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Error message should remind correct usage of indexing with logical vector column #1844

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dracodoc opened this issue Sep 10, 2016 · 0 comments
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@dracodoc
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@dracodoc dracodoc commented Sep 10, 2016

It's pretty common in R to use a logical vector for selecting, but this fails:

> x <- data.table(b = c(FALSE, TRUE), d = 1:2)
> x
       b d
1: FALSE 1
2:  TRUE 2
> x[b]
Error in eval(expr, envir, enclos) : object 'b' not found
> x[(b)]
      b d
1: TRUE 2

This is a frequent mistake, see here and here

The problem can be avoided with more user friendly error message. For example:

Error in eval(expr, envir, enclos) : object 'b' not found
When 'i' is a single variable name, it is not considered an expression of column names and is instead evaluated in calling scope. 
If 'b' is intended to be used as logical vector to index, wrap 'b' with () as expression.
@mattdowle mattdowle added this to the v1.10.6 milestone Aug 5, 2017
@mattdowle mattdowle closed this in 8231e2c Aug 5, 2017
mattdowle added a commit that referenced this issue Aug 5, 2017
Closes #1844 -- supplement error message when potentially subsetting by a logical vector in i without wrapping in ()
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