The indexing operator, `?[]`, provides a way to encode the process of indexing an expression in a function call.

The first calling sequence evaluates to the expression x, indexed by the entries of the list l. For example, `?[]`(tbl, [a, b]) evaluates to tbl[a, b].

Evaluating the second calling sequence assigns the expression sequence contained in the list m to the entry that the first calling sequence evaluates to. For example, evaluating `?[]`(tbl, [a, b], [c, d]) is the same as running the statement tbl[a, b] := c, d;.

The help page object,operators discusses some of the uses and consequences of overriding the indexing operator in an object. Briefly, it allows the object author to define the behavior when an object is indexed.

$\mathrm{tbl}≔\mathrm{table}\left(\left[\mathrm{one}=1\,\mathrm{two}=2\,\mathrm{three}=3\,\mathrm{four}=4\right]\right)$

$\mathrm{tbl}≔table\left(\left[\mathrm{three}=3\,\mathrm{four}=4\,\mathrm{one}=1\,\mathrm{two}=2\right]\right)$

$\mathrm{tbl}\left[\mathrm{one}\right]=\mathrm{`?[]`}\left(\mathrm{tbl}\,\left[\mathrm{one}\right]\right)$

$1=1$

$\mathrm{tbl}\left[\mathrm{five}\right]≔5$

${\mathrm{tbl}}_{\mathrm{five}}≔5$

$\mathrm{`?[]`}\left(\mathrm{tbl}\,\left[\mathrm{six}\right]\,\left[6\right]\right)$

${\mathrm{tbl}}_{\mathrm{six}}$

$\mathrm{eval}\left(\mathrm{tbl}\right)$

$table\left(\left[\mathrm{six}=6\,\mathrm{five}=5\,\mathrm{three}=3\,\mathrm{four}=4\,\mathrm{one}=1\,\mathrm{two}=2\right]\right)$

$\mathrm{ids}≔\left[\left[\mathrm{two}\right]\,\left[\mathrm{six}\right]\,\left[\mathrm{three}\right]\,\left[\mathrm{two}\right]\right]$

$\mathrm{ids}≔\left[\left[\mathrm{two}\right]\,\left[\mathrm{six}\right]\,\left[\mathrm{three}\right]\,\left[\mathrm{two}\right]\right]$

$\mathrm{map2}\left(\mathrm{`?[]`}\,\mathrm{tbl}\,\mathrm{ids}\right)$

$\left[2\,6\,3\,2\right]$

$\mathrm{lst}≔\left[4\,5\,6\right]$

$\mathrm{lst}≔\left[4\,5\,6\right]$

$\mathrm{vec}≔⟨7\,8\,9⟩$

$\mathrm{vec}≔\left[\begin{array}{c}7\\ 8\\ 9\end{array}\right]$

$\mathrm{map}\left(\mathrm{`?[]`}\,\left[\mathrm{lst}\,\mathrm{vec}\,\mathrm{var}\right]\,\left[2\right]\right)$

$\left[5\,8\,{\mathrm{var}}_2\right]$