I have a 207x7 xts object (called temp). I have a 207x3 matrix (called ac.topn), each row of which contains the columns I'd like from the corresponding row in the xts object. 

For example, given the following top two rows of temp and ac.topn,

```
temp
             v1   v2    v3  v4  v5    v6   v7
1997-09-30 14.5  8.7  -5.8 2.6 4.7   1.9 17.2
1997-10-31  6.0 -2.0 -25.7 2.9 4.9   9.6  8.4

head(ac.topn)
           Rank1 Rank2 Rank3
1997-09-30     7     4     2
1997-10-31     6     5     7

```

I would like to get the result:
 1997-09-30 17.2 2.6 8.7 (elements 7, 4, and 2 from the first row of temp)
 1997-10-31 9.6 4.9 8.4 (elements 6, 5, 7 from the second row of temp)

My first attempt was `temp[,ac.topn]`. I've browsed for help, but am struggling to word my request effectively.

Thank you.