Expand Records to create Edges for igraph - r

Question

I have a dataset that has multiple data points I want to map. iGraph uses 1-1 relationships though so I'm looking for a way to take one long record into many 1-1 records. For Example:
test <- data.frame(
drug1=c("A","B","C","D","E","F","G","H","I","J","K"),
drug2=c("P","O","R","T","L","A","N","D","R","A","D"),
drug3=c("B","O","R","I","S","B","E","C","K","E","R"),
age=c(15,20,35,1,35,58,51,21,54,80,75))
Which gives this output
drug1 drug2 drug3 age
1 A P B 15
2 B O O 20
3 C R R 35
4 D T I 1
5 E L S 35
6 F A B 58
7 G N E 51
8 H D C 21
9 I R K 54
10 J A E 80
11 K D R 75
I'd like to make a new table with drug1-drug2 and then stack drug2-drug3 into the previous column. So it would look like this.
drug1 drug2 age
1 A P 15
2 P B 15
3 C R 20
4 R R 20
5 E L 35
drug2 is held in the drug1 spot and drug3 is moved to drug1. I realize I can do this by creating multiple smaller steps, but was was wondering if anyone new of a way to loop this process. I have up to 11 fields.
Here are the smaller steps.
a <- test[,c("drug1","drug2","age")]
b <- test[,c("drug2","drug3","age")]
names(b) <- c("drug1","drug2","age")
test2 <- rbind(a,b)
drug1 drug2 age
1 A P 15
2 B O 20
3 C R 35
4 D T 1
5 E L 35
6 F A 58
7 G N 51
8 H D 21
9 I R 54
10 J A 80
11 K D 75
12 P B 15
13 O O 20
14 R R 35
15 T I 1
16 L S 35
17 A B 58
18 N E 51
19 D C 21
20 R K 54
21 A E 80
22 D R 75

Answer 1

So if you have many fields, here's a helper function which can pull down the data into pairs.
pulldown <- function(data, cols=1:(min(attr)-1),
attr=ncol(data), newnames=names(data)[c(cols[1:2], attr)]) {
if(is.character(attr)) attr<-match(attr, names(data))
if(is.character(cols)) cols<-match(cols, names(data))
do.call(rbind, lapply(unname(data.frame(t(embed(cols,2)))), function(x) {
`colnames<-`(data[, c(sort(x), attr)], newnames)
}))
}
You can run it with your data with
pulldown(test)
It has a parameter called attr where you can specify the columns (index or names) you would like repeated every row (here I have it default to the last column). Then the cols parameter is a vector of all the columns that you would like to turn into pairs. (The default is the beginning to one before the first attr). You can also specify a vector of newnames for the columns as they come out.
With three columns your method is pretty simple, this might be a better choice for 11 columns.

Answer 2

Slightly more compact and a one-liner would be:
test2 <- rbind( test[c("drug1","drug2","age")],
setNames(test[c("drug3", "drug2", "age")], c("drug1", "drug2", "age"))
)
The setNames function can be useful when column names are missing or need to be coerced to something else.