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Undefined values: Difference between revisions
→[[Undefined values#ALGOL 68]]
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{{task}}For languages which have an explicit notion of an undefined value, identify and exercise those language's mechanisms for identifying and manipulating a variable's value's status as being undefined
=={{header|ALGOL 68}}==
{{works with|ALGOL 68|Revision 1 - no extensions to language used}}
{{works with|ALGOL 68G|Any - tested with release [http://sourceforge.net/projects/algol68/files/algol68g/algol68g-1.18.0/algol68g-1.18.0-9h.tiny.el5.centos.fc11.i386.rpm/download 1.18.0-9h.tiny]}}
{{works with|ELLA ALGOL 68|Any (with appropriate job cards) - tested with release [http://sourceforge.net/projects/algol68/files/algol68toc/algol68toc-1.8.8d/algol68toc-1.8-8d.fc9.i386.rpm/download 1.8-8d]}}
Note: Some implementations (eg [[ALGOL 68C]]) also have a procedure named ''undefined'' that is called to indicated that the behaviour at a particular point in a program is unexpected, undefined, or non-standard.
<lang algol68>MODE X = REAL;
REF X x := NIL;
MODE U = UNION(COMPL, VOID);
U u := EMPTY;
IF x IS REF X(NIL) THEN
print(("x IS NIL", new line))
ELSE
print(("x ISNT NIL", new line))
FI;
CASE u IN
(VOID):print(("u is EMPTY", new line))
OUT print(("u isnt EMPTY", new line))
ESAC</lang>
Output:
<pre>
x IS NIL
u is EMPTY
</pre>
=={{header|C sharp|C#}}==
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