Absolute newbie to R.
I have a dataframe that has some common values in one column(C1), but only one of the corresponding column has a value(C2), so I want to paste that value to all of the empty/NA spaces in C2 based on same value in C1.
This would make more sense:

df:

```
C1    C2  
A      NA  
A      val10  
A      NA  
B      val14  
B      NA  
B      NA  
B      NA  
C      NA  
C      val9  

```

What I wanted it to look like is

```
C1   C2  
A    val10  
A    val10  
A    val10  
B    val14  
B    val14  
B    val14  
B    val14  
C    val9  
C    val9 

```

(C2 and C1 don't have any particular pattern or sequence between each other)

I'm assuming I would do a Group\_by for C1, but I'm bit confused how to copy the values. Using transmute/mutate or paste. I tried a few iterations but wasn't successful.