Editing ALGOL 68 (section)

====Assignation and identity relations, etc.====
These are technically not operators, rather they are considered "[https://web.archive.org/web/20130101062456/http://jmvdveer.home.xs4all.nl/report.html#052 units associated with names]"
{|class="wikitable"
|-
! style="background:#ccc;"|'''PRIO'''rity<br />(Quaternaries)
! style="background:#ccc;"|Algol68 "Worthy characters"<sup>[[#The language of the unrevised report|r0]]&[[#Revisions|r1]]</sup>
! style="background:#ccc;"|+Algol68<sup>[[#The language of the unrevised report|r0]]&[[#Revisions|r1]]</sup>
! style="background:#ccc;"|+Algol68<sup>C,G,R</sup>
! style="background:#ccc;"|+Algol68<sup>[[#The language of the unrevised report|r0]]</sup>
|-
|ALIGN=CENTER| Effectively 0 || {{tt|1=:=, '''IS''' :=:, '''ISNT''' :/=: :~=:, '''AT'''  @, ":", ";"}} || {{tt|1=:≠: :¬=:}} 
| {{tt|1=:=:=<sup>C</sup>, =:=<sup>R</sup>}} || {{tt|1=..=, .=, '''CT''', ::, '''CTAB''', ::=, .., '''is not''', "..", ".,"}}
|}

Note: Quaternaries include names '''SKIP''' and ~.

{{code|1=:=:}} (alternatively {{tt|1='''IS'''}}) tests if two pointers are equal; {{code|1=:/=:}} (alternatively {{tt|1='''ISNT'''}}) tests if they are unequal.

=====Why {{code|1=:=:}} and {{code|1=:/=:}} are needed=====
Consider trying to compare two pointer values, such as the following variables, declared as pointers-to-integer:

: <code>'''REF''' '''INT''' ip, jp </code>

Now consider how to decide whether these two are pointing to the same location, or whether one of them is pointing to '''NIL'''. The following expression

: <code>ip = jp </code>

will dereference both pointers down to values of type {{tt|1='''INT'''}}, and compare those, since the {{char|{{=}}}} operator is defined for {{tt|1='''INT'''}}, but not {{tt|1='''REF''' '''INT'''}}. It is ''not legal'' to define {{char|{{=}}}} for operands of type {{tt|1='''REF''' '''INT'''}} and {{tt|1='''INT'''}} at the same time, because then calls become ambiguous, due to the implicit coercions that can be applied: should the operands be left as {{tt|1='''REF''' '''INT'''}} and that version of the operator called? Or should they be dereferenced further to {{tt|1='''INT'''}} and that version used instead? Therefore the following expression can never be made legal:

: <code>ip = '''NIL''' </code>

Hence the need for separate constructs not subject to the normal coercion rules for operands to operators. But there is a gotcha. The following expressions:

: <code>ip :=: jp </code>
: <code>ip :=: '''NIL''' </code>

while legal, will probably not do what might be expected. They will always return {{tt|1='''FALSE'''}}, because they are comparing the ''actual addresses of the variables ''<code>ip</code>'' and ''<code>jp</code>'', rather than what they point to''. To achieve the right effect, one would have to write

: <code>ip :=: '''REF''' '''INT'''(jp) </code>
: <code>ip :=: '''REF''' '''INT'''('''NIL''') </code>