例2.11_1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
import string
import random
x=string.ascii_letters+string.digits
y=".join([random.choice(x)for i in range(1000)])"
#choice()用于从多个元素中随机选择一个
d= dict()
for ch in y:
d[ch] = d.get(ch,0) + 1
for k,v in sorted(d.items()):
print(k,':',v)
import string
import random
x=string.ascii_letters+string.digits
y=".join([random.choice(x)for i in range(1000)])"
d= dict()
for ch in y:
d[ch] = d.get(ch,0) + 1
for k,v in sorted(d.items()):
print(k,':',v)结果如下图
