Newbie data organisation/structures question...

So my data is in this sort of format:

P  T  I
1  1  (1, 2, 3)
2  1  (2, 4)
1  2  (1, 3, 6, 7)
2  2  (6)

Not knowing why you organized the data as you did, let me suggest  
another approach:

iv <- c(1, 2, 3, 2, 4, 1, 3, 6, 7, 6)
p <- c(1, 1, 1, 2, 2, 1, 1, 1, 1, 2)
t <- rep(1:2, each = 5)
dat <- data.frame(iv, p, t)

And I want to be able to quickly get:

The I when both P and T are given. e.g.:
P = 2, T = 2; I = (6)

subset(dat, p == 2 & t ==2)$iv

The concatenated vector of Is when P and a subset of T is given, e.g.:
P = 1, T = 1:2;  Is = (1, 2, 3, 1, 3, 6, 7)

(iv1 <- subset(dat, p == 1)$iv)

The length of that vector.

length(iv1)

A list of Is when either P or T is given. e.g.:
P = 2: I = (2, 4), (6)
T = 1: I = (1, 2, 3), (1, 3, 6, 7)

list(p2t1 = subset(dat, p == 2 & t ==1)$iv, p2t2 = subset(dat, p == 2  
& t ==2)$iv)
list(p1t1 = subset(dat, p == 1 & t ==1)$iv, p1t2 = subset(dat, p == 1  
& t ==2)$iv) # correcting your requirement to get your result

There are many other ways of getting the results you need as Marc  
Schwartz pointed out in his reply.
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