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How do I search a list of booleans for the first true?

I'm trying to find the index of the first true value:

1b?001b

Expected result: 2
Actual result: ,0b

Two questions:

  • How do I solve this issue?
  • What is my code actually doing?
Solution

  • For your first question, you want to use the where keyword, which when passed a boolean list will return the indices of the true values:

    q)where 00101b
2 4

    Use first in front of the above to obtain what you want.

    For your second question, I believe q is casting the left argument of to an int/long and then 'rolling' on the right argument list (see https://code.kx.com/q/ref/deal/).

    Example:

    q)0b?111b
`boolean$() //return 0 random elements from 111b
q)1b?111b
,1b //return 1 random element from 111b