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.求偶數元素的和[1,2,1,2,3,3,3,3]    
方法一：只適用於當前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：適用於任何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.統計一下所有數字出現的個數[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("數字"+str(i)+"共出現了:"+str(a.count(i))+"次")
...
數字1共出現了:2次
數字2共出現了:2次
數字3共出現了:4次