Exists function in q/kdb

Question

I need to write a function in q/kdb which takes a variable v and returns 1b if v is defined and 0b if it is not:

$ a:2
$ doesExist`a
1b
$ doesExist`b
0b

Any ideas appreciated.

Answer 1

q)doesExist:{x~key x}
q)a:2
q)doesExist`a
    1b
q)doesExist`b
    0b

Answer 2

key`.

Will give you all the variables in the current namespace.

Similarly

key`.foo

Will give you all the variables in the .foo namespace.

By extension:

`a in key`.

Will give you the boolean you're after

Answer 3

Based on MdSalih's answer and pamphlet's comment, perhaps we can test the opposite. Since key outputs an empty list if the variable is not defined, we should test for that, which gets us around the keyed table problem.

q)AnswerToLifeUniverseAndEverything:42
q)doesExist:{not () ~ key x}
q)doesExist[`AnswerToLifeUniverseAndEverything]
1b
q)doesExist[`UltimateQuestionToLifeUniverseAndEverything]
0b