There are too many edge cases to give you a complete answer (and I don't want to do your work for you). Walter has tried to get at some of the edge cases, but your answer does not clarify the problem.
Given
a = [ 2 2 2 2 3 3 3 3 7 7 4 4 4 4 4 7 7 7 7 7 7 1 1 1 ]
you can find all the indices which are some value x with
You probably need to to find all the indices that are 3, 4, and 7, and then look and see if there are any 7's between the 3's and 4's that meet your rules. You can create a set of nested loops that check each 3, 4, and 7 index. In the end you should create an array that has the indices of the 7's you want to replace
You can then replace them with
There are probably faster computational ways to solving the problem than nest for loops, but get the nested loops will potentially help you see the problems.
