２分法

#! usr/bin/env python
# -*- coding: utf-8 -*-

#nibunhou.py
import random
import math
def f(x):
    return x*x*x - x +1
a = float(raw_input("set a as f(a)>0 : "))
b = float(raw_input("set b as f(b)<0 : "))
det = math.fabs((a-b)*1.0 / a )
if math.fabs(a)!=math.fabs(b):
    EPS = 0.000001#収束条件値
    while det > EPS:
        det = math.fabs((a-b)*1.0 / a )
        x = float(a + b) / 2
        print "x=", x
        print "f(x)=",f(x)
        if f(x) > 0:
            a = x
        elif f(x) < 0:
            b = x
    print "approximate solution= ",x
else:
    print "invalid input"