Here is the example data. I want ot select these observations with for example a=2 or 4 or 6 or 8, the I don't know the value and I want to get the random one from available. and desired output in my example with a=2,4,6,8 would be observations 1,2,3,4,5,6,7,8,9,10,12,13,16.
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