I have the following data frame in R which presents the number of purchases per year over a period of four years, for 4 customers. The `member_since` variable shows the year when the customer joined the company. 

```
id<-c(1,2,3,4)
member_since<-c(2014,2016,2015,2014)
X2014<-c(2,0,0,3)
X2015<-c(3,0,4,2)
X2016<-c(3,2,3,4)
X2017<-c(2,3,6,0)
df<-data.frame(id,member_since,X2014,X2015,X2016,X2017)

 id    member_since X2014 X2015 X2016 X2017
 1         2014      2     3     3     2
 2         2016      0     0     2     3
 3         2015      0     4     3     6
 4         2014      3     2     4     0

```

Now I am trying to create a new variable `mean_purchase` to calculate the average number of purchases per year for each customer, from the year s/he joined. This means for example for `customer 2` the total number of purchases should be divided by 2, but for `customer 4` the sum of purchases should be divided by 4. 

```
 id    member_since X2014 X2015 X2016 X2017 mean_purchase
 1         2014      2     3     3     2         2.5
 2         2016      0     0     2     3         2.5
 3         2015      0     4     3     6         4.33
 4         2014      3     2     4     0         2.25

```

Would appreciate your help on this.