[datatable-help] A[B]?

[Rguy]

I am beginning to learn the data.table package. At the outset,
'data.table.pdf' states:

It is inspired by A[B] syntax in R where A is a matrix and B is a 2-column
matrix.

I have used matrices in R but am unfamiliar with the A[B] syntax. When I
check the documentation for 'matrix' I find no discussion of such syntax. So
this "explanation" is in fact a black hole. Please tell your readers what
the package does in such a way that they are not sent on a wild goose chase.
For example:

The data.table package supports an A[B] syntax where A is a data table, B is
a 2 column data table, and the effect of the expression A[B] is...

What does A[B] accomplish?

Thanks.




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