Could anyone please explain to me why the following function call returns 'NULL' instead of 1?
> df <- data.frame(1,2,3)
> df
X1 X2 X3
1 1 2 3
> f <- function(vdf, vx) { return (vdf$vx); }
> f(df, "X1")
NULL
>
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stathisk
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1You could try `f – David Arenburg May 06 '14 at 21:06
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2It happens for the same reason that `col="X1";df$col` returns NULL. R position matches `"X1"` to the argument `vx` and `$` does not evaluate its argument so the function is searching for `df$vx` regardless of the value you assign to the argument `vx`. – Simon O'Hanlon May 06 '14 at 21:09
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[**Here's a bit more info**](http://stackoverflow.com/a/18228613/1478381) and another example. – Simon O'Hanlon May 06 '14 at 21:11
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Thanks folks. Should I delete it ? – stathisk May 06 '14 at 21:28