我正在为我的 Python 类(class)编写一个信用卡支付计算器。作业是为计算器编写一个定义，计算出在 x 个月后使您的余额为零所需的每月付款。
定义有 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找到答案，我还写了另外两个有助于作业的定义:
1) newBalance() - 确定支付后的新余额； 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将列表中的最后一项评估为等于 0(或至少在 0 的 0.005 以内!)并且如果所以，请退还让我到达那里的付款。
如果最终余额为负(我付的太多了!):payment = payment - (payment/2); 如果余额为正(我还没有付够!):payment = payment + (payment/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
"""
感谢您的帮助。除非我用谷歌搜索过的所有内容都在 stackoverflow 上，否则可能不会加入 compsci。
请您参考如下方法:
这不是你平分的方式
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，你要么设置
low_guess = guess
或
high_guess = guess
然后重复
注意:在Python2中，如果分母和分子都是整数，/就是整数除法，所以最好保证初始的guess是float
