创建奇数索引的总和python

Question

我试图创建一个函数等于列表中的其他数字的总和。例如，如果列表是[0,1,2,3,4,5]，则函数应该等于5 + 3 + 1。我怎么能这样做？我对Python的知识并没有比while和for循环更远。谢谢。

Answer 1

下面是一个简单的一行：

In [37]: L 
Out[37]: [0, 1, 2, 3, 4, 5] 

In [38]: sum(L[1::2]) 
Out[38]: 9 

在上面的代码，L[1::2]说：“在L获得有史以来第二个元素，索引1开始”

这里是一个办法做到所有繁重的自己：

L = [0, 1, 2, 3, 4, 5] 
total = 0 
for i in range(len(L)): 
    if i%2: # if this is an odd index 
     total += L[i] 

这里的另一种方式，用enumerate：

L = [0, 1, 2, 3, 4, 5] 
total = 0 
for i,num in enumerate(L): 
    if i%2: 
     total += num 

Answer 2

>>> arr = [0,1,2,3,4,5] 
>>> sum([x for idx, x in enumerate(arr) if idx%2 != 0]) 
9 

这仅仅是一个列表理解仅包括在arr具有奇数索引的元素。

为了说明在传统的for循环：

>>> my_sum = 0 
>>> for idx, x in enumerate(arr): 
...  if idx % 2 != 0: 
...   my_sum += x 
...   print("%d was odd, so %d was added. Current sum is %d" % (idx, x, my_sum)) 
...  else: 
...   print("%d was even, so %d was not added. Current sum is %d" % (idx, x, my_sum)) 
... 
0 was even, so 0 was not added. Current sum is 0 
1 was odd, so 1 was added. Current sum is 1 
2 was even, so 2 was not added. Current sum is 1 
3 was odd, so 3 was added. Current sum is 4 
4 was even, so 4 was not added. Current sum is 4 
5 was odd, so 5 was added. Current sum is 9