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rdata.tableexpressionquote
Guybrush
Guybrush

Reputation: 738

Data.table: Using quotes or expressions as i

I want to use as quote (or expression) as i in a data.table. But it seems to be not possible.

Here is a minimal example:

library(data.table)

dt <- data.table(a = 1:10)

dt[a == 5,]             # Everything well
dt[quote(a == 5),]      # Error: i has not evaluated to logical, integer or double
dt[expression(a == 5),] # Error: i has not evaluated to logical, integer or double

The help page of data.table states for i:

Integer, logical or character vector, single column numeric matrix, expression of column names, list, data.frame or data.table.

So I think, it should work with expressions. What is my mistake? Or is there a way to "unquote" the quote?

Upvotes: 2

Views: 962

Answers (1)

Guybrush
Guybrush

Reputation: 738

For reasons of completeness, here is the answer of @nicola posted as comment below the question:

The expression or quote should be enclosed via eval(). For the examples in the question it looks like:

library(data.table)

dt <- data.table(a = 1:10)

dt[a == 5,]             # Everything well
dt[eval(quote(a == 5)),]      # Now, it works
dt[eval(expression(a == 5)),] # Now, it works

The secret seems to be that R evaluates the arguments within the environment of the function and not within the context the function is called.

Upvotes: 1

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