python - "How to Think Like a Computer Scientist (python)"中的练习 7.9 测量字符串中某个字符的出现次数

Question

问题是如何编写一个程序来测量一个字符在 python 中以通用方式出现在字符串中的次数。

我写的代码:

def countLetters(str, ch):
   count=0
   index=0
   for ch in str:
     if ch==str[index]:
       count=count+1
     index=index+1
   print count

当我使用这个函数时，它测量的是字符串的长度，而不是该字符在字符串中出现的次数。我做错了什么？编写此代码的正确方法是什么？

Answer 1

您正在覆盖“ch”变量:

def countLetters(str, ch):
#                      ^ the character you are looking for
    count=0
    index=0
    for ch in str:
#        ^ the string character you are trying to check
        if ch==str[index]:  # ???
            count=count+1
        index=index+1
    print count

(另外，返回值通常比打印它更有用)。

内置方法是str.count:

"aaabb".count("a")  -> 3

如何重写代码:

def countLetters(search_in, search_for):
    count = 0
    for s in search_in:    # iterate by string char, not by index
        if s==search_for:
            count += 1
    return count

和一个快速的 pythonic 替换:

def countLetters(search_in, search_for):
    return sum(1 for s in search_in if s==search_for)