Rearranging data frame from column names to key value pairs

Question

If I have data like this in R:

> a = data.frame(a = c(1, 2),
               b = c(3, 4))
> a
  a b
1 1 3
2 2 4

but I would like to have it like this:

> b = data.frame(k = c("a", "a", "b", "b"),
+                v = c(1, 2, 3, 4))
> b
  k v
1 a 1
2 a 2
3 b 3
4 b 4

How can I convert the data this way?

Answer 1

This is a simple gather() using package tidyr:

> library(tidyr)
> 
> gather(a, key = "k", value = "v")
  k v
1 a 1
2 a 2
3 b 3
4 b 4

Answer 2

You can alternatively use the melt() function from the reshape2 package

> library(reshape2)
> 
> melt(a, variable.name="k", value.name="v")
No id variables; using all as measure variables
  k v
1 a 1
2 a 2
3 b 3
4 b 4

Hope this is insightful!