flipping specific bits in numeric array

[les ander]

Hi,
suppose i have a numeric array of 1's and 0's and i would like
to flip the 1's --> 0's for only selected entries:
for example

state=array([1,0,0,0,1])

a=array([0.4,0.2,0.5,0.8,0.8])

I_a=nonzero(a<0.5)   # I would like to flip bits for entries of "state"
                     # at index in I_a

for i in I_a:
   if state[i]==0: state[i]=1
   else: state[i]=0


is there are faster way? thanks

furthermore, is it possible to overload the "not" operator for
numeric so that the i could achieve the above with simply "not a"?
thanks

p.s. I know what numarray has array indexing, however, i don't 
want to use numarray since i am using MLab which does not seem to support 
numarray