Dear R-helpers:

Suppose I have a datafram called "test_frame" like this
     col1   col2  col3  col4
r1   x        x        x     x
r2   x        x        x     x
r3   x        x        x     x
..    x        x        x     x
rn   x        x        x     x

I know I can get data of col3 by using test_frame[,3].

My question is, if I want to do this by specifying "col3" rather than 3,
what should I do?

Many thanks....

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