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Function assignment section, with references from HOPL4
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You can assign a value to a variable with the glyph: '←'.
{{Built-in|Assignment|←}} allows associating a name with an [[array]] value. Some dialects also allow assignment of function and operator values using the assignment arrow. In [[defined function]]s, assignment is global by default, but can be made local through explicit mention of the target name in the function header, or through dynamic [[shadow]]ing using <syntaxhighlight lang=apl inline>⎕SHADOW</syntaxhighlight>. In [[dfn]]s, assignments are local by default, but can be made global by explicit mention of the target namespace. Modified/indexed/selective assignment updates the most local definition.
==Examples==
===Basic usage===
Common examples (boxing on, and [[index origin]] is 0):
<syntaxhighlight lang=apl>
      ⎕←mat←(1 2 3)(1 2 3)
┌─────┬─────┐
│1 2 3│1 2 3│
└─────┴─────┘
</syntaxhighlight>
===Indexed assignment===
Individual elements can be updated using index assignment:
<syntaxhighlight lang=apl>
      mat[0]←1
      mat
┌─┬─────┐
│1│1 2 3│
└─┴─────┘
</syntaxhighlight>
A semicolon is necessary when dealing with a [[matrix]]:
<syntaxhighlight lang=apl>
      mat←3 3⍴⍳9
      mat
0 1 2
3 4 5
6 7 8
      mat[0 1;]
0 1 2
3 4 5
      mat[0 1;0 1]←0
      mat
0 0 2
0 0 5
6 7 8
</syntaxhighlight>
For higher-[[rank]] arrays, the number of semicolons needed is one less than the array rank.
===Modified assignment===
Some dialects allow placing a function the the immediate left of the assignment arrow:
<syntaxhighlight lang=apl>
      var←42
      var+←1
      var
43
</syntaxhighlight>
<syntaxhighlight lang=apl inline>var+←1</syntaxhighlight> is essentially equivalent to <syntaxhighlight lang=apl inline>1⊣var←var+1</syntaxhighlight> except that the result is [[shy]].
===Modified indexed assignment===
Modified assignment can also be combined with indexed assignment:
<syntaxhighlight lang=apl>
      mat←3 3⍴0
      mat
0 0 0
0 0 0
0 0 0
      mat[0 1;1]+←1
      mat
0 1 0
0 1 0
0 0 0
      mat[1;1],←'x'
      mat
0 1 0
0 x 0
0 0 0
</syntaxhighlight>


Common examples (boxing on, and ⎕io is 0):
== Function assignment ==
    mat←(1 2 3)(1 2 3)
 
    mat
In modern dialects, the right hand side of assignment may be a [[function]] (or possibly an [[operator]]). This allows [[Anonymous function|anonymous]] and [[tacit]] functions to be named.
┌─────┬─────┐
<syntaxhighlight lang=apl>
│1 2 3│1 2 3│
      sum←+/
└─────┴─────┘
      sum ⍳10
    mat[0]←1  ⍝ indexed assignment
55
    mat
</syntaxhighlight>
┌─┬─────┐
 
│1│1 2 3│
Before the 1980s, only arrays could be assigned: [[defined function]]s included the function name in the header in order to associate it with the function, and [[derived function]]s could not be named directly. Instead the programmer would define a function that explicitly called that derived function on its arguments. [[Iverson]]'s 1978 paper [[Operators and Functions]] used a new [[glyph]], an assignment arrow with a bar above (that is, [[overstruck]] with a [[high minus]]), as [[Adin Falkoff]] objected to the use of the unmodified assignment arrow.<ref>[[Roger Hui]] and [[Morten Kromberg]]. [https://dl.acm.org/doi/abs/10.1145/3386319 ''APL since 1978'']. ACM HOPL IV. 2020-06.</ref> However, Iverson used the assignment arrow directly in several papers he co-authored beginning in 1980.<ref>[[Bob Bernecky]] and [[Ken Iverson]]. [https://www.jsoftware.com/papers/opea.htm Operators and Enclosed Arrays] at [[IPSA '80]]</ref><ref>[[Ken Iverson]] and Peter Wooster. [https://dl.acm.org/doi/abs/10.1145/390007.805349 A function definition operator] at [[APL81]].</ref><ref>[[Ken Iverson]] and [[Arthur Whitney]]. [https://doi.org/10.1145/800071.802236 "Practical uses of a model of APL"] ([https://www.jsoftware.com/papers/APLModel.htm web]) at [[APL82]].</ref> Function assignment was added to [[Dyalog APL]] in version 4.0 in 1986. It was a key feature of [[J]], which supports only anonymous and tacit functions.
└─┴─────┘
 
    mat←3 3⍴⍳9
== External Links ==
    mat
 
0 1 2
=== Documentation ===
3 4 5
 
6 7 8
* [https://help.dyalog.com/latest/index.htm#Language/Primitive%20Functions/Assignment.htm Dyalog]
    mat[0 1;] ⍝ 1 semicolon is necessary when dealing with 2D arrays, 2 semicolons for 3D arrays etc.
* [https://microapl.com/apl_help/ch_020_010_110.htm APLX]
0 1 2
* J: [https://code.jsoftware.com/wiki/Vocabulary/eqdot local], [https://code.jsoftware.com/wiki/Vocabulary/eqco global]
3 4 5
 
    mat[0 1;0 1]←0
== References ==
    mat
<references/>
0 0 2
{{APL built-ins}}[[Category:Primitive functions]]
0 0 5
6 7 8
    ⍝ incrementing (or any dyadic function) parts of an array
    mat←3 3⍴0
    mat
0 0 0
0 0 0
0 0 0
    mat[0 1;1]+←1
    mat
0 1 0
0 1 0
0 0 0
    mat[1;1],←'x'
    mat
0 1 0
0 x 0
0 0 0
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