python - 返回列表中给定数字之前的数字

Question

我正在尝试编写一个二分搜索，它将在有序列表中的给定数字之前产生最大的数字。

y = int(input("Enter a number:"))
numblist = [5, 8, 9, 10, 18, 20, 25, 28, 30, 35]

lowerval = numblist[0]
higherval = numblist[9]
number = 0

mid = (higherval + lowerval)//2

for y in numblist:
   number += 1
if mid == y:
   print(number)
   break

if mid < y:
    lowerval = mid
else:
    higherval = mid
    mid = (higherval + lowerval)//2

例如，如果我输入20，返回的数字应该是18。老实说，我不知道如何调用正确的位置。我对 python 非常陌生，所以任何帮助将不胜感激。

Answer 1

以下代码片段将完成您想要的操作，尽管方式相对笨拙(但更容易理解):

# Ask the user for an input
y = int(input("Enter a number: "))
# List of numbers to be searched
numblist = [5, 8, 9, 10, 18, 20, 25, 28, 30, 35]

# Set the lower bound of the search
lowerval = numblist[0]
# Set the upper bound of the search
higherval = numblist[-1]


# Search Algorithm
while True:
    mid = (lowerval + higherval) // 2
    if mid == y:
        # Here, I am assuming that you will only test against a list with only unique values.
        # It first indexes the value that was found, then subtracts one from it to obtain the value prior to the found value.
        print(numblist[numblist.index(y) - 1])
        # Then it exits the while loop.
        break
    if mid < y:
        lowerval = mid
    if mid > y:
        higherval = mid

希望这有帮助!