python: If loop returns wrong result

I wrote the following script trying to calculate portions in array a and stack the result to the original array:

import numpy


a = numpy.array([[1, 4, 4.555],
                 [2, 3, 9.112],
                 [3, 1, 7.322],
                 [1, 5, 3.911]])

i = 0
b = numpy.ones((4,1))
if i <= 2:
    b[i][0] = float(a[i][2])/(a[i][2]+a[i+1][2])
    b[i+1][0] = float(a[i+1][2])/(a[i][2]+a[i+1][2])
    i = i+2
print b
c = numpy.hstack((a,b))
print c

But the result were strange:

array b:
[[ 0.33328455]
 [ 0.66671545]
 [ 1.        ]
 [ 1.        ]]
array c:
[[ 1.          4.          4.555       0.33328455]
 [ 2.          3.          9.112       0.66671545]
 [ 3.          1.          7.322       1.        ]
 [ 1.          5.          3.911       1.        ]]

The first and second rows of array c was what I want, but the rest rows were strange. The result array I expected is:

[[ 1.          4.          4.555       0.33328455]
 [ 2.          3.          9.112       0.66671545]
 [ 3.          1.          7.322       0.65182943]
 [ 1.          5.          3.911       0.34817057]]

How to fix this? And is there more efficient coding to do this task?