使用遞歸來反轉列表，並使用python計算列表中元素的出現次數

Question

我正在嘗試使用遞歸和列表，方法是編寫一個函數，該函數接受一個字符串，將其拆分為一個列表，然后使用遞歸生成字符串的反向版本，並計算搜索項出現的次數在列表中。 換句話說，如果用戶輸入“ the quick brown fox”，並且想知道“ duck”在該句子中出現的頻率，程序將輸出“ fox brown quick The”，然后輸入“ duck is找到（在本例中為0），似乎我的代碼目前正在運行，但是它不會打印出REVERSE和COUNT函數的結果，有人可以向我解釋為什么嗎？

此函數反轉字符串中元素的順序

def REVERSE(strng):
    new_list=''
#base case
    if len(strng)==0:
        return new_list+""
#recursive call
#returns the first element in the string, and adds it to the end of the rest of the string called recursively from the second element
    else:
        new_list+=REVERSE(strng[1:]) + strng[0]
        return new_list
    print (new_list)

用於計算字符串中子字符串出現次數的函數

def COUNT(strng,srch):
        count=0
    #base case
        if len(strng)==0:
            return count
    #recursive call in event search term found in the first element of the list
        elif strng[0]==srch:
            count+=1
            return COUNT(strng[1:],srch)
    #recursive call in event search term not found in first element of list
        else:
            count+=0
            return COUNT(strng[1:],srch)   
        print ("The term" + srch + "occurs" + count + "times")

這是調用這兩個功能的程序。 我將它們保存在單獨的文件中以練習導入等

from functions import *
def main():
        terms = input ("Enter the list of terms:\n").split(" ")
    query = input("Enter a query term:\n")
    print("List in reverse order:")
    REVERSE(terms)
    print()
    COUNT(terms, query)
main()

Answer 1

REVERSE和COUNT都將永遠不會執行這些打印語句-它們將始終在執行到達print之前return 。

您可能要從REVERSE和COUNT刪除無用的print ，並在main打印它們的返回值：

def main():
terms = input ("Enter the list of terms:\n").split(" ")
query = input("Enter a query term:\n")
print("List in reverse order:")
print(REVERSE(terms))
print()
print(COUNT(terms, query))

Answer 2

到目前為止，在我看來，以下內容可能會對您有所幫助。

#!/usr/bin/python

def REVERSE(strng):
    new_list=''
    #base case
    if len(strng)==0:
        new_list+=""
    #recursive call
    #returns the first element in the string, and adds it to the end of the rest of the string called recursively from the second element
    else:
        new_list+=REVERSE(strng[1:]) + strng[0] + " "
    return new_list


def COUNT(strng,srch):
        count=0
        #base case
        if len(strng)==0:
            count = 0
        #recursive call in event search term found in the first element of the list
        elif strng[0]==srch:
            count+=1
            COUNT(strng[1:],srch)
        #recursive call in event search term not found in first element of list
        else:
            count+=0
            COUNT(strng[1:],srch)
        return count

if __name__ == '__main__':
    terms = input ("Enter the list of terms:\n").split(" ")
    query = input("Enter a query term:\n")
    print("List in reverse order: %s" % REVERSE(terms))
    print ("The term '%s' occurs %d times." % (query, COUNT(terms, query)))