data:
> tmp1
   Date HrMn Temp Q.4
1 19450101 0900 -37.0  1
2 19450101 1000 -35.9  2
3 19450101 1100 -35.9  3
4 19450101 1200 -36.4  4
5 19450101 1300 -36.4  5
6 19450101 1400 -36.4  6
7 19450101 1500 -36.4  7
8 19450101 1600 -37.5  9

Accidentally, I did this (I meant to write Q.4 instead of Q here)
> tmp1$Q[tmp1$Q!="1" & tmp1$Q!= "5"]<-NA

I would have expected it to tank, but still got a new field with the the
values I wanted
> tmp1
   Date HrMn Temp Q.4   Q
1 19450101 0900 -37.0  1   1
2 19450101 1000 -35.9  2 <NA>
3 19450101 1100 -35.9  3 <NA>
4 19450101 1200 -36.4  4 <NA>
5 19450101 1300 -36.4  5   5
6 19450101 1400 -36.4  6 <NA>
7 19450101 1500 -36.4  7 <NA>
8 19450101 1600 -37.5  9 <NA>

Can someone explain this?
Peter

> sessionInfo()
R version 2.10.1 Patched (2010-03-16 r51294)
i386-pc-mingw32

locale:
[1] LC_COLLATE=English_United States.1252 LC_CTYPE=English_United
States.1252  
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                
[5] LC_TIME=English_United States.1252  

attached base packages:
[1] stats   graphics grDevices utils   datasets methods  base  

other attached packages:
[1] lattice_0.18-3 fortunes_1.3-7 zoo_1.6-2  

loaded via a namespace (and not attached):
[1] grid_2.10.1 tools_2.10.1

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