determine location of numbers within a vector

Hi, I have a vector

x=[345 3433 20 5345 3533 21 3453453 34534 345345 22 234];

I'm looking for the fastest way to find the location of a numbers that starts from 20 and increase by one. In this toy example the output would be y=[3 6 10];

In reality the size of vector x for me can be 2 billion so I'm looking for a fast way to get y. Thanks for your suggestions.

Answer 1

[Thorsten]

 ind = arrayfun(@(val) find(val == x), 20:22)