Null object: Difference between revisions
show how to check for undefined names in J
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=={{header|J}}==
J doesn't have NULL. To indicate "missing data", "normal" data is usually pressed into service (e.g. <tt>0</tt> or <tt>_1</tt> in a numeric context, <tt>' '</tt> in a literal context, <tt>a:</tt> in a boxed context, etc). Most frequently, missing data is represented by the empty vector <tt>''</tt>.
However, undefined names in J can be identified:
<lang J>isUndefined=: _1 = nc@boxxopen</lang>
Example use:
<lang J> isUndefined 'foo'
1
foo=:9
isUndefined 'foo'
0</lang>
Note, however, that this "name is not defined" state is not a first class value in J -- you can not create a list of "undefineds".
=={{header|Java}}==
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