Question

我遇到了嵌套在for循环中的while循环问题。它在for循环的第一次迭代中完美执行，但for循环然后在所有其他迭代上跳过while循环。

我正在尝试填充列表nsteps_list，其中while循环执行的次数为nsteps，用于for循环的每次迭代。预期的答案可能是List = [17,16,16,14,15,13,12,15 ......]，但所有发生的事情都是List = [17,1,0,0,0,0] ...]

循环代码：

# Bisection Method

minn = np.arange(.00001, .001, 0.00005)
nsteps_list = [0.0] * (len(minn))  # Rewrite each index with nsteps as iterating through
nsteps = 0

for i in range(0, len(minn) - 1):

    while math.fabs(fx_2) > minn[i]:

        if fx_2 > 0:
            x_3 = x_2
            print "UPDATE: x_3 = " + str(x_2)

        elif fx_2 < 0:
            x_1 = x_2
            print "UPDATE: x_1 = " + str(x_2)

        x_2 = 0.5 * (x_1 + x_3)
        fx_2 = func(x_2)
        nsteps += 1

    print nsteps
    nsteps_list[i] = nsteps
    nsteps = 0

print "List: " + str(nsteps_list)

我从实验中知道它在for循环中迭代很好，但它无法返回到while循环，所以nsteps重置为0永远不会改变，我的列表中填充了0。

以下是完整代码，在上下文中：

#!/usr/bin/python

import matplotlib.pylab as plt
import numpy as np
import math


# Parabola = 3x^2-9x+2 ==> Has minimum and 2 real roots

def func(n):  # Defining function to get f(x) for each x for parabola

    a = 3
    b = -9
    c = 2

    fx = a * n * n + b * n + c
    return fx


# Calling parabola function on values in x
x = np.arange(-2.0, 4.0, 0.2)
y = func(x)

plt.figure(1)
plt.plot(x, y)
plt.plot(x, x * 0)

# Declare Variables for bisection method

x_1 = 2.0
x_3 = 3.0
x_2 = 0.5 * (x_1 + x_3)

fx_1 = func(x_1)
fx_2 = func(x_2)
fx_3 = func(x_3)

if fx_1 >= 0:
    print "Warning: Variable x_1 not initialised correctly."
if fx_3 <= 0:
    print "Warning: Variable x_3 not initialised correctly."

# Bisection Method

minn = np.arange(.00001, .001, 0.00005)
nsteps_list = [0.0] * (len(minn))  # Rewrite each index with nsteps as iterating through
nsteps = 0

for i in range(0, len(minn) - 1):

    while math.fabs(fx_2) > minn[i]:

        if fx_2 > 0:
            x_3 = x_2
            print "UPDATE: x_3 = " + str(x_2)

        elif fx_2 < 0:
            x_1 = x_2
            print "UPDATE: x_1 = " + str(x_2)

        x_2 = 0.5 * (x_1 + x_3)
        fx_2 = func(x_2)
        nsteps += 1

    print nsteps
    nsteps_list[i] = nsteps
    nsteps = 0

print "List: " + str(nsteps_list)

print "x_2 = " + str(x_2) + " and f(x_2) = " + str(fx_2) + "."

plt.figure(2)
plt.plot(np.log10(minn), nsteps_list)
plt.figure(1)
plt.plot(x_2, fx_2, "mo")

plt.show()

所以我需要这个数组在相应的minn值的对数上绘制图形。有什么问题的想法？

Answer 1

我调试了你的程序。在第一个for循环交互之后，while循环运行17次。 i从0递增到1，并尝试再次评估while循环。

此时此fx_2的值为-8.63145396579057e-06 minn[1]是6.0000000000000002e-05

所以fx_2的绝对值小于 min[i] i==1，因此不允许再次输入内循环，吐出零而是nsteps。

由于我不是数学专家，我建议你再看看你想要计算什么。

while循环嵌套for循环仅执行for循环

1 个答案: