I want to calculate mean of elements of a column until NA value and then start again to calculate average for the rest elements and put all averages into a list.
my data is like this (taking mean of column V4 and put into list-of-means:

```
                     V1      V2      V3 V4
1                  chr1 3686375 3686400  6
2                  chr1 3686400 3686425  8
3 Next bedGraph section      NA      NA NA
4                  chr1 3840175 3840200  2
5                  chr1 3840200 3840225  3
6                  chr1 3840225 3840250  4

```

The list-of-means = `(7,3)`