R数据表：根据参考列为列分配值

Question

I would like to assign a value into a column from a larger table, using another column as a reference.

Eg data:

require(data.table)
dt <- data.table(N=c(1:5),GPa1=c(sample(0:5,5)),GPa2=c(sample(5:15,5)),
GPb1=c(sample(0:20,5)),GPb2=c(sample(0:10,5)),id=c("b","a","b","b","a"))

   N GPa1 GPa2 GPb1 GPb2 id
1: 1    4   10    7    0  b
2: 2    5   15   19    7  a
3: 3    1    5   20    5  b
4: 4    0   13    3    4  b
5: 5    3    7    8    1  a

The idea is to get new columns Val1 and Val2 . Any GP column ending in 1 is eligible for Val1 and any ending in 2 is eligible for Val2 . The value to be insterted into the column is determined by the id column, per row.

So you can see for Val1 , you'd draw on the GPb1 column, then GPa1 , GPb1 , GPb1 again and finally GPa1 .

The final result would be;

   N GPa1 GPa2 GPb1 GPb2 id Val1 Val2
1: 1    4   10    7    0  b   7    0
2: 2    5   15   19    7  a   5   15
3: 3    1    5   20    5  b  20    5
4: 4    0   13    3    4  b   3    4
5: 5    3    7    8    1  a   3    7

I did achieve the answer but in quite a few lines after melting it etc, but i'm sure there must be an elegant way to do this in data.table . I was initially frustrated by the fact paste0 doesn't work in data.table ;

dt[1,paste0("GP",id,"1")]

but;

# The following gives a vector that is correct for Val1 (and works for 2)
diag(as.matrix(dt[,.SD,.SDcols=dt[,paste0("GP",id,"1")]]))

# I think the answer lies in `set`, but i've not had any luck.
for (i in 1:nrow(dt)) set(dt, i=dt[i,.SD,.SDcols=dt[,paste0("GP",id,"2")]], j=i, value=0)

The data is quite ugly this way so perhaps it's better to just use the melt method.

Answer 1

dt[id == "a", c("Val1", "Val2") := .(GPa1, GPa2)]
dt[id == "b", c("Val1", "Val2") := .(GPb1, GPb2)]
#   N GPa1 GPa2 GPb1 GPb2 id Val1 Val2
#1: 1    2   13    5    8  b    5    8
#2: 2    3    8    7    2  a    3    8
#3: 3    5   11   19    1  b   19    1
#4: 4    4    5    6    9  b    6    9
#5: 5    1   15    1   10  a    1   15