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This is a trivial but very useful tip:

> x=data.frame(a=1:4, c=5)
> x
  a c
1 1 5
2 2 5
3 3 5
4 4 5
> x[1,] 
  a c
1 1 5
> x[,1]
[1] 1 2 3 4
> x[,1, drop=F]
  a
1 1
2 2
3 3
4 4

where you can see that:

• to avoid a[, i] become a vector, rather than a single-column data frame, we can use drop=F option. 

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