例2.11_2首先生成包含1000个随机字符的字符串，然后统计每个字符的出现次数，注意get()方法的使用
#利用collections模块的Counter()函数直接作出统计
#依次加载三个模块
import string,random,collections
x = string.ascii_letters + string.digits
y = ''.join([random.choice(x) for i in range(1000)])
count=collections.Counter(y)
for k,v in sorted(count.items()):
print(k,':',v)
print('学号：3008')
import string,random,collections
x = string.ascii_letters + string.digits
y = ''.join([random.choice(x) for i in range(1000)])
count=collections.Counter(y)
for k,v in sorted(count.items()):
print(k,':',v)
print('学号：3008')结果如下图所示
