python - 如何将计算句子中多少个字母的字符串更改为计算一个字母的出现？

Question

我的任务是改变这一点：

    sentence = 'The cat sat on the mat.'
    for letter in sentence:
    print(letter)

变成一个统计小写字母a出现次数的代码。我有点明白，但我不知道如何改变它。

Answer 1

最好使用count()：

>>> sentence = 'The cat sat on the mat.'
>>> sentence.count('a')
3

但是，如果您需要使用循环：

sentence = 'The cat sat on the mat.'
c = 0
for letter in sentence:
    if letter == 'a':
        c += 1
print(c)

Answer 2

使用正则表达式的另一种方法：

 import re

 sentence = 'The cat sat on the mat.'
 m = re.findall('a', sentence)
 print len(m)

Answer 3

也许是这样的？

occurrences = {}
sentence = 'The cat sat on the mat.'
for letter in sentence:
    occurrences[letter] = occurrences.get(letter, 0) + 1

print occurrence