Executing loops for specific number of times.

I have vectors x=[3 5 8 10] and v=1:12 starting from last element of vector x i.e 10 8 5 3 in reverse order i want to do following stuff

    for i=1:12
    if x=10
    v(i)=v(i)-[real(p(i))*r(i)];

    next if x=8
    v(i)=v(i)-[real(p(i))*r(i)];
    ...
    ..
    ....
    it has to complete till x=3
    v(i)=v(i)-[real(p(i))*r(i)];
    end

before hand i dont know how many elements are there in vector x and loop should start from last element of x to first...