Topic: `APLX Help` : `Help on APL language` : `APL Fundamentals` : `Axis operator`
[ Previous | Next | Contents | Index | APL Home ] # Axis Operator

A number of primitive functions and operators can be applied to a particular axis (or dimension) of an array. The `[` `]` brackets are used to indicate the axis being specified.

The highest dimension of a data item is considered to be the first dimension and the lowest dimension the last . Thus the first dimension of a matrix is the rows and the last dimension is the columns. In the case of a three-dimensional object, the first dimension is the planes followed by the rows and columns.

Axis numbers are governed by the Index Origin, `⎕IO,` and in Index Origin 1, (the default), the first dimension is represented by `,` the second by `` and so on. In Index Origin 0 the first dimension would be `,` the second `` and so on. The number used to represent the axis is always a whole number, except for the ravel and laminate functions.

The primitive functions and operators which will accept an axis operator include the dyadic forms of the primitive scalar functions :

```     + - × ÷ | ⌈ ⌊ * ⍟ ○ ! ^ ∨ ⍲ ⍱ < ≤ = ≥ > ≠
```

and some primitive mixed functions :

```     , ⍪       Ravel/Catenate/Laminate       (note first axis variant)
⌽ ⊖       Reverse/Rotate                (note first axis variant)
⊂         Enclose/Partition
⊃         Disclose
↑         Take
↓         Drop
⌷         Index
```

as well as the operators:

```     / ⌿       Compress/Replicate            (note first axis variant)
/ ⌿       Reduce                        (note first axis variant)
\ ⍀       Scan                          (note first axis variant)
\ ⍀       Expand                        (note first axis variant)
```

See the reference section entry for Axis `([])` for more details, as well as the reference entries for individual mixed functions and operators listed above.

Topic: `APLX Help` : `Help on APL language` : `APL Fundamentals` : `Axis operator`
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