Vectorization - Adding numpy arrays without loops?

So I have the following numpy arrays:

c = array([[ 1,  2,  3],
           [ 4,  5,  6],
           [ 7,  8,  9],
           [10, 11, 12]])
X = array([[10, 15, 20,  5],
           [ 1,  2,  6, 23]])
y = array([1, 1])

I am trying to add each 1x4 row in the X array to one of the columns in c. The y array specifies which column. The above example, means that we are adding both rows in the X array to column 1 of c. That is, we should expect the result of:

     c = array([[ 1,  2+10+1,  3],  =  array([[ 1,  13,  3],
                [ 4,  5+15+2,  6],            [ 4,  22,  6],
                [ 7,  8+20+6,  9],            [ 7,  34,  9],
                [10, 11+5+23, 12]])           [10,  39, 12]])  

Does anyone know how I can do this without any loops? I tried c[:,y] += X but it seems like this only adds the second row of X to column 1 of c once. With that being said, it should be noted that y does not necessarily have to be [1,1], it can also be [0,1]. In this case, we would add the first row of X to column 0 of c and the second row of X to column 1 of c.