I have three vectors as:

```
low = c(1,2,3)
med = c(3,5)
high = c(9)

```

I would like to create a dataframe like

```
value group
1     low
2     low
3     low
3     med
5     med
9     high

```

I tried to create a dataframe (or a factor) like `x=data.frame(low = low, med=med, high=high)` but it gives three columns.