[R] updating subset of data.frame

[Rnewb]

i have a data frame and a numeric vector indexed as a subset of the rows in
the data.frame.  what command can i use to assign the values in the vector
to the appropriate rows of the data.frame?  here's my failed attempt.  what
i would want is data[1,'z'] == 2, data[5,'z'] == -4, data[8,'z'] == -5,
data[9,'z'] == 5, and for the other values of 'z' to remain 0.

> data
     x   y z
1   -6   4 0
2   -3  19 0
3   15  40 0
4    0  17 0
5    1  12 0
6   17  45 0
7    4  19 0
8  -13 -16 0
9   -7   5 0
10  -5  22 0
> vec
 1  5  8  9 
 2 -4 -5  5 
> data['z']=vec
Error in `[<-.data.frame`(`*tmp*`, "z", value = c(2, -4, -5, 5)) : 
  replacement has 4 rows, data has 10
> 

thanks,
Rnewb
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