不使用内置函数的列表和最大值

Question

我的任务需要一些帮助：

Using a for loop, write a function called getMax4 that takes in a list of numbers. It determines and returns the maximum number that is divisible by 4. The function returns -999 if the argument is an empty list The function returns 0 if no number in the argument list is divisible by 4.

下面显示了调用函数时的输出示例：

在我的代码中，我成功地做到了以下几点：

def getMax4(list):
    highest = 0
    if len(list) == 0:
        return -999
    else:
        for number in list:
            if number % 4 == 0:
                if number < highest:
                    highest = number

                else:
                    highest = 0               
                return number
print(getMax4([]))
print(getMax4([1, 3, 9]))
print(getMax4([-4, 3, -12, -8, 13]))
print(getMax4([1, 16, 18, 12]))           

我的结果如下：

我不明白为什么这个值被确定为None，而它应该是0。你知道吗

Answer 1

还有一点需要提醒的是，不要使用内建名称作为变量，例如list、str、int、float、tuple、type，如果您稍后将这些内建作为函数调用，会给您带来糟糕的结果。所以我建议你把清单改成数字。你知道吗

因为它只在if语句中出现return number：

if number % 4 == 0:

要返回0：

只需将return number标识到for循环。你知道吗

比如

def getMax4(numbers):
    highest = []
    if len(numbers) == 0:
        return -999
    else:
        for number in numbers:
            if number % 4 == 0:
                if highest==[] or number > highest:
                    highest = number             
        return highest if highest!=[] else 0

Answer 2

只是对现有代码的一个小更改，返回0以显式添加。你知道吗

def getMax4(list):
    highest = 0
    if len(list) == 0:
        return -999
    else:
        for number in list:
            if number % 4 == 0:
                if number < highest:
                    highest = number

                else:
                    highest = 0               
                return number

    return 0

Answer 3

我们来看看代码的执行情况。你知道吗

def getMax4(list):
    highest = 0
    if len(list) == 0:
        return -999
    else:
        for number in list:
            if number % 4 == 0:
                if number < highest:
                    highest = number

                else:
                    highest = 0               
                return number

最明显的情况是长度为零的列表，它可以正确地处理它们。呜呼。你知道吗

另一个明显的例子是一个数字可以被4整除的列表。让我们看看：

# imagine getMax4([[1, 16, 20, 12])
# note that this is different from your example, because your example doesn't
# highlight a meaningful bug!

# unroll the for loop:

number = 1
if number % 4 == 0:  # it doesn't, so skip
number = 16
if number % 4 == 0:
    if number < highest:  # it's not, but this should pass. Your comparison is backwards here!
    else:
        highest = 0  # what?? we reset it?? Why??!
    return number

所以我们已经确定了几个问题，包括一旦我们找到一个可以被4整除的数，我们就返回这个数作为解，尽管这里的正确答案是20。这显然是错误的！你知道吗

让我们介入并解决这些问题。你知道吗

def getMax4(list):
    highest = 0
    if len(list) == 0:
        return -999
    else:
        for number in list:
            if number % 4 == 0:
                if number > highest:  # flip comparison
                    highest = number
                # removed the else clause
        return highest  # pulled this all the way out past the for loop's end

现在，让我们用新代码完成同一个调用

# getMax4([[1, 16, 20, 12])

# unroll the for loop
number = 1
if 1 % 4 == 0:  # it's not
number = 16
if 16 % 4 == 0:  # it is!
    if 16 > 0:  # it is!
        highest = 16  # remember it
number = 20
if 20 % 4 == 0:  # bingo!
    if 20 > 16:  # yup-a-roonie
        highest = 20  # remember this one now. We can forget 16
number = 12
if 12 % 4 == 0:  # batting 1.000
    if 12 > 20:  # not this time, buck-o.

# now we're all done with the for loop, so we step outside and find
return number  # which is 20

---

现在我们可以看一下清理一下：

def getMax4(list):
    highest = 0
    if len(list) == 0:
        return -999
    # whenever you have an early-exit conditional, there's no reason to
    # write the rest in an `else`. Just omit the `else` and dedent.
    for number in list:
        if number % 4 == 0:
            if number > highest:
                highest = number
    return highest