Let me try to make this question as general as possible. 

Let's say I have two variables *a* and *b*.

```
a <- as.integer(runif(20, min = 0, max = 10))
a <- as.data.frame(a)
b <- as.data.frame(a[c(-7, -11, -15),])

```

So *b* has 17 observations and is a subset of *a* which has 20 observations. 

My question is the following: how I would use these two variables to generate a third variable *c* which like *a* has 20 observations but for which observations 7, 11 and 15 are missing, and for which the other observations are identical to *b* but in the order of *a*?

Or to put it somewhat differently: how could I squeeze in these missing observations into variable *b* at locations 7, 11 and 15? 

It seems pretty straightforward (and it probably is) but I have been not getting this to work for a bit too long now.