[SciPy-User] find zero

[Alan G Isaac]

On 5/30/2014 5:35 AM, nicky van foreest wrote:
> I am trying to find a zero in an array V that is positive at the left some index i, say, and negative at the right of i.


The insertion point is
1+np.argmax(np.diff(np.sign(a)) != 0)
but a simple bisection algorithm would be much faster for large arrays.
Without testing::

def bisect(a, i1, i2):
   if a[i1]*a[i2] > 0:
     raise ValueError("Sign changing interval required.")
   while abs(i1-i2) > 1:
     midpt = (i1+i2) // 2
     if a[midpt]*a[i1] > 0:
       i1 = midpt
     else:
       i2 = midpt
   return max(i1,i2)

fwiw,
Alan Isaac