Function axis
[ax]

Bracket axis ([ax]
) or function axis is the a special syntax whereby many APLs allow the behavior of a function to be modified, for example ⌽[2]
to Rotate along the second axis. Axis specification was a feature of Iverson notation and was ubiquitous in early APLs; many newer APLs which adhere to leading axis theory reject the use of axis specification in favor of the Rank operator because it is a fully general operator while the behavior of functions with axis must be defined for each function separately.
Contents
Functions
Monadic functions
The following monads may allow an axis:
 Mix accepts a list of axes to specify where the axes of argument elements will be placed in the result.
 Ravel accepts a list of axes which are combined, or a single fractional number to add a length1 axis.
 Enclose accepts a list of axes. Each subarray along these axes is enclosed.
 Split accepts a single axis, and encloses each vector along that axis.
 Reverse reverses along the specified axis.
Dyadic functions
The following dyads may allow one:
 Scalar dyadics accept a list of axes to override conformability rules: it specifies , for each axis in the lowerrank (or left, in case of a tie) argument, which axis in the other argument it is paired with.
 Catenate combines along the selected axis, adding a new axis if a noninteger axis is given.
 Rotate rotates the right argument along the selected axis.
 Replicate and Expand work on the specified right argument axis.
 Take and Drop modify the selected right argument axes.
 Squish takes axes to specify which axis of the right argument corresponds to each left argument element.
 Partition and Partitioned Enclose have complicated and different behavior.
Operators
The following operators may admit axis specification:
 Reduction removes the specified right argument axis.
 Scan works on the specified right argument axis.
In Dyalog APL, a slash with axis retains its functionoperator overloading: it can be applied as an operator or as a dyadic function (Replicate or Expand).