Indirect accessing of data frame variable in R -

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i have r data.frame object called x. r console when str(x), following get.

str (x)

'data.frame':   2776 obs. of  4 variables:    $ date   : factor w/ 4018 levels "2003-01-01","2003-01-02",..: 1 6 11 16 21 26 31 36 41 46 ...   $ sulfate: num  na na na na na na na na na na ...   $ nitrate: num  na na na na na na na na na na ...   $ id     : int  1 1 1 1 1 1 1 1 1 1 ...

i want access values related particular variable. in process when x.$nitrate values corresponding variable (ie nitrate) when following

> frame_variable <- "nitrate"

and then do

> x$frame_variable null

this limitation prohibiting me use feature in function i'm trying create. here.

thanks.

this way access columns variable:

x[, frame_variable]

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