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我正在为我的 Python 课程编写一个信用卡支付计算器。任务是为计算器编写一个定义,该计算器计算出在 x 个月内使您的余额为零所需的每月付款。

该定义采用 3 个参数:initialBalance、apr、months。

据我所知,作业的重点是让我们使用二分法来找到我们的答案,我还写了另外两个有助于作业的定义:

1) newBalance() - 确定付款后的新余额;2) balances() - 在付款后返回余额列表;

鉴于此,balances()[-1] 返回最终余额,因此我的行动计划是评估列表中的最后一项等于 0(或至少在 0 的 0.005 以内!),如果是,则返回让我到达那里的付款。

如果最终余额为负数(我付出了太多!):付款=付款-(付款/ 2);如果余额为正(我还没有付足够的钱!):付款=付款+(付款/2);

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

这是我的代码,(最后是教授的测试定义):

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
    """

谢谢您的帮助。除非我用谷歌搜索的所有内容都在 stackoverflow 上,否则可能不会加入 compsci。

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2 回答 2

5

这不是你如何一分为二

guess = guess - (guess / 2)

通常你会保留一个 low_guess 和一个 high_guess。你试试

guess = (low_guess+high_guess)/2

然后根据结果,您可以设置

low_guess = guess

或者

high_guess = guess

并重复

注意:在 Python2 中,/如果分母和分子都是整数,则为整数除法,因此最好确保初始guess值是浮点数

于 2012-10-09T03:12:40.160 回答
1
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.
    """
    low_guess = 0
    high_guess = initialBalance
    guess = float((low_guess + high_guess) / 2)
    listo = months*[guess]

    while True:

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

        elif float(balances(initialBalance, apr, listo)[-1]) < -0.005:
            high_guess = guess
            print "BOO", guess ##for debugging
            print balances(initialBalance, apr, listo)[-1]

        else:
            low_guess = guess
            print "NAY", guess  ##for debugging
            print balances(initialBalance, apr, listo)[-1]
        guess = float((low_guess + high_guess) / 2)
        listo = months*[guess]
于 2012-10-09T03:51:17.437 回答