牛頓/拉復生法求平方根

def squareRootNR(x, epsilon):
    assert x >= 0
    assert epsilon >= 0
    x = float(x)
    guess = x/2.0
##    guess = 0.001
    diff = guess**2 - x
    ctr = 1
    while abs(diff) > epsilon and ctr <= 100:
        guess = guess - diff/(2.0*guess)
        diff = guess**2 - x
        ctr += 1
    assert ctr <= 100
    print 'NR method, Num, iterations:', ctr
    return guess