Question

下面是我必须为学校工作的一个程序，我必须使用我概述的样式，但无法弄清楚为什么程序总是返回string not found。知道为什么会这样做吗？我应该使用测试函数和调试器，但这超出了我的范围。我修复了一个崩溃程序的递归问题。

def str_search(data, target, start, end):
    """
    str_search : String String NatNum NatNum -> NatNum or NoneType
    Description:
    Search for a target value in a sorted data string.
    The search happens between the start and end indices inclusively.
    This starts searching in the middle. If it finds the target, it is done.
    Otherwise it decides whether to search the first half or the second half.
    preconditions: the data string is in ascending alphanumeric order.
    Parameters:
        data - a string
        target - the target value to find is a single character string e.g. 'Q'
        start - the starting index into the data
        end - the ending index into the data
    Returns:
        index of target in data, if present; otherwise None.
    """

    if start <= end:
        return None

    mid_index = ( start + end ) // 2
    mid_value = data[mid_index]

    # debug statement prints the data.
    #print( "Searching for", target, ":", data[start:mid_index],
    #    "*" + str( mid_value ) + "*", data[mid_index+1:end+1] )

    if target == mid_value:
        return mid_index
    elif target <= mid_value:
        return str_search(data, target, start, mid_index - 1)
    else:
        return str_search(data, target, mid_index, end)


def find_target(data, target):
    """
    find_target : String String -> NatNum or NoneType
    find_target returns the index of target in data or None if not found.
    Parameters:
        data - a string
        target - the target value to find
    Returns:
        The index of the target element in data, if present, or None.
    """

    return str_search(data, target, 0, len(data) - 1)


def makeString():
    """
    makeString : () -> String
    makeString returns a String
    """
    data = ""
    # append characters to make the string
    for num in range(36, 108, 2):
        data += chr(num)
    return data


def main_search():
    """
    main_search : Void -> NoneType
    """

    data = makeString()
    print("Number of elements: ", len(data))

    while True:
        print("\nData: ", data)
        target = input("Enter a character to find: ")

        if target == "":
            break
        else:
            index = find_target(data, target)
            print()
            if index != None:
                print(target, "found at index", index)
            else:
                print(target, "not found")
                # end while


def test_str_search():
    """
    a suite of pass/fail test cases to validate that str_search works.
    """
    # Complete this test function.
    pass

#######################################################################
# 3.3. Document your debugging results trying to fix the str_search code.
# Enter answers to the questions below inside the triple-quoted string.
"""
    Were you able to completely fix str_search?
    If not, explain in detail the cases that still fail.
    What tool(s) did you use?
    What went well?
    What problems did you have?
"""
#######################################################################

if __name__ == "__main__":
    #
    # Run the test functions for problem 1, problem 2, and problem 3.
    #
    #test_create_multiplication_table()
    #test_is_palindrome()
    #test_is_reverse_of()
    test_str_search()
    #
    main_search()

Answer 1

在你的搜索功能中你有这个

if start <= end:
        return None

但你的开始是0，结束是len（数据） - 1，这就是你搜索函数一直没有返回任何内容的原因。

Answer 2

这很有趣，你需要测试你是否已经到了二进制搜索的结尾，前进或后退，然后才宣布没有找到任何内容，你必须确保一旦你到达搜索结尾就得到答案并没有留在任何一端。你可以通过以下方式得到你想要的东西：

if end - start <= 1:
    if target == data[end]:
        return end
    elif target == data[start]:
        return start
    else:
        return None

在python 3中搜索一串字符

2 个答案: