Python循环异常：查找列表最值时提前停止比较的问题排查

问题现象 #

测试输入数字23、34、68、3、566、2、1后，代码输出结果如下：

current list > ['23', '34', '68', '3', '566', '2', '1']
the smallest number is > 1
largest number is > 68

实际最大值应为566，但输出错误，最小值结果正确。

问题代码 #

largest = None
smallest = None
a = []
while True:
    num = input("Enter a number: ")
    if num == "done":
        break
    try:
        FailureCond = int(num)
    except ValueError:
        print("that's not a number")
        continue
    a.append(num)
    continue

for i in range(len(a)):

    if smallest is None:
        smallest = a[i]
        largest = a[i]

    if a[i] < smallest:
        smallest = a[i]

    if a[i] > largest:
        largest = a[i]


print("current list >",a)
print("the smallest number is >",smallest)
print("largest number is >",largest)

错误原因 #

列表a中存储的是字符串类型的数字，而非整数类型。字符串比较是按单个字符的ASCII码值逐位对比：

• '566'的第一个字符是'5'（ASCII码53），'68'的第一个字符是'6'（ASCII码54），所以'566' > '68'的结果为False，导致最大值无法更新为566。
• 最小值正确是因为'1'的ASCII码比其他字符串的首字符都小。

修复方案 #

方案一：存储整数类型 #

将输入的字符串转换为整数后再存入列表，修改a.append(num)为a.append(int(num))：

largest = None
smallest = None
a = []
while True:
    num = input("Enter a number: ")
    if num == "done":
        break
    try:
        num_int = int(num)
    except ValueError:
        print("that's not a number")
        continue
    a.append(num_int)

for i in range(len(a)):
    if smallest is None:
        smallest = a[i]
        largest = a[i]

    if a[i] < smallest:
        smallest = a[i]

    if a[i] > largest:
        largest = a[i]

print("current list >",a)
print("the smallest number is >",smallest)
print("largest number is >",largest)

方案二：比较时转换为整数 #

在循环比较环节，将列表元素转换为整数后再进行判断：

largest = None
smallest = None
a = []
while True:
    num = input("Enter a number: ")
    if num == "done":
        break
    try:
        FailureCond = int(num)
    except ValueError:
        print("that's not a number")
        continue
    a.append(num)

for i in range(len(a)):
    current_num = int(a[i])
    if smallest is None:
        smallest = current_num
        largest = current_num

    if current_num < smallest:
        smallest = current_num

    if current_num > largest:
        largest = current_num

print("current list >",a)
print("the smallest number is >",smallest)
print("largest number is >",largest)

内容的提问来源于stack exchange，提问作者Thanh bui dux