Assignment X←Y

Assignment allocates the result of the expression Y to the name or names in X.

If Y is an array expression, X must contain one or more names which are variables, system variables, or are undefined. Following assignment, the name(s) in X become variable(s) with value(s) taken from the result of the expression Y.

If X contains a single name, the variable assumes the value of Y. If X contains multiple names then Y can be a single-item array of any rank (including a scalar) or a vector. If Y is a single-item array, the scalar value ⊃Y is assigned to all names in X. Otherwise, each element of Y is assigned to the corresponding name in X. Although not mandatory, Dyalog recommends that the names in X are enclosed in parentheses to reduce potential ambiguity in assignment statements.

The assignment arrow (or specification arrow) is often read as 'Is' or 'Gets'.

Examples of single assignment

Examples of multiple assignment using parentheses

      (A B)←2
      A
2
      B
2

      (P ⎕IO Q)←'TEXT' 1 (1 2 3)
      P
TEXT
      ⎕IO
1
      Q
1 2 3

Example of multiple assignment without parentheses

      year month day←2017 05 24
      day
24
      month
5
      year
2017

Implementation note: erroneous expressions such as var 3←5 will result in 5 being assigned to var, even though a SYNTAX ERROR will be generated. In the case of (var 3)←5 no assignment will be made.

Pass-through assignments are permitted. The value of Y is carried through each assignment:

      I←J←K←0
      I,J,K
0 0 0

Function Assignment

If Y is a function expression, X must be a single name which is either undefined, or is the name of an existing function or defined operator. X may not be the name of a system function, or a primitive symbol.

Examples

      PLUS←+
      PLUS
+

      SUM←+/
      SUM
+/

      MEAN←{(+/⍵)÷⍴⍵}

Namespace Reference Assignment

If an expression evaluates to a namespace reference, or ref, you may assign it to a name. A name assigned to a simple scalar ref, has name class 9, whereas one assigned to an array containing refs has name class 2.

      'f1'⎕WC'Form'
      'ns1' ⎕NS ''

      N←ns1
      ⎕NC'N'           ⍝ name class of a scalar ref
9
      F←f1
      ⎕NC'F'           ⍝ name class of a scalar ref
9
      refs←N F         ⍝ vector of refs.
      ⎕NC'refs'        ⍝ nameclass of vector.
2
      F2←2⊃refs
      ⎕NC 'F2'
9

Re-Assignment

A name that already exists may be assigned a new value if the assignment will not alter its name class, or will change it from 2 to 9 or vice versa. The table of permitted re-assignments is as follows:

	Ref	Variable	Function	Operator
Ref	Yes	Yes
Variable	Yes	Yes
Function			Yes	Yes
Operator			Yes	Yes