[R] editing a data.frame

[john.polo]

dear R users,

the data frame (read in from a csv) looks like this:
        TreeTag     Census    Stage     DBH
1     CW-W740   2001 juvenile     5.8
2     CW-W739   2001 juvenile     4.3
3     CW-W738   2001 juvenile     4.7
4     CW-W737   2001 juvenile     5.4
5     CW-W736   2001 juvenile     7.4
6     CW-W735   2001 juvenile     5.4
...
1501 1.00E-20   2001  adult      32.5

i would like to change values under the TreeTag column. as the last 
value shows, some of the tags have decimals followed by 2 decimal 
places. i just want whole numbers, i.e. not 1.00E-20, but 1E-20. i have 
a rough understanding of regexp and grepped all the positions that have 
the inappropriate tags. i tried sub() a couple of different ways, like
yr1bp$TreeTag[1501]<-sub("1.00", "1", yr1bp$TreeTag[1501])
and after turning yr1bp$TreeTag[1501] into <NA>,
yr1bp$TreeTag[1501]<-sub("", "1E-20", yr1pb$TreeTag[1501])
and
sub("", "1E-20", yr1bp$TreeTag[1501])
but it's not working. i guess it has something to do with the data.frame 
characteristics i'm not aware of or don't understand. would i somehow 
have to tear apart the columns, edit them, and then put it back 
together? not that i know how to do that, but i'm wondering out loud.

john