 20
# b 0  0 10  5
# c 0  0  0 30
# d 0  0  0  0

df + t(df)
#    a  b  c  d
# a  0 30 40 20
# b 30  0 10  5
# c 40 10  0 30
# d 20  5 30  0