# single-column data frame

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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 4where you can see that:

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

Here is full explanation for that:

http://stat.ethz.ch/R-manual/R-patched/library/base/html/Extract.data.frame.html

To

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