Python代碼不會在第一次運行時執行

Question

使用Spyder Python 3.6此代碼不會執行，表示ispal方法未定義。但是，當我運行它並首先放入一個整數（比如我的字符串輸入= 0）時，它會在運行之後識別該方法。似乎我必須先通過一個分支，而不是首先調用該方法。感謝批評。

s = input('enter a string: ') 
s1 = s 
s1 = s1.lower() 
s1 = s1.replace(',', '') 
s1 = s1.replace(' ', '') 

if s1.isalpha(): 
    if ispal(s1) == True: 
     print(s,' is a palindrome') 
    else: 
     print(s,' is not a palindrome') 
else: 
    print('you entered illegal chars') 


def ispal(s1): 
    if len(s1) <= 1: 
     return True 
    else: 
     #if the first and last char are the same 
     #and if all 
     return s1[0] == s1[-1] and ispal(s1[1:]) 

Answer 1

首先，如TGKL指出它的定義之前，你打電話ispal。因此調用之前定義，即：

def ispal(s1): 
    if len(s1) <= 1: 
     return True 
    else: 
     #if the first and last char are the same 
     #and if all 
     return s1[0] == s1[-1] and ispal(s1[1:]) 

if s1.isalpha(): 
    if ispal(s1) == True: 
     print(s,' is a palindrome') 
    else: 
     print(s,' is not a palindrome') 
else: 
    print('you entered illegal chars') 

其次你的迴文遞歸函數是正確的，當你調用ispal裏面本身除了。而不是ispal(s1[1:])你應該做ispal(s1[1:-1])這將刪除剛剛測試的第一個和最後一個字符。

Answer 2

你必須首先定義你的方法，然後調用它：

s = raw_input('enter a string: ') #use raw_input so the text it takes will give to you directly a string without "" 
s1 = s 
s1 = s1.lower() 
s1 = s1.replace(',', '') 
s1 = s1.replace(' ', '') 

def ispal(s1): 
    if len(s1) <= 1: 
     return True 
    else: 
     #if the first and last char are the same 
     #and if all 
     return s1[0] == s1[-1] and ispal(s1[2:]) # here you put ispal(s1[1:]) it doesn't work properly :/ 

if s1.isalpha(): 
    if ispal(s1) == True: 
     print(s,' is a palindrome') 
    else: 
     print(s,' is not a palindrome') 
else: 
    print('you entered illegal chars')