如何在Matlab中找到指定索引的最近索引

I have two vectors in MATLAB, A and B. B contains some indices (1 to end). I have a random index, R (within the range of the vector indices). I want to write a function (or statement) to choose A[z], where z is the most nearest number (i.e index) to R that is not included in B.I have two vectors in MATLAB, A and B. B contai