Source code for first_negative

"""This module contains a function to illustrate the use of 
while loops. 

"""

    
def firstnegative(L):
    """ Given a list of values, finds and returns the index of the first 
    negative value in the list. If the list does not contain any 
    negative values, it returns -1.

    Note that the use of negative values when the value is 
    "not found" is not a good idea. A better value would be to return
    None.

    Good test cases are listed below::

     >>> print 'first negative in [1,2,-1] is at index', firstnegative([1,2,-1])
     first negative in [1,2,-1] is at index 2
     >>> print 'first negative in [-1,2,-1] is at index', firstnegative([-1,2,-1])
     first negative in [-1,2,-1] is at index 0
     >>> print 'first negative in [1,2,1] is at index', firstnegative([1,2,1])
     first negative in [1,2,1] is at index -1

    """

    i = 0
    while (i < len(L)):
        val = L[i]
        if val < 0:
            return i
        i += 1
    return -1