changing the form of a list

i have a 2 x n matrix that is a paired list of connections, i am working
with that looks something like:
1 2
1 3
1 5
2 1
2 3
3 1
3 2
4 5
5 1
5 4
for later operations it would be helpful if i could change this into the
form of:
1  2 3 5
2  1 3
3  1 2
4  5
5  1 4
the initial list is randomly generated and the max number of times that any
one number can appear is previoulsy defined by a limiting function (which is
variable depending on the numbers involved)
is there a clean way to change the form of that list?

Suppose the matrix is actually a data.frame (if it isn't, use
as.data.frame) called df with columns called i and x

tapply(df$x, df$i, c)

df

i  x
1  1  1
2  2  2
3  3  3
4  3  4
5  4  5
6  2  6
7  1  7
8  3  8
9  4  9
10 3 10

 tapply(df$x,df$i,c)

$"1"
[1] 1 7

$"2"
[1] 2 6

$"3"
[1]  3  4  8 10

$"4"
[1] 5 9

Or by()

by(df, df$i, function(subset) subset$x)

df$i: 1
[1] 1 7
------------------------------------------------------------
df$i: 2
[1] 2 6
------------------------------------------------------------
df$i: 3
[1]  3  4  8 10
------------------------------------------------------------
df$i: 4
[1] 5 9

Or you might want to leave the data in its current form and use tapply()
or by() to do whatever it is you want to do with it.

	-thomas




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