为什么我的二分算法不起作用？蟒蛇

Question

为什么我的二分算法不起作用？蟒蛇

Tyl*_*our 1 python bisection

我正在为我的Python课程编写信用卡付款计算器.分配是为计算器编写一个定义,计算出每月支付的费用,以便在x个月内将余额归零.

该定义有3个参数:initialBalance,apr,months.

尽管我可以想象,分配的目的是让我们使用二分法来找到答案,并且我写了另外两个有助于赋值的定义:

1)newBalance() - 确定付款后的新余额; 2)余额() - 在付款后返回余额清单;

在这种情况下,balances()[ - 1]返回最终余额,所以我的行动计划是将列表中的最后一项评估为等于0(或至少在0的0.005之内!)如果是,则返回付款让我在那里.

如果最终余额是负数(我付了太多钱!):付款=付款 - (付款/ 2); 如果余额是正数(我没有足够的支付!):付款=付款+(付款/ 2);

尽我所能,我的算法应该最终得出结论,但它永远找不到足够接近的答案......

这是我的代码,(以及教授在最后的测试def):

def newBalance(prevBalance, apr, payment):
    """
    - prevBalance: the balance on the credit card statement.
    - apr: the annual percentage rate (15.9 here means 15.9%).
    - payment: the amount paid this month to the credit card company.
    - returns: the new balance that will be owed on the credit card
      (assumes no purchases are made). 
    """
    interestCharge = float(((apr / 12.0) / 100) * prevBalance)
    return float((interestCharge + prevBalance) - payment)




def balances(initialBalance, apr, payments):
    """
    - initialBalance: the initial balance on the credit card.
    - apr: the annual percentage rate (15.9 here means 15.9%).
    - payments: a list of monthly payments made on the credit card.
    - returns: a list giving the balance on the credit card each
      month. The first number in the list is the initial
      balance, the next number is the balance after the first
      payment is made, and so on. Note that the length of the returned
      list is len(payments) + 1.
      """
    balancelist = []
    balancelist.append(initialBalance)
    for x in range(0, len(payments)):
        balancelist.append(newBalance(balancelist[x], apr, payments[x]))
    return balancelist




def findMonthlyPayment(initialBalance, apr, months):
    """
    - initialBalance: the starting balance on the card.
    - apr: the APR.
    - months: the number of equal monthly payments you wish to
      make in order to reduce the balance to zero.
    - returns: the monthly payment amount needed to reduce the
      balance to zero (well, "zero" to within $0.005, anyway)
      in the given number of months.
    """
    guess = float(initialBalance / months)
    listo = months*[guess]

    while True:

        if abs(float(balances(initialBalance, apr, listo)[-1]) - initialBalance) < 0.006:
            print "BINGO", guess  ##for debugging
            print balances(initialBalance, apr, listo)[-1]
            return guess

        else:
            if float(balances(initialBalance, apr, listo)[-1]) < -0.005:
                guess = guess - (guess / 2)

                print "BOO", guess ##for debugging
                print balances(initialBalance, apr, listo)[-1]

            else:
                guess = guess + (guess / 2)
                print "NAY", guess  ##for debugging
                print balances(initialBalance, apr, listo)[-1]

        listo = months*[guess]



def testFindMonthlyPayment():
    answer = findMonthlyPayment(1000, 18.9, 60)
    print
    myString = "Monthly payment to pay off $%.2f in %d months at %.2f%% APR:"
    print myString % (1000, 60, 18.9)
    print "$%.2f" % answer
    # Output should look approximately like this:
    """
    iteration: 1 guess: 500.0 final bal: -46777.3384635
    iteration: 2   guess: 250.0   final balance: -22111.7016729
    iteration: 3   guess: 125.0   final balance: -9778.88327752
    iteration: 4   guess: 62.5   final balance: -3612.47407985
    iteration: 5   guess: 31.25   final balance: -529.269481021
    iteration: 6   guess: 15.625   final balance: 1012.3328184
    iteration: 7   guess: 23.4375   final balance: 241.531668687
    iteration: 8   guess: 27.34375   final balance: -143.868906167
    iteration: 9   guess: 25.390625   final balance: 48.83138126
    iteration: 10   guess: 26.3671875   final balance: -47.5187624535
    iteration: 11   guess: 25.87890625   final balance: 0.656309403241
    iteration: 12   guess: 26.123046875   final balance: -23.4312265251
    iteration: 13   guess: 26.0009765625   final balance: -11.387458561
    iteration: 14   guess: 25.9399414062   final balance: -5.36557457885
    iteration: 15   guess: 25.9094238281   final balance: -2.35463258781
    iteration: 16   guess: 25.8941650391   final balance: -0.849161592282
    iteration: 17   guess: 25.8865356445   final balance: -0.0964260945206
    iteration: 18   guess: 25.8827209473   final balance: 0.27994165436
    iteration: 19   guess: 25.8846282959   final balance: 0.0917577799204
    iteration: 20   guess: 25.8855819702   final balance: -0.00233415730086

    Monthly payment to pay off $1000.00 in 60 months at 18.90 APR:
    $25.89
    """

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谢谢您的帮助.可能不会加入compsci,除非我用google搜索的所有东西都是在stackoverflow上.