在Python 3

Question

以下搜索字符的字符串，是一个程序，我必须为学校工作的，在此我们必须用我所概述的风格，但想不通为什么程序总是返回string not found。任何想法，为什么这是这样做？我应该使用测试功能和一个调试器，但是这是超越我。我修正了崩溃的程序的递归问题。

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，你有这样的

if start <= end: 
     return None 

但你开始为0，并最终被LEN（数据） - 1，这就是为什么你搜索函数返回什么所有的时间。

Answer 2

这是有趣的，你需要测试，如果你已经达到了目的，向前或向后的二进制搜索您声明没有被发现之前，你必须确保一旦你已经达到了搜索结束了答案并不是隐藏在任何一端。你应该得到你想要的以下内容：

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