有什么方法可以在Python中提取数字的符号吗？

Question

I have am coding the algorithm of Bolzano in Python. This is my code for now:

def Bolzano(fonction, a, b, tol=0.000001):
   while abs(b-a)>tol:
       m=(a+b)/2
       if cmp(fonction(m))==cmp(fonction(a)):
           a=m
       else:
           b=m
   return a, b

It works until it encounters cmp, which it doesn't recognise. However I don't see another way to do it, since Python doesn't have a sign function. Is there any other way to extract the sign of a number?

Answer 1

Is there any other way to extract the sign of a number?

How about writing your own?

Implementation

def sign(num):
    return -1 if num < 0 else 1

Example

>>> sign(10)
1
>>> sign(-10)
-1

Ohh and cmp is a built-in that requires two parameters (numbers) and simply compares them and checks which of them is larger. You should have used it as follows

def Bolzano(fonction, a, b, tol=0.000001):
   while abs(b-a)>tol:
       m=(a+b)/2
       if cmp(fonction(m), fonction(a)) == 0:
           a=m
       else:
           b=m
   return a, b

Answer 2

可能使用：

if cmp(fonction(m),fonction(a)) == 0:

Answer 3

def same_sign(a, b):
    return (a * b) >= 0

Examples:

>>> same_sign(3, 4)
True

>>> same_sign(-3, 4)
False

>>> same_sign(3, -4)
False

>>> same_sign(-3, -4)
True

>>> same_sign(-3, 0)
True