If I have a data frame where I am adding columns, and I would like one column to sum them up. I will not know the names of the columns ahead of time, so I guess I would need some kind of function that would count the number of columns and then sum them up.

If my data is like this:

```
w=1:10
x=11:20
z=data.frame(w,x)

```

I would like the total for `z$w` and `z$x`. But then if I were to add `z$y`, I would like to have that incorporated into the sum as well.