为什么在此 Python 代码中无法进行记忆？

问题描述

我正在关注一个动态编程视频。但是，我的代码的记忆功能不起作用。当我打印（备忘录）它是空白的时，它不存储任何真/假。请指教

def cansum(targetsum,numbers):
      memo = dict()
      print(memo)
      # check in memory
      if targetsum in memo:
            return memo[targetsum]
      if targetsum < 0: return False
      if targetsum == 0: return True
      for number in numbers:
            remainder = targetsum - number
            if cansum(remainder,numbers) == True:
                  memo[targetsum] = True
                  return True
      memo[targetsum] = False
      return False


print(cansum(7,[2,3])) #True
print(cansum(7,[5,3,4,7])) #True

解决方法

我认为这就是您可能想要做的：

def cansum(targetsum,numbers):
    memo = dict()

    def cansum_helper(targetsum,numbers):
        # check in memory
        if targetsum in memo:
            return memo[targetsum]
        if targetsum < 0:
            return False
        if targetsum == 0:
            return True
        for number in numbers:
            remainder = targetsum - number
            if cansum_helper(remainder,numbers) == True:
                memo[targetsum] = True
                return True
        memo[targetsum] = False
        return False

    result = cansum_helper(targetsum,numbers)

    print(memo)

    return result


print(cansum(7,[2,3]))  # True
print(cansum(7,[5,3,4,7]))  # True

如果你把

memo = dict()

进入递归函数，您为每个递归函数创建一个字典，一旦设置了备忘录，return 语句就跟在后面，因此您将无法看到更改。但我们的目的是你的整个函数只需要一个字典。

memoization python