python3-列表字典简单练习题

1.求全部元素的和[1,2,1,2,3,3,3,3]
>>> a=[1,2,1,2,3,3,3,3]
>>> result=0
>>> for i in a:
...     result+=i
...
>>> print (result)
18

2.求偶数元素的和[1,2,1,2,3,3,3,3]    
方法一：只适用于当前list
>>> a=[1, 2, 1, 2, 3, 3, 3, 3]
>>> res=0
>>> for i in a[1:4:2]:
...     res+=i
...
>>> print (res)
4
方法二：适用于任何list
>>> a=[1, 2, 1, 2, 3, 3, 3, 3]
>>> res=0
>>> for i in a:
...     if i%2==0:
...        res+=i
...
>>> print(res)
4

3.统计一下所有数字出现的个数[1,2,1,2,3,3,3,3]
>>> d={}
>>> for i in a:
...     d[i]="1"
...
>>> d
{1: '1', 2: '1', 3: '1'}
>>> d.keys()
dict_keys([1, 2, 3])
>>> list(d.keys())
[1, 2, 3]
>>> for i in list(d.keys()):
...     print("数字"+str(i)+"共出现了:"+str(a.count(i))+"次")
...
数字1共出现了:2次
数字2共出现了:2次
数字3共出现了:4次