第一次迭代后如何阻止它退出？

Question

我试图返回一个用新字符替换给定旧字符的语句，但循环在第一次迭代后继续退出。

def test(s, old_ch, new_ch): 
    """returns the given input and replaces an old character with a new character""" 
    newstring = "" 
    for ch in s: 
        while (ch == old_ch): 
            newstring += new_ch  
            break
        while (ch != old_ch):
            newstring += ch 
            break 
        return newstring

我知道在 python 中已经定义了替换函数，但这是我被告知这样做的方式。 （与for ch in s 位相同）

Answer 1

你需要改变两件事。

1. 不要使用 while 循环和中断，只需使用 if 语句。您的代码仍会以相同的方式工作，但它会更具可读性。

2. 将 return 语句移到 for 循环之外，即取消缩进。这样newstring 将在整个 for 循环执行完毕之前返回，这是期望的行为。

完整的更正代码如下：

def test(s, old_ch, new_ch): 
    """returns the given input and replaces an old character with a new character""" 
    newstring = "" 
    for ch in s: 
        if (ch == old_ch): 
            newstring += new_ch  
        else:
            newstring += ch 
    return newstring

print(test("Megalovania", "a", "o"))
# Prints Megolovonio

Answer 2

for ch in s 语句用于遍历字符串的每个字符。 因此，不需要 for 循环中的 while 循环。

def test(s, old_ch, new_ch): 
    """returns the given input and replaces an old character with a new character""" 
    newstring = "" 
    for ch in s: 
        if(ch == old_ch): 
            newstring = newstring+new_ch  
            
        if(ch != old_ch):
            newstring = newstring + ch 
            
    return newstring

此代码执行您想要完成的预期工作。

Answer 3

试试这个，这样你会更好地理解迭代。

def test(s, old_ch, new_ch): 
    newstring = "" 
    for ch in s:
        print(f"ch is {ch}")
        if (ch == old_ch):
            print(f"if condition is True for ch {ch} and old_ch {old_ch}")
            newstring += new_ch  
        else:
            print(f"elif condition is True for ch {ch} and old_ch {old_ch}")
            newstring += ch
    print(newstring)
    return newstring

调用这个函数

test("New to Python", "t", "y")

输出是

ch is N
elif condition is True for ch N and old_ch t
ch is e
elif condition is True for ch e and old_ch t
ch is w
elif condition is True for ch w and old_ch t
ch is  
elif condition is True for ch   and old_ch t
ch is t
if condition is True for ch t and old_ch t
ch is o
elif condition is True for ch o and old_ch t
ch is  
elif condition is True for ch   and old_ch t
ch is P
elif condition is True for ch P and old_ch t
ch is y
elif condition is True for ch y and old_ch t
ch is t
if condition is True for ch t and old_ch t
ch is h
elif condition is True for ch h and old_ch t
ch is o
elif condition is True for ch o and old_ch t
ch is n
elif condition is True for ch n and old_ch t