stupid question about array

I have an array ..ex: a=array(1,3,4,7,8,9)
During the execution of some code a variable b gets value of 5
I want to check if there is an element of a array which is equal to b
and I want to do this without checking every element of a in a for/next
cycle
Is there operator to do this ? and can I get the index of a ?
Thanx!

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