Comparing the data and Pairing a data - Problem occurs - Needed help

Hello all, inorder to do MRC i need to do a pairing process as per my project

Consider some vectors

a1 =[1 2 3 4 5 6 7 8]'; b1= [3 3 2 3 2 3 3 2]'; c1=zeros[8,1];

I need to check b1(1)==3 if it is 3 then allocate a1(1) value in c1(1),

then i need to check b1(2)==3 if it is 3 then allocate a1(1) (SAME VALUE OF a1(1)) value in c1(2).

Now c1(1) & c1(2) = a1(1) ie. 1

Like this where ever 3 comes i need to take 3 as a pair and respective values to be repeated twice.

Then I need to check b1(3)==3 if it is not equal to 3 then allocate a1(2) (NEXT VALUE OF a1 vector) value in c1(3)

Now c1(3)= a1(2) ie.2

Then i need to check b1(4)==3 if it is equal then allocate a1(3) (NEXT VALUE OF a1 vector) value in c1(4)

Now c1(4)=a1(3) ie.3

Then i need to check b1(5)==3 if it is not equal to 3 then allocate a1(4) (NEXT VALUE OF a1 vector) value in c1(5)

Now c1(5)=a1(4) ie.4

Then i need to check b1(6)==3 if it is equal then allocate a1(3) (SAME VALUE OF a1 vector which is not paired before) value in c1(6)

Now c1(6)=a1(3) ie.3

c1(4) & c1(6)=a1(3) ie. 3 should be allocated

this is the code i have written

for i=1:2:8
    for j=i:i+1
        if b1(j)==3
            c1(j)=a1(i);
        else
            c1(j)=a1(i);
        end
    end
end

MY EXPECTED OUTPUT IS c1=[1 1 2 3 4 3 5 6]'
The output I am getting is c1 =[ 1 1 3 3 5 5 7 7]' 

NOTE: In my program vector b1 is randomised with the values 2,3 so i cannot predict the position.

Kindly suggest a solution to my problem, Thanks in advance.

[Sreedhar Arumugam]

Could you please provide another sample test case?